Paper Cut<\/h1>\r\n<\/div>\r\nIt\u2019s just a paper cut, but the break in your skin could provide an easy way for [pb_glossary id=\"5799\"]pathogens<\/span>[\/pb_glossary]<\/span>\u00a0to enter your body. If\u00a0[pb_glossary id=\"2588\"]bacteria[\/pb_glossary]\u00a0were to enter through the cut and infect the wound, your innate immune system would quickly respond with a dizzying array of general defenses.<\/span>\r\n\r\nWhat Is the Innate Immune System?<\/h1>\r\n<\/div>\r\nThe\u00a0[pb_glossary id=\"4820\"]innate immune system[\/pb_glossary]<\/strong>\u00a0is a subset of the human immune system that produces rapid, but non-specific responses to pathogens. Innate responses are generic, rather than tailored to a particular\u00a0pathogen. The innate system responds in the same general way to every pathogen\u00a0it encounters. Although the innate immune system provides immediate and rapid defenses against pathogens, it does not confer long-lasting\u00a0immunity\u00a0to them. In most organisms, the innate immune system is the dominant system of host defense. Other than most vertebrates (including humans), the innate immune system is the only system of host defense.\r\n\r\nIn humans, the innate immune system includes surface barriers, inflammation, the complement system, and a variety of cellular responses. Surface barriers of various types generally keep most pathogens out of the body. If these barriers fail, then other innate defenses are triggered. The triggering event is usually the identification of pathogens by pattern-recognition receptors on\u00a0cells\u00a0of the innate immune system. These receptors recognize molecules that are broadly shared by pathogens, but distinguishable from host molecules. Alternatively, the other innate defenses may be triggered when damaged, injured, or stressed cells send out alarm signals, many of which are recognized by the same receptors as those that recognize pathogens.\r\n\r\nBarriers to Pathogens<\/h1>\r\n<\/div>\r\nThe body\u2019s first line of defense consists of three different types of barriers that keep most pathogens out of\u00a0body tissues. The types of barriers are mechanical, chemical, and biological barriers.\r\nMechanical Barriers<\/h2>\r\n[caption id=\"attachment_4858\" align=\"alignleft\" width=\"343\"]![\"17.4.2](\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/hairy-nose-2.jpg\")
Figure 17.4.2 Nasal hairs are a mechanical barrier to larger particles in the air.<\/em>[\/caption]\r\n\r\n[pb_glossary id=\"4857\"]Mechanical barriers[\/pb_glossary] are the first line of defense against pathogens, and they physically block pathogens from entering the body. The [pb_glossary id=\"3521\"]skin[\/pb_glossary] is the most important mechanical barrier. In fact, it is the single most important defense the body has. The outer layer of skin \u2014 the\u00a0[pb_glossary id=\"5991\"]epidermis[\/pb_glossary]\u00a0\u2014 is tough, and very difficult for pathogens to penetrate. It consists of dead\u00a0cells\u00a0that are constantly shed from the body surface, a process that helps remove\u00a0bacteria\u00a0and other infectious agents that have adhered to the skin. The epidermis also lacks\u00a0blood vessels\u00a0and is usually lacking moisture, so it does not provide a suitable environment for most pathogens.\u00a0[pb_glossary id=\"3531\"]Hair[\/pb_glossary] \u2014 which is an accessory organ of the skin \u2014 also helps keep out pathogens. Hairs inside the nose may trap larger pathogens and other particles in the air before they can enter the airways of the respiratory system (see Figure 17.4.2).\r\n\r\n[caption id=\"attachment_4859\" align=\"alignright\" width=\"422\"]![\"17.4.3](\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/1024px-Sneeze-2.jpg\")
Figure 17.4.3 A sneeze can expel many pathogens from the respiratory tract, which is why you should always cover your mouth and nose and when you sneeze.<\/em>[\/caption]\r\n\r\n[pb_glossary id=\"3538\"]Mucous membranes<\/span>[\/pb_glossary]<\/span> provide a mechanical barrier to pathogens and other particles at body openings. These membranes also line the respiratory, gastrointestinal, urinary, and reproductive tracts. Mucous membranes secrete mucus, which is a slimy and somewhat sticky substance that traps pathogens. Many mucous membranes also have hair-like [pb_glossary id=\"1597\"]cilia[\/pb_glossary] that sweep mucus and trapped pathogens toward body openings, where they can be removed from the body. When you sneeze or cough, mucus and pathogens are mechanically ejected from the nose and throat, as you can see in Figure 17.4.3.\u00a0 A sneeze can travel as fast as 160 Km\/hr (about 99 mi\/hour) and expel as many as 100,000 droplets into the air around you (a good reason to cover your sneezes!). Other mechanical defenses include tears, which wash pathogens from the eyes, and urine, which flushes pathogens out of the urinary tract.<\/span>\r\nChemical Barriers<\/h2>\r\nChemical barriers also protect against infection by pathogens. They destroy pathogens on the outer body surface, at body openings, and on inner body linings. Sweat, mucus, tears, saliva, and breastmilk all contain antimicrobial substances (such as the\u00a0enzyme\u00a0lysozyme) that kill pathogens, especially\u00a0bacteria. Sebaceous glands in the\u00a0dermis\u00a0of the skin secrete acids that form a very fine, slightly acidic film on the surface of the skin. This film acts as a barrier to bacteria, viruses, and other potential contaminants that might penetrate the skin. Urine and vaginal secretions are also too acidic for many pathogens to endure. Semen contains zinc \u2014 which most pathogens cannot tolerate \u2014 as well as defensins, which are antimicrobial\u00a0proteins\u00a0that act mainly by disrupting bacterial cell membranes. In the stomach, stomach\u00a0acid\u00a0and digestive\u00a0enzymes\u00a0called proteases (which break down proteins) kill most of the pathogens that enter the gastrointestinal tract in food or\u00a0water.\r\nBiological Barriers<\/h2>\r\nBiological barriers are living organisms that help protect the body from pathogens. Trillions of harmless bacteria normally live on the human skin and in the urinary, reproductive, and gastrointestinal tracts. These bacteria use up food and surface space that help prevent pathogenic bacteria from colonizing the body. Some of these harmless bacteria also secrete substances that change the conditions of their environment, making it less hospitable to potentially harmful bacteria.\u00a0They may release toxins or change the\u00a0pH, for example. All of these effects of harmless bacteria reduce the chances that pathogenic microorganisms will be able to reach sufficient numbers and cause illness.\r\n\r\nInflammation<\/h1>\r\n<\/div>\r\nIf pathogens manage to breach the barriers protecting the body, one of the first active responses of the innate immune system kicks in. This response is\u00a0[pb_glossary id=\"4860\"]inflammation[\/pb_glossary].<\/strong>\u00a0The main function of inflammation is to establish a physical barrier against the spread of infection. It also eliminates the initial cause of cell injury, clears out dead\u00a0cells\u00a0and tissues damaged from the original insult and the inflammatory process, and initiates tissue repair. Inflammation is often a response to infection by pathogens, but there are other possible causes, including burns, frostbite, and exposure to toxins.\r\n\r\nThe signs and symptoms of inflammation include redness, swelling, warmth, pain, and frequently some loss of function. These symptoms are caused by increased blood flow into infected tissue, and a number of other processes, illustrated in Figure 17.4.4.\r\n\r\n[caption id=\"attachment_4864\" align=\"alignnone\" width=\"1024\"]![\"17.4.4](\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/OSC_Microbio_17_06_Erythema-2.jpg\")
Figure 17.4.4 This drawing shows what happens during the inflammatory response.<\/em>[\/caption]\r\n\r\nInflammation is triggered by chemicals such as\u00a0[pb_glossary id=\"4865\"]cytokines[\/pb_glossary]<\/strong>\u00a0and\u00a0[pb_glossary id=\"4866\"]histamines[\/pb_glossary],<\/strong>which are released by injured or infected cells, or by immune system cells such as macrophages (described below) that are already present in tissues. These chemicals cause capillaries to dilate and become leaky, increasing blood flow to the infected area and allowing blood to enter the tissues. Pathogen-destroying leukocytes and tissue-repairing proteins migrate into tissue spaces from the bloodstream to attack pathogens and repair their damage. Cytokines also promote [pb_glossary id=\"4867\"]chemotaxis[\/pb_glossary], which is migration to the site of infection by pathogen-destroying leukocytes. Some cytokines have anti-viral effects. They may shut down protein synthesis in host cells, which viruses need in order to survive and replicate.\r\n\r\nSee the video \"The inflammatory response\" by Neural Academy to learn about inflammatory response in more detail:\r\n\r\nhttps:\/\/youtu.be\/Fbzb75HA9M8\r\nThe inflammatory response, Neural Academy, 2019.<\/p>\r\n\r\n
\r\nComplement System<\/h1>\r\n<\/div>\r\nThe\u00a0[pb_glossary id=\"4868\"]complement system[\/pb_glossary]<\/strong>\u00a0is a complex biochemical mechanism named for its ability to \u201ccomplement\u201d the killing of pathogens by [pb_glossary id=\"5659\"]antibodies[\/pb_glossary], which are produced as part of an adaptive immune response. The complement system consists of more than two dozen\u00a0proteins\u00a0normally found in the\u00a0blood\u00a0and synthesized in the [pb_glossary id=\"2989\"]liver[\/pb_glossary]. The proteins usually circulate as non-functional precursor molecules until activated.\r\n\r\nAs shown in Figure 17.4.5, when the first protein in the complement series is activated \u2014typically by the binding of an antibody to an [pb_glossary id=\"2704\"]antigen[\/pb_glossary] on a pathogen \u2014 it sets in motion a domino effect. Each component takes its turn in a precise chain of steps known as the complement cascade. The end product is a cylinder that punctures a hole in the pathogen\u2019s cell membrane. This allows fluids and molecules to flow in and out of the cell, which swells and bursts.\r\n\r\n[caption id=\"attachment_4869\" align=\"aligncenter\" width=\"643\"]![\"17.4.5](\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/2212_Complement_Cascade_and_Function-2.jpg\")
Figure 17.4.5 The complement system is a cascade of proteins that complements the killing of pathogen cells by antibodies.<\/em>[\/caption]\r\n\r\n\r\nCellular Responses<\/h1>\r\n<\/div>\r\nCellular responses of the innate immune system involve a variety of different types of [pb_glossary id=\"5623\"]leukocytes<\/span>[\/pb_glossary]<\/span>. Many of these leukocytes circulate in the blood and act like independent, single-celled organisms, searching out and destroying pathogens in the human host. These and other immune cells of the innate system identify pathogens or debris, and then help to eliminate them in some way. One way is by [pb_glossary id=\"1680\"]phagocytosis[\/pb_glossary].<\/span>\r\nPhagocytosis<\/h2>\r\n[pb_glossary id=\"1680\"]Phagocytosis[\/pb_glossary]<\/strong>\u00a0is an important feature of innate\u00a0immunity\u00a0that is performed by cells classified as phagocytes. In the process of phagocytosis, phagocytes engulf and digest pathogens or other harmful particles. Phagocytes generally patrol the body searching for pathogens, but they can also be called to specific locations by the release of [pb_glossary id=\"4865\"]cytokines<\/span>[\/pb_glossary]<\/span>\u00a0when [pb_glossary id=\"4860\"]inflammation[\/pb_glossary] occurs. Some phagocytes reside permanently in certain tissues.<\/span>\r\n\r\nAs shown in Figure 17.4.6, when a pathogen such as a bacterium is encountered by a phagocyte, the phagocyte extends a portion of its plasma membrane, wrapping the membrane around the pathogen until it is enveloped. Once inside the phagocyte, the pathogen becomes enclosed within an intracellular vesicle called a phagosome. The phagosome then fuses with another vesicle called a [pb_glossary id=\"4870\"]lysosome[\/pb_glossary], forming a phagolysosome. Digestive enzymes and acids from the lysosome kill and digest the pathogen in the phagolysosome. The final step of phagocytosis is excretion of soluble debris from the destroyed pathogen through [pb_glossary id=\"1682\"]exocytosis[\/pb_glossary].\r\n\r\n[caption id=\"attachment_4871\" align=\"aligncenter\" width=\"512\"]![\"17.4.6](\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/512px-Phagocytosis2-2.png\")
Figure 17.4.6 Phagocytosis is a multi-step process in which a pathogen is engulfed and digested by immune cells called phagocytes.<\/em>[\/caption]\r\n\r\nTypes of leukocytes that kill pathogens by phagocytosis include neutrophils, macrophages, and dendritic cells. You can see illustrations of these and other leukocytes involved in innate immune responses in Figure 17.4.7.\r\n\r\n[caption id=\"attachment_4872\" align=\"alignright\" width=\"292\"]![\"17.4.7](\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Innate_Immune_cells.svg_-2.png\")
Figure 17.4.7 Types of leukocytes evolved in innate immune responses are illustrated here.<\/em>[\/caption]\r\nNeutrophils<\/h2>\r\n[pb_glossary id=\"5545\"]Neutrophils[\/pb_glossary]<\/strong>\u00a0are leukocytes that travel throughout the body in the blood. They are usually the first immune cells to arrive at the site of an infection. They are the most numerous types of phagocytes, and they normally make up at least half of the total circulating leukocytes. The [pb_glossary id=\"4021\"]bone marrow[\/pb_glossary] of a normal healthy adult produces more than 100 billion neutrophils per day. During acute inflammation, more than\u00a0ten times<\/em>\u00a0that many neutrophils may be produced each day. Many neutrophils are needed to fight infections, because after a neutrophil phagocytizes just a few pathogens, it generally dies.\r\nMacrophages<\/h2>\r\n[pb_glossary id=\"4340\"]Macrophage<\/strong>s<\/strong>[\/pb_glossary]<\/strong>\u00a0are large phagocytic leukocytes that develop from monocytes. Macrophages spend much of their time within the [pb_glossary id=\"4874\"]interstitial fluid[\/pb_glossary] in\u00a0body tissues. They are the most efficient phagocytes, and they can phagocytize substantial numbers of pathogens or other cells. Macrophages are also versatile cells that produce a wide array of chemicals\u00a0\u2014\u00a0including\u00a0enzymes, complement proteins, and [pb_glossary id=\"4865\"]cytokine<\/span>s<\/span>[\/pb_glossary]<\/span>\u00a0\u2014 in addition to their phagocytic action. As phagocytes, macrophages act as scavengers that rid tissues of worn-out cells and other debris, as well as pathogens. In addition, macrophages act as antigen-presenting cells that activate the\u00a0[pb_glossary id=\"4822\"]adaptive immune system[\/pb_glossary].<\/span>\r\nDendritic Cells<\/h2>\r\nLike macrophages,\u00a0[pb_glossary id=\"4875\"]dendritic cells[\/pb_glossary]<\/strong>\u00a0develop from monocytes. They reside in tissues that have contact with the external environment, so they are located mainly in the skin, nose, lungs, stomach, and intestines. Besides engulfing and digesting pathogens, dendritic cells also act as antigen-presenting cells that trigger adaptive immune responses.\r\nEosinophils<\/h2>\r\n[pb_glossary id=\"5529\"]Eosinophils[\/pb_glossary]<\/strong> are non-phagocytic leukocytes that are related to [pb_glossary id=\"5545\"]neutrophil[\/pb_glossary]. They specialize in defending against [pb_glossary id=\"4812\"]parasites[\/pb_glossary]. They are very effective in killing large parasites (such as worms) by secreting a range of highly-toxic substances when activated. Eosinophils may become overactive and cause [pb_glossary id=\"4877\"]allergies[\/pb_glossary] or [pb_glossary id=\"4348\"]asthma[\/pb_glossary].\r\nBasophils<\/h2>\r\n[pb_glossary id=\"5573\"]Basophils[\/pb_glossary]<\/strong>\u00a0are non-phagocytic leukocytes that are also related to neutrophils. They are the least numerous of all white blood cells. Basophils secrete two types of chemicals that aid in body defenses: [pb_glossary id=\"4879\"]histamines<\/span><\/strong>[\/pb_glossary]<\/span> and heparin. <\/span>Histamines\u00a0are responsible for dilating\u00a0blood vessels\u00a0and increasing their permeability in inflammation.\u00a0<\/span>[pb_glossary id=\"4880\"]Heparin[\/pb_glossary]<\/strong>\u00a0inhibits blood clotting, and also promotes the movement of leukocytes into an area of infection.<\/span>\r\nMast Cells<\/h2>\r\n[pb_glossary id=\"4881\"]Mast cells[\/pb_glossary]<\/strong>\u00a0are non-phagocytic leukocytes that help initiate [pb_glossary id=\"4860\"]inflammation[\/pb_glossary] by secreting [pb_glossary id=\"4879\"]histamines<\/span>[\/pb_glossary]<\/span>. In some people, histamines trigger [pb_glossary id=\"4877\"]allergic reactions[\/pb_glossary], as well as inflammation. Mast cells may also secrete chemicals that help defend against parasites.<\/span>\r\nNatural Killer Cells<\/h2>\r\n[pb_glossary id=\"5515\"]Natural killer cells[\/pb_glossary]<\/strong>\u00a0are in the subset of leukocytes called [pb_glossary id=\"4832\"]lymphocytes<\/span>[\/pb_glossary]<\/span>, which are produced by the\u00a0lymphatic system. Natural killer cells destroy [pb_glossary id=\"5605\"]cancer<\/span>ous<\/span>[\/pb_glossary]<\/span>\u00a0or [pb_glossary id=\"4283\"]virus[\/pb_glossary]-infected host cells, although they do not directly attack invading pathogens. Natural killer cells recognize these host cells by a condition they exhibit called \u201cmissing self.\u201d Cells with missing self have abnormally low levels of cell-surface proteins of the [pb_glossary id=\"4825\"]major histocompatibility complex (MHC)[\/pb_glossary], which normally identify\u00a0body cells\u00a0as self.<\/span>\r\n\r\nInnate Immune Evasion<\/h1>\r\n<\/div>\r\nMany pathogens have evolved mechanisms that allow them to evade\u00a0human hosts'\u00a0innate immune systems. Some of these mechanisms include:\r\n\r\n \t- Invading host cells to replicate so they are \u201chidden\u201d from the immune system.<\/strong>\u00a0The bacterium that causes tuberculosis<\/a> uses this mechanism.<\/li>\r\n \t
- Forming a protective capsule around themselves to avoid being destroyed by immune system cells.<\/strong>\u00a0This defense occurs in bacteria, such as\u00a0Salmonella<\/em><\/a>\u00a0species.<\/li>\r\n \t
- Mimicking host cells so the immune system does not recognize them as foreign.<\/strong>\u00a0Some\u00a0species\u00a0of\u00a0Staphylococcus<\/em><\/a>\u00a0bacteria use this mechanism.<\/li>\r\n \t
- Directly killing phagocytes.<\/strong>\u00a0This ability evolved in several\u00a0species\u00a0of bacteria, including the species that causes anthrax<\/a>.<\/li>\r\n \t
- Producing molecules that prevent the formation of interferons, which are immune chemicals that fight viruses.<\/strong>\u00a0Some influenza<\/a> viruses have this capability.<\/li>\r\n \t
- Forming complex biofilms that provide protection from the cells and proteins of the immune system.<\/strong> This characterizes some species of [pb_glossary id=\"2588\"]bacteria[\/pb_glossary] and fungi. You can see an example of a bacterial biofilm on teeth in Figure 17.4.8.<\/li>\r\n<\/ul>\r\n[caption id=\"attachment_4883\" align=\"aligncenter\" width=\"400\"]
![\"17.4.8](\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/1024px-Gingivitis-before-and-after-3-2.jpg\")
Figure 17.4.8 The dental plaque on the top set of teeth is a biofilm that sticks to the teeth and consists of many species of bacteria. The plaque biofilm is difficult to remove, and it subjects the teeth and gums to high concentrations of bacterial metabolites, which result in dental disease. The same teeth after plaque removal are shown in the bottom picture.<\/em>[\/caption]\r\n\r\n\r\n\r\n \r\n\r\n<\/div>\r\n\r\n\r\n17.4 Summary<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- The [pb_glossary id=\"4820\"]innate immune system[\/pb_glossary] is a subset of the human immune system that produces rapid, but non-specific responses to pathogens. Unlike the\u00a0[pb_glossary id=\"4822\"]adaptive immune system[\/pb_glossary], the innate system does not confer\u00a0[pb_glossary id=\"4884\"]immunity[\/pb_glossary]. The innate immune system includes surface barriers, [pb_glossary id=\"4860\"]inflammation[\/pb_glossary], the [pb_glossary id=\"4868\"]complement system[\/pb_glossary], and a variety of cellular responses.<\/li>\r\n \t
- The body\u2019s first line of defense consists of three different types of barriers that keep most pathogens out of\u00a0body tissues. The types of barriers are mechanical, chemical, and biological barriers.<\/li>\r\n \t
- Mechanical barriers\u00a0\u2014\u00a0which include the [pb_glossary id=\"3521\"]skin[\/pb_glossary], [pb_glossary id=\"3538\"]mucous membranes[\/pb_glossary], and fluids such as tears and [pb_glossary id=\"4717\"]urine[\/pb_glossary] \u2014 physically block pathogens from entering the body. Chemical barriers\u00a0\u2014\u00a0such as enzymes in [pb_glossary id=\"4711\"]sweat[\/pb_glossary], [pb_glossary id=\"4595\"]saliva[\/pb_glossary], and [pb_glossary id=\"4885\"]semen[\/pb_glossary]\u00a0\u2014\u00a0kill pathogens on body surfaces. Biological barriers are harmless bacteria that use up food and space so pathogenic bacteria cannot colonize the body.<\/li>\r\n \t
- If pathogens breach protective barriers, [pb_glossary id=\"4860\"]inflammation[\/pb_glossary] occurs. This creates a physical barrier against the spread of infection, and repairs tissue damage. Inflammation is triggered by chemicals such as [pb_glossary id=\"4865\"]cytokines[\/pb_glossary]\u00a0and [pb_glossary id=\"4879\"]histamines[\/pb_glossary], and it causes swelling, redness, and warmth.<\/li>\r\n \t
- The [pb_glossary id=\"4868\"]complement system[\/pb_glossary] is a complex biochemical mechanism that helps [pb_glossary id=\"5659\"]antibodies[\/pb_glossary] kill pathogens. Once activated, the complement system consists of more than two dozen proteins that lead to disruption of the\u00a0cell membrane\u00a0of pathogens and bursting of the cells.<\/li>\r\n \t
- Cellular responses of the innate immune system involve various types of [pb_glossary id=\"5623\"]leukocytes[\/pb_glossary]. For example, [pb_glossary id=\"5545\"]neutrophils[\/pb_glossary], [pb_glossary id=\"4340\"]macrophages[\/pb_glossary], and [pb_glossary id=\"4875\"]dendritic cells[\/pb_glossary]\u00a0phagocytize pathogens. [pb_glossary id=\"5573\"]Basophils[\/pb_glossary]\u00a0and [pb_glossary id=\"4881\"]mast cells[\/pb_glossary]\u00a0release chemicals that trigger inflammation. [pb_glossary id=\"5515\"]Natural killer cells[\/pb_glossary]\u00a0destroy cancerous or virus-infected cells, and eosinophils kill parasites.<\/li>\r\n \t
- Many pathogens have evolved mechanisms that help them evade the innate immune system. For example, some pathogens form a protective capsule around themselves, and some mimic host cells so the immune system does not recognize them as foreign.<\/li>\r\n<\/ul>\r\n<\/div>\r\n<\/div>\r\n
\r\n17.4 Review Questions<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- What is the innate immune system?<\/li>\r\n \t
- Identify the body\u2019s first line of defense.<\/li>\r\n \t
- [h5p id=\"638\"]<\/li>\r\n \t
- What are biological barriers? How do they protect the body?<\/li>\r\n \t
- State the purposes of inflammation. What triggers inflammation, and what signs and symptoms does it cause?<\/li>\r\n \t
- Define the complement system. How does it help destroy pathogens?<\/li>\r\n \t
- Describe two ways that pathogens can evade the innate immune system.<\/li>\r\n \t
- What are the ways in which phagocytes can encounter pathogens in the body?<\/li>\r\n \t
- Describe two different ways in which enzymes play a role in the innate immune response.<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n
\r\n17.4 Explore More<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\nhttps:\/\/youtu.be\/WW4skW6gucU\r\nHow mucus keeps us healthy - Katharina Ribbeck, TED-Ed, 2015.<\/p>\r\nhttps:\/\/youtu.be\/sYjtMP67vyk\r\n
Human Physiology - Innate Immune System, Janux, 2015.<\/p>\r\nhttps:\/\/youtu.be\/c64M1tZyWPM\r\n
Myriam Sidibe: The simple power of handwashing, TED, 2014.<\/p>\r\nhttps:\/\/youtu.be\/shEPwQPQG4I\r\n
Everything You Didn't Want To Know About Snot, Gross Science, 2017.<\/p>\r\nhttps:\/\/youtu.be\/dy1D3d1FBcw\r\n
Cough Grosser Than Sneeze? | Curiosity - World's Dirtiest Man, Discovery, 2011.<\/p>\r\n\r\n<\/div>\r\n<\/div>\r\n \r\n\r\n<\/div>\r\n
Attributions<\/h2>\r\nFigure 17.4.1<\/strong>\r\n\r\nOww_Papercut_14365<\/a> by Laurence Facun<\/a> on Wikimedia Commons is used under a CC BY 2.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/2.0) license.\r\n\r\nFigure 17.4.2<\/strong>\r\n\r\nhairy-nose<\/a> by Piotr Siedlecki\u00a0 on publicdomainpictures.net<\/a> is used under a CC0 1.0<\/a> Universal Public Domain Dedication (http:\/\/creativecommons.org\/publicdomain\/zero\/1.0\/) license.\r\n\r\nFigure 17.4.3<\/strong>\r\n\r\n1024px-Sneeze<\/a> by James Gathany\/ CDC Public Health Image library (PHIL) ID# 11162<\/a> on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/public_domain).\r\n\r\nFigure 17.4.4<\/strong>\r\n\r\nOSC_Microbio_17_06_Erythema<\/a> by CNX OpenStax<\/a> on Wikimedia Commons is used under a CC BY 4.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/4.0) license.\r\n\r\nFigure 17.4.5<\/strong>\r\n\r\n2212_Complement_Cascade_and_Function<\/a>\u00a0by OpenStax College<\/a> on Wikimedia Commons is used under a\u00a0CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\r\n\r\nFigure 17.4.6<\/strong>\r\n\r\n512px-Phagocytosis2<\/a> by Graham\u00a0Colm<\/span><\/a> at English Wikipedia<\/a><\/span>\u00a0on Wikimedia Commons is used under a CC BY-SA 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-sa\/3.0) license.\r\n\r\nFigure 17.4.7<\/strong>\r\n\r\nInnate_Immune_cells.svg<\/a> by Fred the Oyster<\/a> on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).\r\n\r\nFigure 17.4.8<\/strong>\r\n\r\n1024px-Gingivitis-before-and-after-3<\/a> by Onetimeuseaccount<\/a> on Wikimedia Commons is used under a CC0 1.0<\/a> Universal Public Domain Dedication (http:\/\/creativecommons.org\/publicdomain\/zero\/1.0\/) license.\r\nReferences<\/h2>\r\n
Betts, J. G., Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E., Womble, M., DeSaix, P. (2013, June 19). Figure 21.13 Complement cascade and function [digital image].\u00a0 In Anatomy and Physiology<\/em> (Section 21.2). OpenStax. https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/21-2-barrier-defenses-and-the-innate-immune-response<\/p>\r\nDiscovery. (2011, October 27). Cough grosser than sneeze? | Curiosity - World's dirtiest man. YouTube. https:\/\/www.youtube.com\/watch?v=dy1D3d1FBcw&feature=youtu.be<\/p>\r\n
Gross Science. (2017, January 31). Everything you didn't want to know about snot. YouTube. https:\/\/www.youtube.com\/watch?v=shEPwQPQG4I&feature=youtu.be<\/p>\r\n
Janux. (2015, January 10). Human physiology - Innate immune system. YouTube. https:\/\/www.youtube.com\/watch?v=sYjtMP67vyk&feature=youtu.be<\/p>\r\n
Mayo Clinic Staff. (n.d.). Anthrax [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/anthrax\/symptoms-causes\/syc-20356203<\/p>\r\n
Mayo Clinic Staff. (n.d.). Influenza (flu) [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/flu\/symptoms-causes\/syc-20351719<\/p>\r\n
Mayo Clinic Staff. (n.d.). Salmonella infection [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/salmonella\/symptoms-causes\/syc-20355329<\/p>\r\n
Mayo Clinic Staff. (n.d.). Staph infection [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/staph-infections\/multimedia\/staph-infection\/img-20008600<\/p>\r\n
Mayo Clinic Staff. (n.d.). Tuberculosis [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/tuberculosis\/symptoms-causes\/syc-20351250<\/p>\r\n
OpenStax. (2016, November 11). Figure\u00a0<\/span>17.23<\/span> A typical case of acute inflammation at the site of a skin wound - Erythema [digital image]. In OpenStax, Microbiology<\/em> (Section 17.5). https:\/\/openstax.org\/details\/books\/microbiology?Bookdetails<\/p>\r\nTED. (2014, October 14). Myriam Sidibe: The simple power of handwashing. YouTube. https:\/\/www.youtube.com\/watch?v=c64M1tZyWPM&feature=youtu.be<\/p>\r\n
TED-Ed. (2015, November 5). How mucus keeps us healthy - Katharina Ribbeck. YouTube. https:\/\/www.youtube.com\/watch?v=WW4skW6gucU&feature=youtu.be<\/p>","rendered":"
<\/p>\n![\"17.4.1](\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2019\/06\/Oww_Papercut_14365-2.jpg\")
Figure 17.4.1 Darn it!\u00a0 Paper cuts are the worst!<\/em><\/figcaption><\/figure>\n\nPaper Cut<\/h1>\n<\/div>\n
It\u2019s just a paper cut, but the break in your skin could provide an easy way for pathogens<\/span><\/a><\/span>\u00a0to enter your body. If\u00a0bacteria<\/a>\u00a0were to enter through the cut and infect the wound, your innate immune system would quickly respond with a dizzying array of general defenses.<\/span><\/p>\n\nWhat Is the Innate Immune System?<\/h1>\n<\/div>\n
The\u00a0innate immune system<\/a><\/strong>\u00a0is a subset of the human immune system that produces rapid, but non-specific responses to pathogens. Innate responses are generic, rather than tailored to a particular\u00a0pathogen. The innate system responds in the same general way to every pathogen\u00a0it encounters. Although the innate immune system provides immediate and rapid defenses against pathogens, it does not confer long-lasting\u00a0immunity\u00a0to them. In most organisms, the innate immune system is the dominant system of host defense. Other than most vertebrates (including humans), the innate immune system is the only system of host defense.<\/p>\nIn humans, the innate immune system includes surface barriers, inflammation, the complement system, and a variety of cellular responses. Surface barriers of various types generally keep most pathogens out of the body. If these barriers fail, then other innate defenses are triggered. The triggering event is usually the identification of pathogens by pattern-recognition receptors on\u00a0cells\u00a0of the innate immune system. These receptors recognize molecules that are broadly shared by pathogens, but distinguishable from host molecules. Alternatively, the other innate defenses may be triggered when damaged, injured, or stressed cells send out alarm signals, many of which are recognized by the same receptors as those that recognize pathogens.<\/p>\n
\nBarriers to Pathogens<\/h1>\n<\/div>\n
The body\u2019s first line of defense consists of three different types of barriers that keep most pathogens out of\u00a0body tissues. The types of barriers are mechanical, chemical, and biological barriers.<\/p>\n
Mechanical Barriers<\/h2>\nFigure 17.4.2 Nasal hairs are a mechanical barrier to larger particles in the air.<\/em><\/figcaption><\/figure>\nMechanical barriers<\/a> are the first line of defense against pathogens, and they physically block pathogens from entering the body. The skin<\/a> is the most important mechanical barrier. In fact, it is the single most important defense the body has. The outer layer of skin \u2014 the\u00a0epidermis<\/a>\u00a0\u2014 is tough, and very difficult for pathogens to penetrate. It consists of dead\u00a0cells\u00a0that are constantly shed from the body surface, a process that helps remove\u00a0bacteria\u00a0and other infectious agents that have adhered to the skin. The epidermis also lacks\u00a0blood vessels\u00a0and is usually lacking moisture, so it does not provide a suitable environment for most pathogens.\u00a0Hair<\/a> \u2014 which is an accessory organ of the skin \u2014 also helps keep out pathogens. Hairs inside the nose may trap larger pathogens and other particles in the air before they can enter the airways of the respiratory system (see Figure 17.4.2).<\/p>\nFigure 17.4.3 A sneeze can expel many pathogens from the respiratory tract, which is why you should always cover your mouth and nose and when you sneeze.<\/em><\/figcaption><\/figure>\nMucous membranes<\/span><\/a><\/span> provide a mechanical barrier to pathogens and other particles at body openings. These membranes also line the respiratory, gastrointestinal, urinary, and reproductive tracts. Mucous membranes secrete mucus, which is a slimy and somewhat sticky substance that traps pathogens. Many mucous membranes also have hair-like cilia<\/a> that sweep mucus and trapped pathogens toward body openings, where they can be removed from the body. When you sneeze or cough, mucus and pathogens are mechanically ejected from the nose and throat, as you can see in Figure 17.4.3.\u00a0 A sneeze can travel as fast as 160 Km\/hr (about 99 mi\/hour) and expel as many as 100,000 droplets into the air around you (a good reason to cover your sneezes!). Other mechanical defenses include tears, which wash pathogens from the eyes, and urine, which flushes pathogens out of the urinary tract.<\/span><\/p>\nChemical Barriers<\/h2>\n
Chemical barriers also protect against infection by pathogens. They destroy pathogens on the outer body surface, at body openings, and on inner body linings. Sweat, mucus, tears, saliva, and breastmilk all contain antimicrobial substances (such as the\u00a0enzyme\u00a0lysozyme) that kill pathogens, especially\u00a0bacteria. Sebaceous glands in the\u00a0dermis\u00a0of the skin secrete acids that form a very fine, slightly acidic film on the surface of the skin. This film acts as a barrier to bacteria, viruses, and other potential contaminants that might penetrate the skin. Urine and vaginal secretions are also too acidic for many pathogens to endure. Semen contains zinc \u2014 which most pathogens cannot tolerate \u2014 as well as defensins, which are antimicrobial\u00a0proteins\u00a0that act mainly by disrupting bacterial cell membranes. In the stomach, stomach\u00a0acid\u00a0and digestive\u00a0enzymes\u00a0called proteases (which break down proteins) kill most of the pathogens that enter the gastrointestinal tract in food or\u00a0water.<\/p>\n
Biological Barriers<\/h2>\n
Biological barriers are living organisms that help protect the body from pathogens. Trillions of harmless bacteria normally live on the human skin and in the urinary, reproductive, and gastrointestinal tracts. These bacteria use up food and surface space that help prevent pathogenic bacteria from colonizing the body. Some of these harmless bacteria also secrete substances that change the conditions of their environment, making it less hospitable to potentially harmful bacteria.\u00a0They may release toxins or change the\u00a0pH, for example. All of these effects of harmless bacteria reduce the chances that pathogenic microorganisms will be able to reach sufficient numbers and cause illness.<\/p>\n
\nInflammation<\/h1>\n<\/div>\n
If pathogens manage to breach the barriers protecting the body, one of the first active responses of the innate immune system kicks in. This response is\u00a0inflammation<\/a>.<\/strong>\u00a0The main function of inflammation is to establish a physical barrier against the spread of infection. It also eliminates the initial cause of cell injury, clears out dead\u00a0cells\u00a0and tissues damaged from the original insult and the inflammatory process, and initiates tissue repair. Inflammation is often a response to infection by pathogens, but there are other possible causes, including burns, frostbite, and exposure to toxins.<\/p>\nThe signs and symptoms of inflammation include redness, swelling, warmth, pain, and frequently some loss of function. These symptoms are caused by increased blood flow into infected tissue, and a number of other processes, illustrated in Figure 17.4.4.<\/p>\nFigure 17.4.4 This drawing shows what happens during the inflammatory response.<\/em><\/figcaption><\/figure>\nInflammation is triggered by chemicals such as\u00a0cytokines<\/a><\/strong>\u00a0and\u00a0histamines<\/a>,<\/strong>which are released by injured or infected cells, or by immune system cells such as macrophages (described below) that are already present in tissues. These chemicals cause capillaries to dilate and become leaky, increasing blood flow to the infected area and allowing blood to enter the tissues. Pathogen-destroying leukocytes and tissue-repairing proteins migrate into tissue spaces from the bloodstream to attack pathogens and repair their damage. Cytokines also promote chemotaxis<\/a>, which is migration to the site of infection by pathogen-destroying leukocytes. Some cytokines have anti-viral effects. They may shut down protein synthesis in host cells, which viruses need in order to survive and replicate.<\/p>\nSee the video “The inflammatory response” by Neural Academy to learn about inflammatory response in more detail:<\/p>\n
What Is the Innate Immune System?<\/h1>\r\n<\/div>\r\nThe\u00a0[pb_glossary id=\"4820\"]innate immune system[\/pb_glossary]<\/strong>\u00a0is a subset of the human immune system that produces rapid, but non-specific responses to pathogens. Innate responses are generic, rather than tailored to a particular\u00a0pathogen. The innate system responds in the same general way to every pathogen\u00a0it encounters. Although the innate immune system provides immediate and rapid defenses against pathogens, it does not confer long-lasting\u00a0immunity\u00a0to them. In most organisms, the innate immune system is the dominant system of host defense. Other than most vertebrates (including humans), the innate immune system is the only system of host defense.\r\n\r\nIn humans, the innate immune system includes surface barriers, inflammation, the complement system, and a variety of cellular responses. Surface barriers of various types generally keep most pathogens out of the body. If these barriers fail, then other innate defenses are triggered. The triggering event is usually the identification of pathogens by pattern-recognition receptors on\u00a0cells\u00a0of the innate immune system. These receptors recognize molecules that are broadly shared by pathogens, but distinguishable from host molecules. Alternatively, the other innate defenses may be triggered when damaged, injured, or stressed cells send out alarm signals, many of which are recognized by the same receptors as those that recognize pathogens.\r\n\r\nBarriers to Pathogens<\/h1>\r\n<\/div>\r\nThe body\u2019s first line of defense consists of three different types of barriers that keep most pathogens out of\u00a0body tissues. The types of barriers are mechanical, chemical, and biological barriers.\r\nMechanical Barriers<\/h2>\r\n[caption id=\"attachment_4858\" align=\"alignleft\" width=\"343\"] Figure 17.4.2 Nasal hairs are a mechanical barrier to larger particles in the air.<\/em>[\/caption]\r\n\r\n[pb_glossary id=\"4857\"]Mechanical barriers[\/pb_glossary] are the first line of defense against pathogens, and they physically block pathogens from entering the body. The [pb_glossary id=\"3521\"]skin[\/pb_glossary] is the most important mechanical barrier. In fact, it is the single most important defense the body has. The outer layer of skin \u2014 the\u00a0[pb_glossary id=\"5991\"]epidermis[\/pb_glossary]\u00a0\u2014 is tough, and very difficult for pathogens to penetrate. It consists of dead\u00a0cells\u00a0that are constantly shed from the body surface, a process that helps remove\u00a0bacteria\u00a0and other infectious agents that have adhered to the skin. The epidermis also lacks\u00a0blood vessels\u00a0and is usually lacking moisture, so it does not provide a suitable environment for most pathogens.\u00a0[pb_glossary id=\"3531\"]Hair[\/pb_glossary] \u2014 which is an accessory organ of the skin \u2014 also helps keep out pathogens. Hairs inside the nose may trap larger pathogens and other particles in the air before they can enter the airways of the respiratory system (see Figure 17.4.2).\r\n\r\n[caption id=\"attachment_4859\" align=\"alignright\" width=\"422\"] Figure 17.4.3 A sneeze can expel many pathogens from the respiratory tract, which is why you should always cover your mouth and nose and when you sneeze.<\/em>[\/caption]\r\n\r\n[pb_glossary id=\"3538\"]Mucous membranes<\/span>[\/pb_glossary]<\/span> provide a mechanical barrier to pathogens and other particles at body openings. These membranes also line the respiratory, gastrointestinal, urinary, and reproductive tracts. Mucous membranes secrete mucus, which is a slimy and somewhat sticky substance that traps pathogens. Many mucous membranes also have hair-like [pb_glossary id=\"1597\"]cilia[\/pb_glossary] that sweep mucus and trapped pathogens toward body openings, where they can be removed from the body. When you sneeze or cough, mucus and pathogens are mechanically ejected from the nose and throat, as you can see in Figure 17.4.3.\u00a0 A sneeze can travel as fast as 160 Km\/hr (about 99 mi\/hour) and expel as many as 100,000 droplets into the air around you (a good reason to cover your sneezes!). Other mechanical defenses include tears, which wash pathogens from the eyes, and urine, which flushes pathogens out of the urinary tract.<\/span>\r\nChemical Barriers<\/h2>\r\nChemical barriers also protect against infection by pathogens. They destroy pathogens on the outer body surface, at body openings, and on inner body linings. Sweat, mucus, tears, saliva, and breastmilk all contain antimicrobial substances (such as the\u00a0enzyme\u00a0lysozyme) that kill pathogens, especially\u00a0bacteria. Sebaceous glands in the\u00a0dermis\u00a0of the skin secrete acids that form a very fine, slightly acidic film on the surface of the skin. This film acts as a barrier to bacteria, viruses, and other potential contaminants that might penetrate the skin. Urine and vaginal secretions are also too acidic for many pathogens to endure. Semen contains zinc \u2014 which most pathogens cannot tolerate \u2014 as well as defensins, which are antimicrobial\u00a0proteins\u00a0that act mainly by disrupting bacterial cell membranes. In the stomach, stomach\u00a0acid\u00a0and digestive\u00a0enzymes\u00a0called proteases (which break down proteins) kill most of the pathogens that enter the gastrointestinal tract in food or\u00a0water.\r\nBiological Barriers<\/h2>\r\nBiological barriers are living organisms that help protect the body from pathogens. Trillions of harmless bacteria normally live on the human skin and in the urinary, reproductive, and gastrointestinal tracts. These bacteria use up food and surface space that help prevent pathogenic bacteria from colonizing the body. Some of these harmless bacteria also secrete substances that change the conditions of their environment, making it less hospitable to potentially harmful bacteria.\u00a0They may release toxins or change the\u00a0pH, for example. All of these effects of harmless bacteria reduce the chances that pathogenic microorganisms will be able to reach sufficient numbers and cause illness.\r\n\r\nInflammation<\/h1>\r\n<\/div>\r\nIf pathogens manage to breach the barriers protecting the body, one of the first active responses of the innate immune system kicks in. This response is\u00a0[pb_glossary id=\"4860\"]inflammation[\/pb_glossary].<\/strong>\u00a0The main function of inflammation is to establish a physical barrier against the spread of infection. It also eliminates the initial cause of cell injury, clears out dead\u00a0cells\u00a0and tissues damaged from the original insult and the inflammatory process, and initiates tissue repair. Inflammation is often a response to infection by pathogens, but there are other possible causes, including burns, frostbite, and exposure to toxins.\r\n\r\nThe signs and symptoms of inflammation include redness, swelling, warmth, pain, and frequently some loss of function. These symptoms are caused by increased blood flow into infected tissue, and a number of other processes, illustrated in Figure 17.4.4.\r\n\r\n[caption id=\"attachment_4864\" align=\"alignnone\" width=\"1024\"] Figure 17.4.4 This drawing shows what happens during the inflammatory response.<\/em>[\/caption]\r\n\r\nInflammation is triggered by chemicals such as\u00a0[pb_glossary id=\"4865\"]cytokines[\/pb_glossary]<\/strong>\u00a0and\u00a0[pb_glossary id=\"4866\"]histamines[\/pb_glossary],<\/strong>which are released by injured or infected cells, or by immune system cells such as macrophages (described below) that are already present in tissues. These chemicals cause capillaries to dilate and become leaky, increasing blood flow to the infected area and allowing blood to enter the tissues. Pathogen-destroying leukocytes and tissue-repairing proteins migrate into tissue spaces from the bloodstream to attack pathogens and repair their damage. Cytokines also promote [pb_glossary id=\"4867\"]chemotaxis[\/pb_glossary], which is migration to the site of infection by pathogen-destroying leukocytes. Some cytokines have anti-viral effects. They may shut down protein synthesis in host cells, which viruses need in order to survive and replicate.\r\n\r\nSee the video \"The inflammatory response\" by Neural Academy to learn about inflammatory response in more detail:\r\n\r\nhttps:\/\/youtu.be\/Fbzb75HA9M8\r\nThe inflammatory response, Neural Academy, 2019.<\/p>\r\n\r\n
\r\nComplement System<\/h1>\r\n<\/div>\r\nThe\u00a0[pb_glossary id=\"4868\"]complement system[\/pb_glossary]<\/strong>\u00a0is a complex biochemical mechanism named for its ability to \u201ccomplement\u201d the killing of pathogens by [pb_glossary id=\"5659\"]antibodies[\/pb_glossary], which are produced as part of an adaptive immune response. The complement system consists of more than two dozen\u00a0proteins\u00a0normally found in the\u00a0blood\u00a0and synthesized in the [pb_glossary id=\"2989\"]liver[\/pb_glossary]. The proteins usually circulate as non-functional precursor molecules until activated.\r\n\r\nAs shown in Figure 17.4.5, when the first protein in the complement series is activated \u2014typically by the binding of an antibody to an [pb_glossary id=\"2704\"]antigen[\/pb_glossary] on a pathogen \u2014 it sets in motion a domino effect. Each component takes its turn in a precise chain of steps known as the complement cascade. The end product is a cylinder that punctures a hole in the pathogen\u2019s cell membrane. This allows fluids and molecules to flow in and out of the cell, which swells and bursts.\r\n\r\n[caption id=\"attachment_4869\" align=\"aligncenter\" width=\"643\"] Figure 17.4.5 The complement system is a cascade of proteins that complements the killing of pathogen cells by antibodies.<\/em>[\/caption]\r\n\r\n\r\nCellular Responses<\/h1>\r\n<\/div>\r\nCellular responses of the innate immune system involve a variety of different types of [pb_glossary id=\"5623\"]leukocytes<\/span>[\/pb_glossary]<\/span>. Many of these leukocytes circulate in the blood and act like independent, single-celled organisms, searching out and destroying pathogens in the human host. These and other immune cells of the innate system identify pathogens or debris, and then help to eliminate them in some way. One way is by [pb_glossary id=\"1680\"]phagocytosis[\/pb_glossary].<\/span>\r\nPhagocytosis<\/h2>\r\n[pb_glossary id=\"1680\"]Phagocytosis[\/pb_glossary]<\/strong>\u00a0is an important feature of innate\u00a0immunity\u00a0that is performed by cells classified as phagocytes. In the process of phagocytosis, phagocytes engulf and digest pathogens or other harmful particles. Phagocytes generally patrol the body searching for pathogens, but they can also be called to specific locations by the release of [pb_glossary id=\"4865\"]cytokines<\/span>[\/pb_glossary]<\/span>\u00a0when [pb_glossary id=\"4860\"]inflammation[\/pb_glossary] occurs. Some phagocytes reside permanently in certain tissues.<\/span>\r\n\r\nAs shown in Figure 17.4.6, when a pathogen such as a bacterium is encountered by a phagocyte, the phagocyte extends a portion of its plasma membrane, wrapping the membrane around the pathogen until it is enveloped. Once inside the phagocyte, the pathogen becomes enclosed within an intracellular vesicle called a phagosome. The phagosome then fuses with another vesicle called a [pb_glossary id=\"4870\"]lysosome[\/pb_glossary], forming a phagolysosome. Digestive enzymes and acids from the lysosome kill and digest the pathogen in the phagolysosome. The final step of phagocytosis is excretion of soluble debris from the destroyed pathogen through [pb_glossary id=\"1682\"]exocytosis[\/pb_glossary].\r\n\r\n[caption id=\"attachment_4871\" align=\"aligncenter\" width=\"512\"] Figure 17.4.6 Phagocytosis is a multi-step process in which a pathogen is engulfed and digested by immune cells called phagocytes.<\/em>[\/caption]\r\n\r\nTypes of leukocytes that kill pathogens by phagocytosis include neutrophils, macrophages, and dendritic cells. You can see illustrations of these and other leukocytes involved in innate immune responses in Figure 17.4.7.\r\n\r\n[caption id=\"attachment_4872\" align=\"alignright\" width=\"292\"] Figure 17.4.7 Types of leukocytes evolved in innate immune responses are illustrated here.<\/em>[\/caption]\r\nNeutrophils<\/h2>\r\n[pb_glossary id=\"5545\"]Neutrophils[\/pb_glossary]<\/strong>\u00a0are leukocytes that travel throughout the body in the blood. They are usually the first immune cells to arrive at the site of an infection. They are the most numerous types of phagocytes, and they normally make up at least half of the total circulating leukocytes. The [pb_glossary id=\"4021\"]bone marrow[\/pb_glossary] of a normal healthy adult produces more than 100 billion neutrophils per day. During acute inflammation, more than\u00a0ten times<\/em>\u00a0that many neutrophils may be produced each day. Many neutrophils are needed to fight infections, because after a neutrophil phagocytizes just a few pathogens, it generally dies.\r\nMacrophages<\/h2>\r\n[pb_glossary id=\"4340\"]Macrophage<\/strong>s<\/strong>[\/pb_glossary]<\/strong>\u00a0are large phagocytic leukocytes that develop from monocytes. Macrophages spend much of their time within the [pb_glossary id=\"4874\"]interstitial fluid[\/pb_glossary] in\u00a0body tissues. They are the most efficient phagocytes, and they can phagocytize substantial numbers of pathogens or other cells. Macrophages are also versatile cells that produce a wide array of chemicals\u00a0\u2014\u00a0including\u00a0enzymes, complement proteins, and [pb_glossary id=\"4865\"]cytokine<\/span>s<\/span>[\/pb_glossary]<\/span>\u00a0\u2014 in addition to their phagocytic action. As phagocytes, macrophages act as scavengers that rid tissues of worn-out cells and other debris, as well as pathogens. In addition, macrophages act as antigen-presenting cells that activate the\u00a0[pb_glossary id=\"4822\"]adaptive immune system[\/pb_glossary].<\/span>\r\nDendritic Cells<\/h2>\r\nLike macrophages,\u00a0[pb_glossary id=\"4875\"]dendritic cells[\/pb_glossary]<\/strong>\u00a0develop from monocytes. They reside in tissues that have contact with the external environment, so they are located mainly in the skin, nose, lungs, stomach, and intestines. Besides engulfing and digesting pathogens, dendritic cells also act as antigen-presenting cells that trigger adaptive immune responses.\r\nEosinophils<\/h2>\r\n[pb_glossary id=\"5529\"]Eosinophils[\/pb_glossary]<\/strong> are non-phagocytic leukocytes that are related to [pb_glossary id=\"5545\"]neutrophil[\/pb_glossary]. They specialize in defending against [pb_glossary id=\"4812\"]parasites[\/pb_glossary]. They are very effective in killing large parasites (such as worms) by secreting a range of highly-toxic substances when activated. Eosinophils may become overactive and cause [pb_glossary id=\"4877\"]allergies[\/pb_glossary] or [pb_glossary id=\"4348\"]asthma[\/pb_glossary].\r\nBasophils<\/h2>\r\n[pb_glossary id=\"5573\"]Basophils[\/pb_glossary]<\/strong>\u00a0are non-phagocytic leukocytes that are also related to neutrophils. They are the least numerous of all white blood cells. Basophils secrete two types of chemicals that aid in body defenses: [pb_glossary id=\"4879\"]histamines<\/span><\/strong>[\/pb_glossary]<\/span> and heparin. <\/span>Histamines\u00a0are responsible for dilating\u00a0blood vessels\u00a0and increasing their permeability in inflammation.\u00a0<\/span>[pb_glossary id=\"4880\"]Heparin[\/pb_glossary]<\/strong>\u00a0inhibits blood clotting, and also promotes the movement of leukocytes into an area of infection.<\/span>\r\nMast Cells<\/h2>\r\n[pb_glossary id=\"4881\"]Mast cells[\/pb_glossary]<\/strong>\u00a0are non-phagocytic leukocytes that help initiate [pb_glossary id=\"4860\"]inflammation[\/pb_glossary] by secreting [pb_glossary id=\"4879\"]histamines<\/span>[\/pb_glossary]<\/span>. In some people, histamines trigger [pb_glossary id=\"4877\"]allergic reactions[\/pb_glossary], as well as inflammation. Mast cells may also secrete chemicals that help defend against parasites.<\/span>\r\nNatural Killer Cells<\/h2>\r\n[pb_glossary id=\"5515\"]Natural killer cells[\/pb_glossary]<\/strong>\u00a0are in the subset of leukocytes called [pb_glossary id=\"4832\"]lymphocytes<\/span>[\/pb_glossary]<\/span>, which are produced by the\u00a0lymphatic system. Natural killer cells destroy [pb_glossary id=\"5605\"]cancer<\/span>ous<\/span>[\/pb_glossary]<\/span>\u00a0or [pb_glossary id=\"4283\"]virus[\/pb_glossary]-infected host cells, although they do not directly attack invading pathogens. Natural killer cells recognize these host cells by a condition they exhibit called \u201cmissing self.\u201d Cells with missing self have abnormally low levels of cell-surface proteins of the [pb_glossary id=\"4825\"]major histocompatibility complex (MHC)[\/pb_glossary], which normally identify\u00a0body cells\u00a0as self.<\/span>\r\n\r\nInnate Immune Evasion<\/h1>\r\n<\/div>\r\nMany pathogens have evolved mechanisms that allow them to evade\u00a0human hosts'\u00a0innate immune systems. Some of these mechanisms include:\r\n\r\n \t- Invading host cells to replicate so they are \u201chidden\u201d from the immune system.<\/strong>\u00a0The bacterium that causes tuberculosis<\/a> uses this mechanism.<\/li>\r\n \t
- Forming a protective capsule around themselves to avoid being destroyed by immune system cells.<\/strong>\u00a0This defense occurs in bacteria, such as\u00a0Salmonella<\/em><\/a>\u00a0species.<\/li>\r\n \t
- Mimicking host cells so the immune system does not recognize them as foreign.<\/strong>\u00a0Some\u00a0species\u00a0of\u00a0Staphylococcus<\/em><\/a>\u00a0bacteria use this mechanism.<\/li>\r\n \t
- Directly killing phagocytes.<\/strong>\u00a0This ability evolved in several\u00a0species\u00a0of bacteria, including the species that causes anthrax<\/a>.<\/li>\r\n \t
- Producing molecules that prevent the formation of interferons, which are immune chemicals that fight viruses.<\/strong>\u00a0Some influenza<\/a> viruses have this capability.<\/li>\r\n \t
- Forming complex biofilms that provide protection from the cells and proteins of the immune system.<\/strong> This characterizes some species of [pb_glossary id=\"2588\"]bacteria[\/pb_glossary] and fungi. You can see an example of a bacterial biofilm on teeth in Figure 17.4.8.<\/li>\r\n<\/ul>\r\n[caption id=\"attachment_4883\" align=\"aligncenter\" width=\"400\"] Figure 17.4.8 The dental plaque on the top set of teeth is a biofilm that sticks to the teeth and consists of many species of bacteria. The plaque biofilm is difficult to remove, and it subjects the teeth and gums to high concentrations of bacterial metabolites, which result in dental disease. The same teeth after plaque removal are shown in the bottom picture.<\/em>[\/caption]\r\n\r\n\r\n\r\n \r\n\r\n<\/div>\r\n\r\n
\r\n17.4 Summary<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- The [pb_glossary id=\"4820\"]innate immune system[\/pb_glossary] is a subset of the human immune system that produces rapid, but non-specific responses to pathogens. Unlike the\u00a0[pb_glossary id=\"4822\"]adaptive immune system[\/pb_glossary], the innate system does not confer\u00a0[pb_glossary id=\"4884\"]immunity[\/pb_glossary]. The innate immune system includes surface barriers, [pb_glossary id=\"4860\"]inflammation[\/pb_glossary], the [pb_glossary id=\"4868\"]complement system[\/pb_glossary], and a variety of cellular responses.<\/li>\r\n \t
- The body\u2019s first line of defense consists of three different types of barriers that keep most pathogens out of\u00a0body tissues. The types of barriers are mechanical, chemical, and biological barriers.<\/li>\r\n \t
- Mechanical barriers\u00a0\u2014\u00a0which include the [pb_glossary id=\"3521\"]skin[\/pb_glossary], [pb_glossary id=\"3538\"]mucous membranes[\/pb_glossary], and fluids such as tears and [pb_glossary id=\"4717\"]urine[\/pb_glossary] \u2014 physically block pathogens from entering the body. Chemical barriers\u00a0\u2014\u00a0such as enzymes in [pb_glossary id=\"4711\"]sweat[\/pb_glossary], [pb_glossary id=\"4595\"]saliva[\/pb_glossary], and [pb_glossary id=\"4885\"]semen[\/pb_glossary]\u00a0\u2014\u00a0kill pathogens on body surfaces. Biological barriers are harmless bacteria that use up food and space so pathogenic bacteria cannot colonize the body.<\/li>\r\n \t
- If pathogens breach protective barriers, [pb_glossary id=\"4860\"]inflammation[\/pb_glossary] occurs. This creates a physical barrier against the spread of infection, and repairs tissue damage. Inflammation is triggered by chemicals such as [pb_glossary id=\"4865\"]cytokines[\/pb_glossary]\u00a0and [pb_glossary id=\"4879\"]histamines[\/pb_glossary], and it causes swelling, redness, and warmth.<\/li>\r\n \t
- The [pb_glossary id=\"4868\"]complement system[\/pb_glossary] is a complex biochemical mechanism that helps [pb_glossary id=\"5659\"]antibodies[\/pb_glossary] kill pathogens. Once activated, the complement system consists of more than two dozen proteins that lead to disruption of the\u00a0cell membrane\u00a0of pathogens and bursting of the cells.<\/li>\r\n \t
- Cellular responses of the innate immune system involve various types of [pb_glossary id=\"5623\"]leukocytes[\/pb_glossary]. For example, [pb_glossary id=\"5545\"]neutrophils[\/pb_glossary], [pb_glossary id=\"4340\"]macrophages[\/pb_glossary], and [pb_glossary id=\"4875\"]dendritic cells[\/pb_glossary]\u00a0phagocytize pathogens. [pb_glossary id=\"5573\"]Basophils[\/pb_glossary]\u00a0and [pb_glossary id=\"4881\"]mast cells[\/pb_glossary]\u00a0release chemicals that trigger inflammation. [pb_glossary id=\"5515\"]Natural killer cells[\/pb_glossary]\u00a0destroy cancerous or virus-infected cells, and eosinophils kill parasites.<\/li>\r\n \t
- Many pathogens have evolved mechanisms that help them evade the innate immune system. For example, some pathogens form a protective capsule around themselves, and some mimic host cells so the immune system does not recognize them as foreign.<\/li>\r\n<\/ul>\r\n<\/div>\r\n<\/div>\r\n
\r\n17.4 Review Questions<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- What is the innate immune system?<\/li>\r\n \t
- Identify the body\u2019s first line of defense.<\/li>\r\n \t
- [h5p id=\"638\"]<\/li>\r\n \t
- What are biological barriers? How do they protect the body?<\/li>\r\n \t
- State the purposes of inflammation. What triggers inflammation, and what signs and symptoms does it cause?<\/li>\r\n \t
- Define the complement system. How does it help destroy pathogens?<\/li>\r\n \t
- Describe two ways that pathogens can evade the innate immune system.<\/li>\r\n \t
- What are the ways in which phagocytes can encounter pathogens in the body?<\/li>\r\n \t
- Describe two different ways in which enzymes play a role in the innate immune response.<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n
\r\n17.4 Explore More<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\nhttps:\/\/youtu.be\/WW4skW6gucU\r\nHow mucus keeps us healthy - Katharina Ribbeck, TED-Ed, 2015.<\/p>\r\nhttps:\/\/youtu.be\/sYjtMP67vyk\r\n
Human Physiology - Innate Immune System, Janux, 2015.<\/p>\r\nhttps:\/\/youtu.be\/c64M1tZyWPM\r\n
Myriam Sidibe: The simple power of handwashing, TED, 2014.<\/p>\r\nhttps:\/\/youtu.be\/shEPwQPQG4I\r\n
Everything You Didn't Want To Know About Snot, Gross Science, 2017.<\/p>\r\nhttps:\/\/youtu.be\/dy1D3d1FBcw\r\n
Cough Grosser Than Sneeze? | Curiosity - World's Dirtiest Man, Discovery, 2011.<\/p>\r\n\r\n<\/div>\r\n<\/div>\r\n \r\n\r\n<\/div>\r\n
Attributions<\/h2>\r\nFigure 17.4.1<\/strong>\r\n\r\nOww_Papercut_14365<\/a> by Laurence Facun<\/a> on Wikimedia Commons is used under a CC BY 2.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/2.0) license.\r\n\r\nFigure 17.4.2<\/strong>\r\n\r\nhairy-nose<\/a> by Piotr Siedlecki\u00a0 on publicdomainpictures.net<\/a> is used under a CC0 1.0<\/a> Universal Public Domain Dedication (http:\/\/creativecommons.org\/publicdomain\/zero\/1.0\/) license.\r\n\r\nFigure 17.4.3<\/strong>\r\n\r\n1024px-Sneeze<\/a> by James Gathany\/ CDC Public Health Image library (PHIL) ID# 11162<\/a> on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/public_domain).\r\n\r\nFigure 17.4.4<\/strong>\r\n\r\nOSC_Microbio_17_06_Erythema<\/a> by CNX OpenStax<\/a> on Wikimedia Commons is used under a CC BY 4.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/4.0) license.\r\n\r\nFigure 17.4.5<\/strong>\r\n\r\n2212_Complement_Cascade_and_Function<\/a>\u00a0by OpenStax College<\/a> on Wikimedia Commons is used under a\u00a0CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\r\n\r\nFigure 17.4.6<\/strong>\r\n\r\n512px-Phagocytosis2<\/a> by Graham\u00a0Colm<\/span><\/a> at English Wikipedia<\/a><\/span>\u00a0on Wikimedia Commons is used under a CC BY-SA 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-sa\/3.0) license.\r\n\r\nFigure 17.4.7<\/strong>\r\n\r\nInnate_Immune_cells.svg<\/a> by Fred the Oyster<\/a> on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).\r\n\r\nFigure 17.4.8<\/strong>\r\n\r\n1024px-Gingivitis-before-and-after-3<\/a> by Onetimeuseaccount<\/a> on Wikimedia Commons is used under a CC0 1.0<\/a> Universal Public Domain Dedication (http:\/\/creativecommons.org\/publicdomain\/zero\/1.0\/) license.\r\nReferences<\/h2>\r\n
Betts, J. G., Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E., Womble, M., DeSaix, P. (2013, June 19). Figure 21.13 Complement cascade and function [digital image].\u00a0 In Anatomy and Physiology<\/em> (Section 21.2). OpenStax. https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/21-2-barrier-defenses-and-the-innate-immune-response<\/p>\r\nDiscovery. (2011, October 27). Cough grosser than sneeze? | Curiosity - World's dirtiest man. YouTube. https:\/\/www.youtube.com\/watch?v=dy1D3d1FBcw&feature=youtu.be<\/p>\r\n
Gross Science. (2017, January 31). Everything you didn't want to know about snot. YouTube. https:\/\/www.youtube.com\/watch?v=shEPwQPQG4I&feature=youtu.be<\/p>\r\n
Janux. (2015, January 10). Human physiology - Innate immune system. YouTube. https:\/\/www.youtube.com\/watch?v=sYjtMP67vyk&feature=youtu.be<\/p>\r\n
Mayo Clinic Staff. (n.d.). Anthrax [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/anthrax\/symptoms-causes\/syc-20356203<\/p>\r\n
Mayo Clinic Staff. (n.d.). Influenza (flu) [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/flu\/symptoms-causes\/syc-20351719<\/p>\r\n
Mayo Clinic Staff. (n.d.). Salmonella infection [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/salmonella\/symptoms-causes\/syc-20355329<\/p>\r\n
Mayo Clinic Staff. (n.d.). Staph infection [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/staph-infections\/multimedia\/staph-infection\/img-20008600<\/p>\r\n
Mayo Clinic Staff. (n.d.). Tuberculosis [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/tuberculosis\/symptoms-causes\/syc-20351250<\/p>\r\n
OpenStax. (2016, November 11). Figure\u00a0<\/span>17.23<\/span> A typical case of acute inflammation at the site of a skin wound - Erythema [digital image]. In OpenStax, Microbiology<\/em> (Section 17.5). https:\/\/openstax.org\/details\/books\/microbiology?Bookdetails<\/p>\r\nTED. (2014, October 14). Myriam Sidibe: The simple power of handwashing. YouTube. https:\/\/www.youtube.com\/watch?v=c64M1tZyWPM&feature=youtu.be<\/p>\r\n
TED-Ed. (2015, November 5). How mucus keeps us healthy - Katharina Ribbeck. YouTube. https:\/\/www.youtube.com\/watch?v=WW4skW6gucU&feature=youtu.be<\/p>","rendered":"
<\/p>\nFigure 17.4.1 Darn it!\u00a0 Paper cuts are the worst!<\/em><\/figcaption><\/figure>\n\nPaper Cut<\/h1>\n<\/div>\n
It\u2019s just a paper cut, but the break in your skin could provide an easy way for pathogens<\/span><\/a><\/span>\u00a0to enter your body. If\u00a0bacteria<\/a>\u00a0were to enter through the cut and infect the wound, your innate immune system would quickly respond with a dizzying array of general defenses.<\/span><\/p>\n\nWhat Is the Innate Immune System?<\/h1>\n<\/div>\n
The\u00a0innate immune system<\/a><\/strong>\u00a0is a subset of the human immune system that produces rapid, but non-specific responses to pathogens. Innate responses are generic, rather than tailored to a particular\u00a0pathogen. The innate system responds in the same general way to every pathogen\u00a0it encounters. Although the innate immune system provides immediate and rapid defenses against pathogens, it does not confer long-lasting\u00a0immunity\u00a0to them. In most organisms, the innate immune system is the dominant system of host defense. Other than most vertebrates (including humans), the innate immune system is the only system of host defense.<\/p>\nIn humans, the innate immune system includes surface barriers, inflammation, the complement system, and a variety of cellular responses. Surface barriers of various types generally keep most pathogens out of the body. If these barriers fail, then other innate defenses are triggered. The triggering event is usually the identification of pathogens by pattern-recognition receptors on\u00a0cells\u00a0of the innate immune system. These receptors recognize molecules that are broadly shared by pathogens, but distinguishable from host molecules. Alternatively, the other innate defenses may be triggered when damaged, injured, or stressed cells send out alarm signals, many of which are recognized by the same receptors as those that recognize pathogens.<\/p>\n
\nBarriers to Pathogens<\/h1>\n<\/div>\n
The body\u2019s first line of defense consists of three different types of barriers that keep most pathogens out of\u00a0body tissues. The types of barriers are mechanical, chemical, and biological barriers.<\/p>\n
Mechanical Barriers<\/h2>\nFigure 17.4.2 Nasal hairs are a mechanical barrier to larger particles in the air.<\/em><\/figcaption><\/figure>\nMechanical barriers<\/a> are the first line of defense against pathogens, and they physically block pathogens from entering the body. The skin<\/a> is the most important mechanical barrier. In fact, it is the single most important defense the body has. The outer layer of skin \u2014 the\u00a0epidermis<\/a>\u00a0\u2014 is tough, and very difficult for pathogens to penetrate. It consists of dead\u00a0cells\u00a0that are constantly shed from the body surface, a process that helps remove\u00a0bacteria\u00a0and other infectious agents that have adhered to the skin. The epidermis also lacks\u00a0blood vessels\u00a0and is usually lacking moisture, so it does not provide a suitable environment for most pathogens.\u00a0Hair<\/a> \u2014 which is an accessory organ of the skin \u2014 also helps keep out pathogens. Hairs inside the nose may trap larger pathogens and other particles in the air before they can enter the airways of the respiratory system (see Figure 17.4.2).<\/p>\nFigure 17.4.3 A sneeze can expel many pathogens from the respiratory tract, which is why you should always cover your mouth and nose and when you sneeze.<\/em><\/figcaption><\/figure>\nMucous membranes<\/span><\/a><\/span> provide a mechanical barrier to pathogens and other particles at body openings. These membranes also line the respiratory, gastrointestinal, urinary, and reproductive tracts. Mucous membranes secrete mucus, which is a slimy and somewhat sticky substance that traps pathogens. Many mucous membranes also have hair-like cilia<\/a> that sweep mucus and trapped pathogens toward body openings, where they can be removed from the body. When you sneeze or cough, mucus and pathogens are mechanically ejected from the nose and throat, as you can see in Figure 17.4.3.\u00a0 A sneeze can travel as fast as 160 Km\/hr (about 99 mi\/hour) and expel as many as 100,000 droplets into the air around you (a good reason to cover your sneezes!). Other mechanical defenses include tears, which wash pathogens from the eyes, and urine, which flushes pathogens out of the urinary tract.<\/span><\/p>\nChemical Barriers<\/h2>\n
Chemical barriers also protect against infection by pathogens. They destroy pathogens on the outer body surface, at body openings, and on inner body linings. Sweat, mucus, tears, saliva, and breastmilk all contain antimicrobial substances (such as the\u00a0enzyme\u00a0lysozyme) that kill pathogens, especially\u00a0bacteria. Sebaceous glands in the\u00a0dermis\u00a0of the skin secrete acids that form a very fine, slightly acidic film on the surface of the skin. This film acts as a barrier to bacteria, viruses, and other potential contaminants that might penetrate the skin. Urine and vaginal secretions are also too acidic for many pathogens to endure. Semen contains zinc \u2014 which most pathogens cannot tolerate \u2014 as well as defensins, which are antimicrobial\u00a0proteins\u00a0that act mainly by disrupting bacterial cell membranes. In the stomach, stomach\u00a0acid\u00a0and digestive\u00a0enzymes\u00a0called proteases (which break down proteins) kill most of the pathogens that enter the gastrointestinal tract in food or\u00a0water.<\/p>\n
Biological Barriers<\/h2>\n
Biological barriers are living organisms that help protect the body from pathogens. Trillions of harmless bacteria normally live on the human skin and in the urinary, reproductive, and gastrointestinal tracts. These bacteria use up food and surface space that help prevent pathogenic bacteria from colonizing the body. Some of these harmless bacteria also secrete substances that change the conditions of their environment, making it less hospitable to potentially harmful bacteria.\u00a0They may release toxins or change the\u00a0pH, for example. All of these effects of harmless bacteria reduce the chances that pathogenic microorganisms will be able to reach sufficient numbers and cause illness.<\/p>\n
\nInflammation<\/h1>\n<\/div>\n
If pathogens manage to breach the barriers protecting the body, one of the first active responses of the innate immune system kicks in. This response is\u00a0inflammation<\/a>.<\/strong>\u00a0The main function of inflammation is to establish a physical barrier against the spread of infection. It also eliminates the initial cause of cell injury, clears out dead\u00a0cells\u00a0and tissues damaged from the original insult and the inflammatory process, and initiates tissue repair. Inflammation is often a response to infection by pathogens, but there are other possible causes, including burns, frostbite, and exposure to toxins.<\/p>\nThe signs and symptoms of inflammation include redness, swelling, warmth, pain, and frequently some loss of function. These symptoms are caused by increased blood flow into infected tissue, and a number of other processes, illustrated in Figure 17.4.4.<\/p>\nFigure 17.4.4 This drawing shows what happens during the inflammatory response.<\/em><\/figcaption><\/figure>\nInflammation is triggered by chemicals such as\u00a0cytokines<\/a><\/strong>\u00a0and\u00a0histamines<\/a>,<\/strong>which are released by injured or infected cells, or by immune system cells such as macrophages (described below) that are already present in tissues. These chemicals cause capillaries to dilate and become leaky, increasing blood flow to the infected area and allowing blood to enter the tissues. Pathogen-destroying leukocytes and tissue-repairing proteins migrate into tissue spaces from the bloodstream to attack pathogens and repair their damage. Cytokines also promote chemotaxis<\/a>, which is migration to the site of infection by pathogen-destroying leukocytes. Some cytokines have anti-viral effects. They may shut down protein synthesis in host cells, which viruses need in order to survive and replicate.<\/p>\nSee the video “The inflammatory response” by Neural Academy to learn about inflammatory response in more detail:<\/p>\n
Barriers to Pathogens<\/h1>\r\n<\/div>\r\nThe body\u2019s first line of defense consists of three different types of barriers that keep most pathogens out of\u00a0body tissues. The types of barriers are mechanical, chemical, and biological barriers.\r\nMechanical Barriers<\/h2>\r\n[caption id=\"attachment_4858\" align=\"alignleft\" width=\"343\"] Figure 17.4.2 Nasal hairs are a mechanical barrier to larger particles in the air.<\/em>[\/caption]\r\n\r\n[pb_glossary id=\"4857\"]Mechanical barriers[\/pb_glossary] are the first line of defense against pathogens, and they physically block pathogens from entering the body. The [pb_glossary id=\"3521\"]skin[\/pb_glossary] is the most important mechanical barrier. In fact, it is the single most important defense the body has. The outer layer of skin \u2014 the\u00a0[pb_glossary id=\"5991\"]epidermis[\/pb_glossary]\u00a0\u2014 is tough, and very difficult for pathogens to penetrate. It consists of dead\u00a0cells\u00a0that are constantly shed from the body surface, a process that helps remove\u00a0bacteria\u00a0and other infectious agents that have adhered to the skin. The epidermis also lacks\u00a0blood vessels\u00a0and is usually lacking moisture, so it does not provide a suitable environment for most pathogens.\u00a0[pb_glossary id=\"3531\"]Hair[\/pb_glossary] \u2014 which is an accessory organ of the skin \u2014 also helps keep out pathogens. Hairs inside the nose may trap larger pathogens and other particles in the air before they can enter the airways of the respiratory system (see Figure 17.4.2).\r\n\r\n[caption id=\"attachment_4859\" align=\"alignright\" width=\"422\"] Figure 17.4.3 A sneeze can expel many pathogens from the respiratory tract, which is why you should always cover your mouth and nose and when you sneeze.<\/em>[\/caption]\r\n\r\n[pb_glossary id=\"3538\"]Mucous membranes<\/span>[\/pb_glossary]<\/span> provide a mechanical barrier to pathogens and other particles at body openings. These membranes also line the respiratory, gastrointestinal, urinary, and reproductive tracts. Mucous membranes secrete mucus, which is a slimy and somewhat sticky substance that traps pathogens. Many mucous membranes also have hair-like [pb_glossary id=\"1597\"]cilia[\/pb_glossary] that sweep mucus and trapped pathogens toward body openings, where they can be removed from the body. When you sneeze or cough, mucus and pathogens are mechanically ejected from the nose and throat, as you can see in Figure 17.4.3.\u00a0 A sneeze can travel as fast as 160 Km\/hr (about 99 mi\/hour) and expel as many as 100,000 droplets into the air around you (a good reason to cover your sneezes!). Other mechanical defenses include tears, which wash pathogens from the eyes, and urine, which flushes pathogens out of the urinary tract.<\/span>\r\nChemical Barriers<\/h2>\r\nChemical barriers also protect against infection by pathogens. They destroy pathogens on the outer body surface, at body openings, and on inner body linings. Sweat, mucus, tears, saliva, and breastmilk all contain antimicrobial substances (such as the\u00a0enzyme\u00a0lysozyme) that kill pathogens, especially\u00a0bacteria. Sebaceous glands in the\u00a0dermis\u00a0of the skin secrete acids that form a very fine, slightly acidic film on the surface of the skin. This film acts as a barrier to bacteria, viruses, and other potential contaminants that might penetrate the skin. Urine and vaginal secretions are also too acidic for many pathogens to endure. Semen contains zinc \u2014 which most pathogens cannot tolerate \u2014 as well as defensins, which are antimicrobial\u00a0proteins\u00a0that act mainly by disrupting bacterial cell membranes. In the stomach, stomach\u00a0acid\u00a0and digestive\u00a0enzymes\u00a0called proteases (which break down proteins) kill most of the pathogens that enter the gastrointestinal tract in food or\u00a0water.\r\nBiological Barriers<\/h2>\r\nBiological barriers are living organisms that help protect the body from pathogens. Trillions of harmless bacteria normally live on the human skin and in the urinary, reproductive, and gastrointestinal tracts. These bacteria use up food and surface space that help prevent pathogenic bacteria from colonizing the body. Some of these harmless bacteria also secrete substances that change the conditions of their environment, making it less hospitable to potentially harmful bacteria.\u00a0They may release toxins or change the\u00a0pH, for example. All of these effects of harmless bacteria reduce the chances that pathogenic microorganisms will be able to reach sufficient numbers and cause illness.\r\n\r\nInflammation<\/h1>\r\n<\/div>\r\nIf pathogens manage to breach the barriers protecting the body, one of the first active responses of the innate immune system kicks in. This response is\u00a0[pb_glossary id=\"4860\"]inflammation[\/pb_glossary].<\/strong>\u00a0The main function of inflammation is to establish a physical barrier against the spread of infection. It also eliminates the initial cause of cell injury, clears out dead\u00a0cells\u00a0and tissues damaged from the original insult and the inflammatory process, and initiates tissue repair. Inflammation is often a response to infection by pathogens, but there are other possible causes, including burns, frostbite, and exposure to toxins.\r\n\r\nThe signs and symptoms of inflammation include redness, swelling, warmth, pain, and frequently some loss of function. These symptoms are caused by increased blood flow into infected tissue, and a number of other processes, illustrated in Figure 17.4.4.\r\n\r\n[caption id=\"attachment_4864\" align=\"alignnone\" width=\"1024\"] Figure 17.4.4 This drawing shows what happens during the inflammatory response.<\/em>[\/caption]\r\n\r\nInflammation is triggered by chemicals such as\u00a0[pb_glossary id=\"4865\"]cytokines[\/pb_glossary]<\/strong>\u00a0and\u00a0[pb_glossary id=\"4866\"]histamines[\/pb_glossary],<\/strong>which are released by injured or infected cells, or by immune system cells such as macrophages (described below) that are already present in tissues. These chemicals cause capillaries to dilate and become leaky, increasing blood flow to the infected area and allowing blood to enter the tissues. Pathogen-destroying leukocytes and tissue-repairing proteins migrate into tissue spaces from the bloodstream to attack pathogens and repair their damage. Cytokines also promote [pb_glossary id=\"4867\"]chemotaxis[\/pb_glossary], which is migration to the site of infection by pathogen-destroying leukocytes. Some cytokines have anti-viral effects. They may shut down protein synthesis in host cells, which viruses need in order to survive and replicate.\r\n\r\nSee the video \"The inflammatory response\" by Neural Academy to learn about inflammatory response in more detail:\r\n\r\nhttps:\/\/youtu.be\/Fbzb75HA9M8\r\nThe inflammatory response, Neural Academy, 2019.<\/p>\r\n\r\n
\r\nComplement System<\/h1>\r\n<\/div>\r\nThe\u00a0[pb_glossary id=\"4868\"]complement system[\/pb_glossary]<\/strong>\u00a0is a complex biochemical mechanism named for its ability to \u201ccomplement\u201d the killing of pathogens by [pb_glossary id=\"5659\"]antibodies[\/pb_glossary], which are produced as part of an adaptive immune response. The complement system consists of more than two dozen\u00a0proteins\u00a0normally found in the\u00a0blood\u00a0and synthesized in the [pb_glossary id=\"2989\"]liver[\/pb_glossary]. The proteins usually circulate as non-functional precursor molecules until activated.\r\n\r\nAs shown in Figure 17.4.5, when the first protein in the complement series is activated \u2014typically by the binding of an antibody to an [pb_glossary id=\"2704\"]antigen[\/pb_glossary] on a pathogen \u2014 it sets in motion a domino effect. Each component takes its turn in a precise chain of steps known as the complement cascade. The end product is a cylinder that punctures a hole in the pathogen\u2019s cell membrane. This allows fluids and molecules to flow in and out of the cell, which swells and bursts.\r\n\r\n[caption id=\"attachment_4869\" align=\"aligncenter\" width=\"643\"] Figure 17.4.5 The complement system is a cascade of proteins that complements the killing of pathogen cells by antibodies.<\/em>[\/caption]\r\n\r\n\r\nCellular Responses<\/h1>\r\n<\/div>\r\nCellular responses of the innate immune system involve a variety of different types of [pb_glossary id=\"5623\"]leukocytes<\/span>[\/pb_glossary]<\/span>. Many of these leukocytes circulate in the blood and act like independent, single-celled organisms, searching out and destroying pathogens in the human host. These and other immune cells of the innate system identify pathogens or debris, and then help to eliminate them in some way. One way is by [pb_glossary id=\"1680\"]phagocytosis[\/pb_glossary].<\/span>\r\nPhagocytosis<\/h2>\r\n[pb_glossary id=\"1680\"]Phagocytosis[\/pb_glossary]<\/strong>\u00a0is an important feature of innate\u00a0immunity\u00a0that is performed by cells classified as phagocytes. In the process of phagocytosis, phagocytes engulf and digest pathogens or other harmful particles. Phagocytes generally patrol the body searching for pathogens, but they can also be called to specific locations by the release of [pb_glossary id=\"4865\"]cytokines<\/span>[\/pb_glossary]<\/span>\u00a0when [pb_glossary id=\"4860\"]inflammation[\/pb_glossary] occurs. Some phagocytes reside permanently in certain tissues.<\/span>\r\n\r\nAs shown in Figure 17.4.6, when a pathogen such as a bacterium is encountered by a phagocyte, the phagocyte extends a portion of its plasma membrane, wrapping the membrane around the pathogen until it is enveloped. Once inside the phagocyte, the pathogen becomes enclosed within an intracellular vesicle called a phagosome. The phagosome then fuses with another vesicle called a [pb_glossary id=\"4870\"]lysosome[\/pb_glossary], forming a phagolysosome. Digestive enzymes and acids from the lysosome kill and digest the pathogen in the phagolysosome. The final step of phagocytosis is excretion of soluble debris from the destroyed pathogen through [pb_glossary id=\"1682\"]exocytosis[\/pb_glossary].\r\n\r\n[caption id=\"attachment_4871\" align=\"aligncenter\" width=\"512\"] Figure 17.4.6 Phagocytosis is a multi-step process in which a pathogen is engulfed and digested by immune cells called phagocytes.<\/em>[\/caption]\r\n\r\nTypes of leukocytes that kill pathogens by phagocytosis include neutrophils, macrophages, and dendritic cells. You can see illustrations of these and other leukocytes involved in innate immune responses in Figure 17.4.7.\r\n\r\n[caption id=\"attachment_4872\" align=\"alignright\" width=\"292\"] Figure 17.4.7 Types of leukocytes evolved in innate immune responses are illustrated here.<\/em>[\/caption]\r\nNeutrophils<\/h2>\r\n[pb_glossary id=\"5545\"]Neutrophils[\/pb_glossary]<\/strong>\u00a0are leukocytes that travel throughout the body in the blood. They are usually the first immune cells to arrive at the site of an infection. They are the most numerous types of phagocytes, and they normally make up at least half of the total circulating leukocytes. The [pb_glossary id=\"4021\"]bone marrow[\/pb_glossary] of a normal healthy adult produces more than 100 billion neutrophils per day. During acute inflammation, more than\u00a0ten times<\/em>\u00a0that many neutrophils may be produced each day. Many neutrophils are needed to fight infections, because after a neutrophil phagocytizes just a few pathogens, it generally dies.\r\nMacrophages<\/h2>\r\n[pb_glossary id=\"4340\"]Macrophage<\/strong>s<\/strong>[\/pb_glossary]<\/strong>\u00a0are large phagocytic leukocytes that develop from monocytes. Macrophages spend much of their time within the [pb_glossary id=\"4874\"]interstitial fluid[\/pb_glossary] in\u00a0body tissues. They are the most efficient phagocytes, and they can phagocytize substantial numbers of pathogens or other cells. Macrophages are also versatile cells that produce a wide array of chemicals\u00a0\u2014\u00a0including\u00a0enzymes, complement proteins, and [pb_glossary id=\"4865\"]cytokine<\/span>s<\/span>[\/pb_glossary]<\/span>\u00a0\u2014 in addition to their phagocytic action. As phagocytes, macrophages act as scavengers that rid tissues of worn-out cells and other debris, as well as pathogens. In addition, macrophages act as antigen-presenting cells that activate the\u00a0[pb_glossary id=\"4822\"]adaptive immune system[\/pb_glossary].<\/span>\r\nDendritic Cells<\/h2>\r\nLike macrophages,\u00a0[pb_glossary id=\"4875\"]dendritic cells[\/pb_glossary]<\/strong>\u00a0develop from monocytes. They reside in tissues that have contact with the external environment, so they are located mainly in the skin, nose, lungs, stomach, and intestines. Besides engulfing and digesting pathogens, dendritic cells also act as antigen-presenting cells that trigger adaptive immune responses.\r\nEosinophils<\/h2>\r\n[pb_glossary id=\"5529\"]Eosinophils[\/pb_glossary]<\/strong> are non-phagocytic leukocytes that are related to [pb_glossary id=\"5545\"]neutrophil[\/pb_glossary]. They specialize in defending against [pb_glossary id=\"4812\"]parasites[\/pb_glossary]. They are very effective in killing large parasites (such as worms) by secreting a range of highly-toxic substances when activated. Eosinophils may become overactive and cause [pb_glossary id=\"4877\"]allergies[\/pb_glossary] or [pb_glossary id=\"4348\"]asthma[\/pb_glossary].\r\nBasophils<\/h2>\r\n[pb_glossary id=\"5573\"]Basophils[\/pb_glossary]<\/strong>\u00a0are non-phagocytic leukocytes that are also related to neutrophils. They are the least numerous of all white blood cells. Basophils secrete two types of chemicals that aid in body defenses: [pb_glossary id=\"4879\"]histamines<\/span><\/strong>[\/pb_glossary]<\/span> and heparin. <\/span>Histamines\u00a0are responsible for dilating\u00a0blood vessels\u00a0and increasing their permeability in inflammation.\u00a0<\/span>[pb_glossary id=\"4880\"]Heparin[\/pb_glossary]<\/strong>\u00a0inhibits blood clotting, and also promotes the movement of leukocytes into an area of infection.<\/span>\r\nMast Cells<\/h2>\r\n[pb_glossary id=\"4881\"]Mast cells[\/pb_glossary]<\/strong>\u00a0are non-phagocytic leukocytes that help initiate [pb_glossary id=\"4860\"]inflammation[\/pb_glossary] by secreting [pb_glossary id=\"4879\"]histamines<\/span>[\/pb_glossary]<\/span>. In some people, histamines trigger [pb_glossary id=\"4877\"]allergic reactions[\/pb_glossary], as well as inflammation. Mast cells may also secrete chemicals that help defend against parasites.<\/span>\r\nNatural Killer Cells<\/h2>\r\n[pb_glossary id=\"5515\"]Natural killer cells[\/pb_glossary]<\/strong>\u00a0are in the subset of leukocytes called [pb_glossary id=\"4832\"]lymphocytes<\/span>[\/pb_glossary]<\/span>, which are produced by the\u00a0lymphatic system. Natural killer cells destroy [pb_glossary id=\"5605\"]cancer<\/span>ous<\/span>[\/pb_glossary]<\/span>\u00a0or [pb_glossary id=\"4283\"]virus[\/pb_glossary]-infected host cells, although they do not directly attack invading pathogens. Natural killer cells recognize these host cells by a condition they exhibit called \u201cmissing self.\u201d Cells with missing self have abnormally low levels of cell-surface proteins of the [pb_glossary id=\"4825\"]major histocompatibility complex (MHC)[\/pb_glossary], which normally identify\u00a0body cells\u00a0as self.<\/span>\r\n\r\nInnate Immune Evasion<\/h1>\r\n<\/div>\r\nMany pathogens have evolved mechanisms that allow them to evade\u00a0human hosts'\u00a0innate immune systems. Some of these mechanisms include:\r\n\r\n \t- Invading host cells to replicate so they are \u201chidden\u201d from the immune system.<\/strong>\u00a0The bacterium that causes tuberculosis<\/a> uses this mechanism.<\/li>\r\n \t
- Forming a protective capsule around themselves to avoid being destroyed by immune system cells.<\/strong>\u00a0This defense occurs in bacteria, such as\u00a0Salmonella<\/em><\/a>\u00a0species.<\/li>\r\n \t
- Mimicking host cells so the immune system does not recognize them as foreign.<\/strong>\u00a0Some\u00a0species\u00a0of\u00a0Staphylococcus<\/em><\/a>\u00a0bacteria use this mechanism.<\/li>\r\n \t
- Directly killing phagocytes.<\/strong>\u00a0This ability evolved in several\u00a0species\u00a0of bacteria, including the species that causes anthrax<\/a>.<\/li>\r\n \t
- Producing molecules that prevent the formation of interferons, which are immune chemicals that fight viruses.<\/strong>\u00a0Some influenza<\/a> viruses have this capability.<\/li>\r\n \t
- Forming complex biofilms that provide protection from the cells and proteins of the immune system.<\/strong> This characterizes some species of [pb_glossary id=\"2588\"]bacteria[\/pb_glossary] and fungi. You can see an example of a bacterial biofilm on teeth in Figure 17.4.8.<\/li>\r\n<\/ul>\r\n[caption id=\"attachment_4883\" align=\"aligncenter\" width=\"400\"] Figure 17.4.8 The dental plaque on the top set of teeth is a biofilm that sticks to the teeth and consists of many species of bacteria. The plaque biofilm is difficult to remove, and it subjects the teeth and gums to high concentrations of bacterial metabolites, which result in dental disease. The same teeth after plaque removal are shown in the bottom picture.<\/em>[\/caption]\r\n\r\n\r\n\r\n \r\n\r\n<\/div>\r\n\r\n
\r\n17.4 Summary<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- The [pb_glossary id=\"4820\"]innate immune system[\/pb_glossary] is a subset of the human immune system that produces rapid, but non-specific responses to pathogens. Unlike the\u00a0[pb_glossary id=\"4822\"]adaptive immune system[\/pb_glossary], the innate system does not confer\u00a0[pb_glossary id=\"4884\"]immunity[\/pb_glossary]. The innate immune system includes surface barriers, [pb_glossary id=\"4860\"]inflammation[\/pb_glossary], the [pb_glossary id=\"4868\"]complement system[\/pb_glossary], and a variety of cellular responses.<\/li>\r\n \t
- The body\u2019s first line of defense consists of three different types of barriers that keep most pathogens out of\u00a0body tissues. The types of barriers are mechanical, chemical, and biological barriers.<\/li>\r\n \t
- Mechanical barriers\u00a0\u2014\u00a0which include the [pb_glossary id=\"3521\"]skin[\/pb_glossary], [pb_glossary id=\"3538\"]mucous membranes[\/pb_glossary], and fluids such as tears and [pb_glossary id=\"4717\"]urine[\/pb_glossary] \u2014 physically block pathogens from entering the body. Chemical barriers\u00a0\u2014\u00a0such as enzymes in [pb_glossary id=\"4711\"]sweat[\/pb_glossary], [pb_glossary id=\"4595\"]saliva[\/pb_glossary], and [pb_glossary id=\"4885\"]semen[\/pb_glossary]\u00a0\u2014\u00a0kill pathogens on body surfaces. Biological barriers are harmless bacteria that use up food and space so pathogenic bacteria cannot colonize the body.<\/li>\r\n \t
- If pathogens breach protective barriers, [pb_glossary id=\"4860\"]inflammation[\/pb_glossary] occurs. This creates a physical barrier against the spread of infection, and repairs tissue damage. Inflammation is triggered by chemicals such as [pb_glossary id=\"4865\"]cytokines[\/pb_glossary]\u00a0and [pb_glossary id=\"4879\"]histamines[\/pb_glossary], and it causes swelling, redness, and warmth.<\/li>\r\n \t
- The [pb_glossary id=\"4868\"]complement system[\/pb_glossary] is a complex biochemical mechanism that helps [pb_glossary id=\"5659\"]antibodies[\/pb_glossary] kill pathogens. Once activated, the complement system consists of more than two dozen proteins that lead to disruption of the\u00a0cell membrane\u00a0of pathogens and bursting of the cells.<\/li>\r\n \t
- Cellular responses of the innate immune system involve various types of [pb_glossary id=\"5623\"]leukocytes[\/pb_glossary]. For example, [pb_glossary id=\"5545\"]neutrophils[\/pb_glossary], [pb_glossary id=\"4340\"]macrophages[\/pb_glossary], and [pb_glossary id=\"4875\"]dendritic cells[\/pb_glossary]\u00a0phagocytize pathogens. [pb_glossary id=\"5573\"]Basophils[\/pb_glossary]\u00a0and [pb_glossary id=\"4881\"]mast cells[\/pb_glossary]\u00a0release chemicals that trigger inflammation. [pb_glossary id=\"5515\"]Natural killer cells[\/pb_glossary]\u00a0destroy cancerous or virus-infected cells, and eosinophils kill parasites.<\/li>\r\n \t
- Many pathogens have evolved mechanisms that help them evade the innate immune system. For example, some pathogens form a protective capsule around themselves, and some mimic host cells so the immune system does not recognize them as foreign.<\/li>\r\n<\/ul>\r\n<\/div>\r\n<\/div>\r\n
\r\n17.4 Review Questions<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- What is the innate immune system?<\/li>\r\n \t
- Identify the body\u2019s first line of defense.<\/li>\r\n \t
- [h5p id=\"638\"]<\/li>\r\n \t
- What are biological barriers? How do they protect the body?<\/li>\r\n \t
- State the purposes of inflammation. What triggers inflammation, and what signs and symptoms does it cause?<\/li>\r\n \t
- Define the complement system. How does it help destroy pathogens?<\/li>\r\n \t
- Describe two ways that pathogens can evade the innate immune system.<\/li>\r\n \t
- What are the ways in which phagocytes can encounter pathogens in the body?<\/li>\r\n \t
- Describe two different ways in which enzymes play a role in the innate immune response.<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n
\r\n17.4 Explore More<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\nhttps:\/\/youtu.be\/WW4skW6gucU\r\nHow mucus keeps us healthy - Katharina Ribbeck, TED-Ed, 2015.<\/p>\r\nhttps:\/\/youtu.be\/sYjtMP67vyk\r\n
Human Physiology - Innate Immune System, Janux, 2015.<\/p>\r\nhttps:\/\/youtu.be\/c64M1tZyWPM\r\n
Myriam Sidibe: The simple power of handwashing, TED, 2014.<\/p>\r\nhttps:\/\/youtu.be\/shEPwQPQG4I\r\n
Everything You Didn't Want To Know About Snot, Gross Science, 2017.<\/p>\r\nhttps:\/\/youtu.be\/dy1D3d1FBcw\r\n
Cough Grosser Than Sneeze? | Curiosity - World's Dirtiest Man, Discovery, 2011.<\/p>\r\n\r\n<\/div>\r\n<\/div>\r\n \r\n\r\n<\/div>\r\n
Attributions<\/h2>\r\nFigure 17.4.1<\/strong>\r\n\r\nOww_Papercut_14365<\/a> by Laurence Facun<\/a> on Wikimedia Commons is used under a CC BY 2.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/2.0) license.\r\n\r\nFigure 17.4.2<\/strong>\r\n\r\nhairy-nose<\/a> by Piotr Siedlecki\u00a0 on publicdomainpictures.net<\/a> is used under a CC0 1.0<\/a> Universal Public Domain Dedication (http:\/\/creativecommons.org\/publicdomain\/zero\/1.0\/) license.\r\n\r\nFigure 17.4.3<\/strong>\r\n\r\n1024px-Sneeze<\/a> by James Gathany\/ CDC Public Health Image library (PHIL) ID# 11162<\/a> on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/public_domain).\r\n\r\nFigure 17.4.4<\/strong>\r\n\r\nOSC_Microbio_17_06_Erythema<\/a> by CNX OpenStax<\/a> on Wikimedia Commons is used under a CC BY 4.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/4.0) license.\r\n\r\nFigure 17.4.5<\/strong>\r\n\r\n2212_Complement_Cascade_and_Function<\/a>\u00a0by OpenStax College<\/a> on Wikimedia Commons is used under a\u00a0CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\r\n\r\nFigure 17.4.6<\/strong>\r\n\r\n512px-Phagocytosis2<\/a> by Graham\u00a0Colm<\/span><\/a> at English Wikipedia<\/a><\/span>\u00a0on Wikimedia Commons is used under a CC BY-SA 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-sa\/3.0) license.\r\n\r\nFigure 17.4.7<\/strong>\r\n\r\nInnate_Immune_cells.svg<\/a> by Fred the Oyster<\/a> on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).\r\n\r\nFigure 17.4.8<\/strong>\r\n\r\n1024px-Gingivitis-before-and-after-3<\/a> by Onetimeuseaccount<\/a> on Wikimedia Commons is used under a CC0 1.0<\/a> Universal Public Domain Dedication (http:\/\/creativecommons.org\/publicdomain\/zero\/1.0\/) license.\r\nReferences<\/h2>\r\n
Betts, J. G., Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E., Womble, M., DeSaix, P. (2013, June 19). Figure 21.13 Complement cascade and function [digital image].\u00a0 In Anatomy and Physiology<\/em> (Section 21.2). OpenStax. https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/21-2-barrier-defenses-and-the-innate-immune-response<\/p>\r\nDiscovery. (2011, October 27). Cough grosser than sneeze? | Curiosity - World's dirtiest man. YouTube. https:\/\/www.youtube.com\/watch?v=dy1D3d1FBcw&feature=youtu.be<\/p>\r\n
Gross Science. (2017, January 31). Everything you didn't want to know about snot. YouTube. https:\/\/www.youtube.com\/watch?v=shEPwQPQG4I&feature=youtu.be<\/p>\r\n
Janux. (2015, January 10). Human physiology - Innate immune system. YouTube. https:\/\/www.youtube.com\/watch?v=sYjtMP67vyk&feature=youtu.be<\/p>\r\n
Mayo Clinic Staff. (n.d.). Anthrax [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/anthrax\/symptoms-causes\/syc-20356203<\/p>\r\n
Mayo Clinic Staff. (n.d.). Influenza (flu) [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/flu\/symptoms-causes\/syc-20351719<\/p>\r\n
Mayo Clinic Staff. (n.d.). Salmonella infection [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/salmonella\/symptoms-causes\/syc-20355329<\/p>\r\n
Mayo Clinic Staff. (n.d.). Staph infection [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/staph-infections\/multimedia\/staph-infection\/img-20008600<\/p>\r\n
Mayo Clinic Staff. (n.d.). Tuberculosis [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/tuberculosis\/symptoms-causes\/syc-20351250<\/p>\r\n
OpenStax. (2016, November 11). Figure\u00a0<\/span>17.23<\/span> A typical case of acute inflammation at the site of a skin wound - Erythema [digital image]. In OpenStax, Microbiology<\/em> (Section 17.5). https:\/\/openstax.org\/details\/books\/microbiology?Bookdetails<\/p>\r\nTED. (2014, October 14). Myriam Sidibe: The simple power of handwashing. YouTube. https:\/\/www.youtube.com\/watch?v=c64M1tZyWPM&feature=youtu.be<\/p>\r\n
TED-Ed. (2015, November 5). How mucus keeps us healthy - Katharina Ribbeck. YouTube. https:\/\/www.youtube.com\/watch?v=WW4skW6gucU&feature=youtu.be<\/p>","rendered":"
<\/p>\nFigure 17.4.1 Darn it!\u00a0 Paper cuts are the worst!<\/em><\/figcaption><\/figure>\n\nPaper Cut<\/h1>\n<\/div>\n
It\u2019s just a paper cut, but the break in your skin could provide an easy way for pathogens<\/span><\/a><\/span>\u00a0to enter your body. If\u00a0bacteria<\/a>\u00a0were to enter through the cut and infect the wound, your innate immune system would quickly respond with a dizzying array of general defenses.<\/span><\/p>\n\nWhat Is the Innate Immune System?<\/h1>\n<\/div>\n
The\u00a0innate immune system<\/a><\/strong>\u00a0is a subset of the human immune system that produces rapid, but non-specific responses to pathogens. Innate responses are generic, rather than tailored to a particular\u00a0pathogen. The innate system responds in the same general way to every pathogen\u00a0it encounters. Although the innate immune system provides immediate and rapid defenses against pathogens, it does not confer long-lasting\u00a0immunity\u00a0to them. In most organisms, the innate immune system is the dominant system of host defense. Other than most vertebrates (including humans), the innate immune system is the only system of host defense.<\/p>\nIn humans, the innate immune system includes surface barriers, inflammation, the complement system, and a variety of cellular responses. Surface barriers of various types generally keep most pathogens out of the body. If these barriers fail, then other innate defenses are triggered. The triggering event is usually the identification of pathogens by pattern-recognition receptors on\u00a0cells\u00a0of the innate immune system. These receptors recognize molecules that are broadly shared by pathogens, but distinguishable from host molecules. Alternatively, the other innate defenses may be triggered when damaged, injured, or stressed cells send out alarm signals, many of which are recognized by the same receptors as those that recognize pathogens.<\/p>\n
\nBarriers to Pathogens<\/h1>\n<\/div>\n
The body\u2019s first line of defense consists of three different types of barriers that keep most pathogens out of\u00a0body tissues. The types of barriers are mechanical, chemical, and biological barriers.<\/p>\n
Mechanical Barriers<\/h2>\nFigure 17.4.2 Nasal hairs are a mechanical barrier to larger particles in the air.<\/em><\/figcaption><\/figure>\nMechanical barriers<\/a> are the first line of defense against pathogens, and they physically block pathogens from entering the body. The skin<\/a> is the most important mechanical barrier. In fact, it is the single most important defense the body has. The outer layer of skin \u2014 the\u00a0epidermis<\/a>\u00a0\u2014 is tough, and very difficult for pathogens to penetrate. It consists of dead\u00a0cells\u00a0that are constantly shed from the body surface, a process that helps remove\u00a0bacteria\u00a0and other infectious agents that have adhered to the skin. The epidermis also lacks\u00a0blood vessels\u00a0and is usually lacking moisture, so it does not provide a suitable environment for most pathogens.\u00a0Hair<\/a> \u2014 which is an accessory organ of the skin \u2014 also helps keep out pathogens. Hairs inside the nose may trap larger pathogens and other particles in the air before they can enter the airways of the respiratory system (see Figure 17.4.2).<\/p>\nFigure 17.4.3 A sneeze can expel many pathogens from the respiratory tract, which is why you should always cover your mouth and nose and when you sneeze.<\/em><\/figcaption><\/figure>\nMucous membranes<\/span><\/a><\/span> provide a mechanical barrier to pathogens and other particles at body openings. These membranes also line the respiratory, gastrointestinal, urinary, and reproductive tracts. Mucous membranes secrete mucus, which is a slimy and somewhat sticky substance that traps pathogens. Many mucous membranes also have hair-like cilia<\/a> that sweep mucus and trapped pathogens toward body openings, where they can be removed from the body. When you sneeze or cough, mucus and pathogens are mechanically ejected from the nose and throat, as you can see in Figure 17.4.3.\u00a0 A sneeze can travel as fast as 160 Km\/hr (about 99 mi\/hour) and expel as many as 100,000 droplets into the air around you (a good reason to cover your sneezes!). Other mechanical defenses include tears, which wash pathogens from the eyes, and urine, which flushes pathogens out of the urinary tract.<\/span><\/p>\nChemical Barriers<\/h2>\n
Chemical barriers also protect against infection by pathogens. They destroy pathogens on the outer body surface, at body openings, and on inner body linings. Sweat, mucus, tears, saliva, and breastmilk all contain antimicrobial substances (such as the\u00a0enzyme\u00a0lysozyme) that kill pathogens, especially\u00a0bacteria. Sebaceous glands in the\u00a0dermis\u00a0of the skin secrete acids that form a very fine, slightly acidic film on the surface of the skin. This film acts as a barrier to bacteria, viruses, and other potential contaminants that might penetrate the skin. Urine and vaginal secretions are also too acidic for many pathogens to endure. Semen contains zinc \u2014 which most pathogens cannot tolerate \u2014 as well as defensins, which are antimicrobial\u00a0proteins\u00a0that act mainly by disrupting bacterial cell membranes. In the stomach, stomach\u00a0acid\u00a0and digestive\u00a0enzymes\u00a0called proteases (which break down proteins) kill most of the pathogens that enter the gastrointestinal tract in food or\u00a0water.<\/p>\n
Biological Barriers<\/h2>\n
Biological barriers are living organisms that help protect the body from pathogens. Trillions of harmless bacteria normally live on the human skin and in the urinary, reproductive, and gastrointestinal tracts. These bacteria use up food and surface space that help prevent pathogenic bacteria from colonizing the body. Some of these harmless bacteria also secrete substances that change the conditions of their environment, making it less hospitable to potentially harmful bacteria.\u00a0They may release toxins or change the\u00a0pH, for example. All of these effects of harmless bacteria reduce the chances that pathogenic microorganisms will be able to reach sufficient numbers and cause illness.<\/p>\n
\nInflammation<\/h1>\n<\/div>\n
If pathogens manage to breach the barriers protecting the body, one of the first active responses of the innate immune system kicks in. This response is\u00a0inflammation<\/a>.<\/strong>\u00a0The main function of inflammation is to establish a physical barrier against the spread of infection. It also eliminates the initial cause of cell injury, clears out dead\u00a0cells\u00a0and tissues damaged from the original insult and the inflammatory process, and initiates tissue repair. Inflammation is often a response to infection by pathogens, but there are other possible causes, including burns, frostbite, and exposure to toxins.<\/p>\nThe signs and symptoms of inflammation include redness, swelling, warmth, pain, and frequently some loss of function. These symptoms are caused by increased blood flow into infected tissue, and a number of other processes, illustrated in Figure 17.4.4.<\/p>\nFigure 17.4.4 This drawing shows what happens during the inflammatory response.<\/em><\/figcaption><\/figure>\nInflammation is triggered by chemicals such as\u00a0cytokines<\/a><\/strong>\u00a0and\u00a0histamines<\/a>,<\/strong>which are released by injured or infected cells, or by immune system cells such as macrophages (described below) that are already present in tissues. These chemicals cause capillaries to dilate and become leaky, increasing blood flow to the infected area and allowing blood to enter the tissues. Pathogen-destroying leukocytes and tissue-repairing proteins migrate into tissue spaces from the bloodstream to attack pathogens and repair their damage. Cytokines also promote chemotaxis<\/a>, which is migration to the site of infection by pathogen-destroying leukocytes. Some cytokines have anti-viral effects. They may shut down protein synthesis in host cells, which viruses need in order to survive and replicate.<\/p>\nSee the video “The inflammatory response” by Neural Academy to learn about inflammatory response in more detail:<\/p>\n
Figure 17.4.2 Nasal hairs are a mechanical barrier to larger particles in the air.<\/em>[\/caption]\r\n\r\n[pb_glossary id=\"4857\"]Mechanical barriers[\/pb_glossary] are the first line of defense against pathogens, and they physically block pathogens from entering the body. The [pb_glossary id=\"3521\"]skin[\/pb_glossary] is the most important mechanical barrier. In fact, it is the single most important defense the body has. The outer layer of skin \u2014 the\u00a0[pb_glossary id=\"5991\"]epidermis[\/pb_glossary]\u00a0\u2014 is tough, and very difficult for pathogens to penetrate. It consists of dead\u00a0cells\u00a0that are constantly shed from the body surface, a process that helps remove\u00a0bacteria\u00a0and other infectious agents that have adhered to the skin. The epidermis also lacks\u00a0blood vessels\u00a0and is usually lacking moisture, so it does not provide a suitable environment for most pathogens.\u00a0[pb_glossary id=\"3531\"]Hair[\/pb_glossary] \u2014 which is an accessory organ of the skin \u2014 also helps keep out pathogens. Hairs inside the nose may trap larger pathogens and other particles in the air before they can enter the airways of the respiratory system (see Figure 17.4.2).\r\n\r\n[caption id=\"attachment_4859\" align=\"alignright\" width=\"422\"] Figure 17.4.3 A sneeze can expel many pathogens from the respiratory tract, which is why you should always cover your mouth and nose and when you sneeze.<\/em>[\/caption]\r\n\r\n[pb_glossary id=\"3538\"]Mucous membranes<\/span>[\/pb_glossary]<\/span> provide a mechanical barrier to pathogens and other particles at body openings. These membranes also line the respiratory, gastrointestinal, urinary, and reproductive tracts. Mucous membranes secrete mucus, which is a slimy and somewhat sticky substance that traps pathogens. Many mucous membranes also have hair-like [pb_glossary id=\"1597\"]cilia[\/pb_glossary] that sweep mucus and trapped pathogens toward body openings, where they can be removed from the body. When you sneeze or cough, mucus and pathogens are mechanically ejected from the nose and throat, as you can see in Figure 17.4.3.\u00a0 A sneeze can travel as fast as 160 Km\/hr (about 99 mi\/hour) and expel as many as 100,000 droplets into the air around you (a good reason to cover your sneezes!). Other mechanical defenses include tears, which wash pathogens from the eyes, and urine, which flushes pathogens out of the urinary tract.<\/span>\r\nChemical Barriers<\/h2>\r\nChemical barriers also protect against infection by pathogens. They destroy pathogens on the outer body surface, at body openings, and on inner body linings. Sweat, mucus, tears, saliva, and breastmilk all contain antimicrobial substances (such as the\u00a0enzyme\u00a0lysozyme) that kill pathogens, especially\u00a0bacteria. Sebaceous glands in the\u00a0dermis\u00a0of the skin secrete acids that form a very fine, slightly acidic film on the surface of the skin. This film acts as a barrier to bacteria, viruses, and other potential contaminants that might penetrate the skin. Urine and vaginal secretions are also too acidic for many pathogens to endure. Semen contains zinc \u2014 which most pathogens cannot tolerate \u2014 as well as defensins, which are antimicrobial\u00a0proteins\u00a0that act mainly by disrupting bacterial cell membranes. In the stomach, stomach\u00a0acid\u00a0and digestive\u00a0enzymes\u00a0called proteases (which break down proteins) kill most of the pathogens that enter the gastrointestinal tract in food or\u00a0water.\r\nBiological Barriers<\/h2>\r\nBiological barriers are living organisms that help protect the body from pathogens. Trillions of harmless bacteria normally live on the human skin and in the urinary, reproductive, and gastrointestinal tracts. These bacteria use up food and surface space that help prevent pathogenic bacteria from colonizing the body. Some of these harmless bacteria also secrete substances that change the conditions of their environment, making it less hospitable to potentially harmful bacteria.\u00a0They may release toxins or change the\u00a0pH, for example. All of these effects of harmless bacteria reduce the chances that pathogenic microorganisms will be able to reach sufficient numbers and cause illness.\r\n\r\nInflammation<\/h1>\r\n<\/div>\r\nIf pathogens manage to breach the barriers protecting the body, one of the first active responses of the innate immune system kicks in. This response is\u00a0[pb_glossary id=\"4860\"]inflammation[\/pb_glossary].<\/strong>\u00a0The main function of inflammation is to establish a physical barrier against the spread of infection. It also eliminates the initial cause of cell injury, clears out dead\u00a0cells\u00a0and tissues damaged from the original insult and the inflammatory process, and initiates tissue repair. Inflammation is often a response to infection by pathogens, but there are other possible causes, including burns, frostbite, and exposure to toxins.\r\n\r\nThe signs and symptoms of inflammation include redness, swelling, warmth, pain, and frequently some loss of function. These symptoms are caused by increased blood flow into infected tissue, and a number of other processes, illustrated in Figure 17.4.4.\r\n\r\n[caption id=\"attachment_4864\" align=\"alignnone\" width=\"1024\"] Figure 17.4.4 This drawing shows what happens during the inflammatory response.<\/em>[\/caption]\r\n\r\nInflammation is triggered by chemicals such as\u00a0[pb_glossary id=\"4865\"]cytokines[\/pb_glossary]<\/strong>\u00a0and\u00a0[pb_glossary id=\"4866\"]histamines[\/pb_glossary],<\/strong>which are released by injured or infected cells, or by immune system cells such as macrophages (described below) that are already present in tissues. These chemicals cause capillaries to dilate and become leaky, increasing blood flow to the infected area and allowing blood to enter the tissues. Pathogen-destroying leukocytes and tissue-repairing proteins migrate into tissue spaces from the bloodstream to attack pathogens and repair their damage. Cytokines also promote [pb_glossary id=\"4867\"]chemotaxis[\/pb_glossary], which is migration to the site of infection by pathogen-destroying leukocytes. Some cytokines have anti-viral effects. They may shut down protein synthesis in host cells, which viruses need in order to survive and replicate.\r\n\r\nSee the video \"The inflammatory response\" by Neural Academy to learn about inflammatory response in more detail:\r\n\r\nhttps:\/\/youtu.be\/Fbzb75HA9M8\r\nThe inflammatory response, Neural Academy, 2019.<\/p>\r\n\r\n
\r\nComplement System<\/h1>\r\n<\/div>\r\nThe\u00a0[pb_glossary id=\"4868\"]complement system[\/pb_glossary]<\/strong>\u00a0is a complex biochemical mechanism named for its ability to \u201ccomplement\u201d the killing of pathogens by [pb_glossary id=\"5659\"]antibodies[\/pb_glossary], which are produced as part of an adaptive immune response. The complement system consists of more than two dozen\u00a0proteins\u00a0normally found in the\u00a0blood\u00a0and synthesized in the [pb_glossary id=\"2989\"]liver[\/pb_glossary]. The proteins usually circulate as non-functional precursor molecules until activated.\r\n\r\nAs shown in Figure 17.4.5, when the first protein in the complement series is activated \u2014typically by the binding of an antibody to an [pb_glossary id=\"2704\"]antigen[\/pb_glossary] on a pathogen \u2014 it sets in motion a domino effect. Each component takes its turn in a precise chain of steps known as the complement cascade. The end product is a cylinder that punctures a hole in the pathogen\u2019s cell membrane. This allows fluids and molecules to flow in and out of the cell, which swells and bursts.\r\n\r\n[caption id=\"attachment_4869\" align=\"aligncenter\" width=\"643\"] Figure 17.4.5 The complement system is a cascade of proteins that complements the killing of pathogen cells by antibodies.<\/em>[\/caption]\r\n\r\n\r\nCellular Responses<\/h1>\r\n<\/div>\r\nCellular responses of the innate immune system involve a variety of different types of [pb_glossary id=\"5623\"]leukocytes<\/span>[\/pb_glossary]<\/span>. Many of these leukocytes circulate in the blood and act like independent, single-celled organisms, searching out and destroying pathogens in the human host. These and other immune cells of the innate system identify pathogens or debris, and then help to eliminate them in some way. One way is by [pb_glossary id=\"1680\"]phagocytosis[\/pb_glossary].<\/span>\r\nPhagocytosis<\/h2>\r\n[pb_glossary id=\"1680\"]Phagocytosis[\/pb_glossary]<\/strong>\u00a0is an important feature of innate\u00a0immunity\u00a0that is performed by cells classified as phagocytes. In the process of phagocytosis, phagocytes engulf and digest pathogens or other harmful particles. Phagocytes generally patrol the body searching for pathogens, but they can also be called to specific locations by the release of [pb_glossary id=\"4865\"]cytokines<\/span>[\/pb_glossary]<\/span>\u00a0when [pb_glossary id=\"4860\"]inflammation[\/pb_glossary] occurs. Some phagocytes reside permanently in certain tissues.<\/span>\r\n\r\nAs shown in Figure 17.4.6, when a pathogen such as a bacterium is encountered by a phagocyte, the phagocyte extends a portion of its plasma membrane, wrapping the membrane around the pathogen until it is enveloped. Once inside the phagocyte, the pathogen becomes enclosed within an intracellular vesicle called a phagosome. The phagosome then fuses with another vesicle called a [pb_glossary id=\"4870\"]lysosome[\/pb_glossary], forming a phagolysosome. Digestive enzymes and acids from the lysosome kill and digest the pathogen in the phagolysosome. The final step of phagocytosis is excretion of soluble debris from the destroyed pathogen through [pb_glossary id=\"1682\"]exocytosis[\/pb_glossary].\r\n\r\n[caption id=\"attachment_4871\" align=\"aligncenter\" width=\"512\"] Figure 17.4.6 Phagocytosis is a multi-step process in which a pathogen is engulfed and digested by immune cells called phagocytes.<\/em>[\/caption]\r\n\r\nTypes of leukocytes that kill pathogens by phagocytosis include neutrophils, macrophages, and dendritic cells. You can see illustrations of these and other leukocytes involved in innate immune responses in Figure 17.4.7.\r\n\r\n[caption id=\"attachment_4872\" align=\"alignright\" width=\"292\"] Figure 17.4.7 Types of leukocytes evolved in innate immune responses are illustrated here.<\/em>[\/caption]\r\nNeutrophils<\/h2>\r\n[pb_glossary id=\"5545\"]Neutrophils[\/pb_glossary]<\/strong>\u00a0are leukocytes that travel throughout the body in the blood. They are usually the first immune cells to arrive at the site of an infection. They are the most numerous types of phagocytes, and they normally make up at least half of the total circulating leukocytes. The [pb_glossary id=\"4021\"]bone marrow[\/pb_glossary] of a normal healthy adult produces more than 100 billion neutrophils per day. During acute inflammation, more than\u00a0ten times<\/em>\u00a0that many neutrophils may be produced each day. Many neutrophils are needed to fight infections, because after a neutrophil phagocytizes just a few pathogens, it generally dies.\r\nMacrophages<\/h2>\r\n[pb_glossary id=\"4340\"]Macrophage<\/strong>s<\/strong>[\/pb_glossary]<\/strong>\u00a0are large phagocytic leukocytes that develop from monocytes. Macrophages spend much of their time within the [pb_glossary id=\"4874\"]interstitial fluid[\/pb_glossary] in\u00a0body tissues. They are the most efficient phagocytes, and they can phagocytize substantial numbers of pathogens or other cells. Macrophages are also versatile cells that produce a wide array of chemicals\u00a0\u2014\u00a0including\u00a0enzymes, complement proteins, and [pb_glossary id=\"4865\"]cytokine<\/span>s<\/span>[\/pb_glossary]<\/span>\u00a0\u2014 in addition to their phagocytic action. As phagocytes, macrophages act as scavengers that rid tissues of worn-out cells and other debris, as well as pathogens. In addition, macrophages act as antigen-presenting cells that activate the\u00a0[pb_glossary id=\"4822\"]adaptive immune system[\/pb_glossary].<\/span>\r\nDendritic Cells<\/h2>\r\nLike macrophages,\u00a0[pb_glossary id=\"4875\"]dendritic cells[\/pb_glossary]<\/strong>\u00a0develop from monocytes. They reside in tissues that have contact with the external environment, so they are located mainly in the skin, nose, lungs, stomach, and intestines. Besides engulfing and digesting pathogens, dendritic cells also act as antigen-presenting cells that trigger adaptive immune responses.\r\nEosinophils<\/h2>\r\n[pb_glossary id=\"5529\"]Eosinophils[\/pb_glossary]<\/strong> are non-phagocytic leukocytes that are related to [pb_glossary id=\"5545\"]neutrophil[\/pb_glossary]. They specialize in defending against [pb_glossary id=\"4812\"]parasites[\/pb_glossary]. They are very effective in killing large parasites (such as worms) by secreting a range of highly-toxic substances when activated. Eosinophils may become overactive and cause [pb_glossary id=\"4877\"]allergies[\/pb_glossary] or [pb_glossary id=\"4348\"]asthma[\/pb_glossary].\r\nBasophils<\/h2>\r\n[pb_glossary id=\"5573\"]Basophils[\/pb_glossary]<\/strong>\u00a0are non-phagocytic leukocytes that are also related to neutrophils. They are the least numerous of all white blood cells. Basophils secrete two types of chemicals that aid in body defenses: [pb_glossary id=\"4879\"]histamines<\/span><\/strong>[\/pb_glossary]<\/span> and heparin. <\/span>Histamines\u00a0are responsible for dilating\u00a0blood vessels\u00a0and increasing their permeability in inflammation.\u00a0<\/span>[pb_glossary id=\"4880\"]Heparin[\/pb_glossary]<\/strong>\u00a0inhibits blood clotting, and also promotes the movement of leukocytes into an area of infection.<\/span>\r\nMast Cells<\/h2>\r\n[pb_glossary id=\"4881\"]Mast cells[\/pb_glossary]<\/strong>\u00a0are non-phagocytic leukocytes that help initiate [pb_glossary id=\"4860\"]inflammation[\/pb_glossary] by secreting [pb_glossary id=\"4879\"]histamines<\/span>[\/pb_glossary]<\/span>. In some people, histamines trigger [pb_glossary id=\"4877\"]allergic reactions[\/pb_glossary], as well as inflammation. Mast cells may also secrete chemicals that help defend against parasites.<\/span>\r\nNatural Killer Cells<\/h2>\r\n[pb_glossary id=\"5515\"]Natural killer cells[\/pb_glossary]<\/strong>\u00a0are in the subset of leukocytes called [pb_glossary id=\"4832\"]lymphocytes<\/span>[\/pb_glossary]<\/span>, which are produced by the\u00a0lymphatic system. Natural killer cells destroy [pb_glossary id=\"5605\"]cancer<\/span>ous<\/span>[\/pb_glossary]<\/span>\u00a0or [pb_glossary id=\"4283\"]virus[\/pb_glossary]-infected host cells, although they do not directly attack invading pathogens. Natural killer cells recognize these host cells by a condition they exhibit called \u201cmissing self.\u201d Cells with missing self have abnormally low levels of cell-surface proteins of the [pb_glossary id=\"4825\"]major histocompatibility complex (MHC)[\/pb_glossary], which normally identify\u00a0body cells\u00a0as self.<\/span>\r\n\r\nInnate Immune Evasion<\/h1>\r\n<\/div>\r\nMany pathogens have evolved mechanisms that allow them to evade\u00a0human hosts'\u00a0innate immune systems. Some of these mechanisms include:\r\n\r\n \t- Invading host cells to replicate so they are \u201chidden\u201d from the immune system.<\/strong>\u00a0The bacterium that causes tuberculosis<\/a> uses this mechanism.<\/li>\r\n \t
- Forming a protective capsule around themselves to avoid being destroyed by immune system cells.<\/strong>\u00a0This defense occurs in bacteria, such as\u00a0Salmonella<\/em><\/a>\u00a0species.<\/li>\r\n \t
- Mimicking host cells so the immune system does not recognize them as foreign.<\/strong>\u00a0Some\u00a0species\u00a0of\u00a0Staphylococcus<\/em><\/a>\u00a0bacteria use this mechanism.<\/li>\r\n \t
- Directly killing phagocytes.<\/strong>\u00a0This ability evolved in several\u00a0species\u00a0of bacteria, including the species that causes anthrax<\/a>.<\/li>\r\n \t
- Producing molecules that prevent the formation of interferons, which are immune chemicals that fight viruses.<\/strong>\u00a0Some influenza<\/a> viruses have this capability.<\/li>\r\n \t
- Forming complex biofilms that provide protection from the cells and proteins of the immune system.<\/strong> This characterizes some species of [pb_glossary id=\"2588\"]bacteria[\/pb_glossary] and fungi. You can see an example of a bacterial biofilm on teeth in Figure 17.4.8.<\/li>\r\n<\/ul>\r\n[caption id=\"attachment_4883\" align=\"aligncenter\" width=\"400\"] Figure 17.4.8 The dental plaque on the top set of teeth is a biofilm that sticks to the teeth and consists of many species of bacteria. The plaque biofilm is difficult to remove, and it subjects the teeth and gums to high concentrations of bacterial metabolites, which result in dental disease. The same teeth after plaque removal are shown in the bottom picture.<\/em>[\/caption]\r\n\r\n\r\n\r\n \r\n\r\n<\/div>\r\n\r\n
\r\n17.4 Summary<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- The [pb_glossary id=\"4820\"]innate immune system[\/pb_glossary] is a subset of the human immune system that produces rapid, but non-specific responses to pathogens. Unlike the\u00a0[pb_glossary id=\"4822\"]adaptive immune system[\/pb_glossary], the innate system does not confer\u00a0[pb_glossary id=\"4884\"]immunity[\/pb_glossary]. The innate immune system includes surface barriers, [pb_glossary id=\"4860\"]inflammation[\/pb_glossary], the [pb_glossary id=\"4868\"]complement system[\/pb_glossary], and a variety of cellular responses.<\/li>\r\n \t
- The body\u2019s first line of defense consists of three different types of barriers that keep most pathogens out of\u00a0body tissues. The types of barriers are mechanical, chemical, and biological barriers.<\/li>\r\n \t
- Mechanical barriers\u00a0\u2014\u00a0which include the [pb_glossary id=\"3521\"]skin[\/pb_glossary], [pb_glossary id=\"3538\"]mucous membranes[\/pb_glossary], and fluids such as tears and [pb_glossary id=\"4717\"]urine[\/pb_glossary] \u2014 physically block pathogens from entering the body. Chemical barriers\u00a0\u2014\u00a0such as enzymes in [pb_glossary id=\"4711\"]sweat[\/pb_glossary], [pb_glossary id=\"4595\"]saliva[\/pb_glossary], and [pb_glossary id=\"4885\"]semen[\/pb_glossary]\u00a0\u2014\u00a0kill pathogens on body surfaces. Biological barriers are harmless bacteria that use up food and space so pathogenic bacteria cannot colonize the body.<\/li>\r\n \t
- If pathogens breach protective barriers, [pb_glossary id=\"4860\"]inflammation[\/pb_glossary] occurs. This creates a physical barrier against the spread of infection, and repairs tissue damage. Inflammation is triggered by chemicals such as [pb_glossary id=\"4865\"]cytokines[\/pb_glossary]\u00a0and [pb_glossary id=\"4879\"]histamines[\/pb_glossary], and it causes swelling, redness, and warmth.<\/li>\r\n \t
- The [pb_glossary id=\"4868\"]complement system[\/pb_glossary] is a complex biochemical mechanism that helps [pb_glossary id=\"5659\"]antibodies[\/pb_glossary] kill pathogens. Once activated, the complement system consists of more than two dozen proteins that lead to disruption of the\u00a0cell membrane\u00a0of pathogens and bursting of the cells.<\/li>\r\n \t
- Cellular responses of the innate immune system involve various types of [pb_glossary id=\"5623\"]leukocytes[\/pb_glossary]. For example, [pb_glossary id=\"5545\"]neutrophils[\/pb_glossary], [pb_glossary id=\"4340\"]macrophages[\/pb_glossary], and [pb_glossary id=\"4875\"]dendritic cells[\/pb_glossary]\u00a0phagocytize pathogens. [pb_glossary id=\"5573\"]Basophils[\/pb_glossary]\u00a0and [pb_glossary id=\"4881\"]mast cells[\/pb_glossary]\u00a0release chemicals that trigger inflammation. [pb_glossary id=\"5515\"]Natural killer cells[\/pb_glossary]\u00a0destroy cancerous or virus-infected cells, and eosinophils kill parasites.<\/li>\r\n \t
- Many pathogens have evolved mechanisms that help them evade the innate immune system. For example, some pathogens form a protective capsule around themselves, and some mimic host cells so the immune system does not recognize them as foreign.<\/li>\r\n<\/ul>\r\n<\/div>\r\n<\/div>\r\n
\r\n17.4 Review Questions<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- What is the innate immune system?<\/li>\r\n \t
- Identify the body\u2019s first line of defense.<\/li>\r\n \t
- [h5p id=\"638\"]<\/li>\r\n \t
- What are biological barriers? How do they protect the body?<\/li>\r\n \t
- State the purposes of inflammation. What triggers inflammation, and what signs and symptoms does it cause?<\/li>\r\n \t
- Define the complement system. How does it help destroy pathogens?<\/li>\r\n \t
- Describe two ways that pathogens can evade the innate immune system.<\/li>\r\n \t
- What are the ways in which phagocytes can encounter pathogens in the body?<\/li>\r\n \t
- Describe two different ways in which enzymes play a role in the innate immune response.<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n
\r\n17.4 Explore More<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\nhttps:\/\/youtu.be\/WW4skW6gucU\r\nHow mucus keeps us healthy - Katharina Ribbeck, TED-Ed, 2015.<\/p>\r\nhttps:\/\/youtu.be\/sYjtMP67vyk\r\n
Human Physiology - Innate Immune System, Janux, 2015.<\/p>\r\nhttps:\/\/youtu.be\/c64M1tZyWPM\r\n
Myriam Sidibe: The simple power of handwashing, TED, 2014.<\/p>\r\nhttps:\/\/youtu.be\/shEPwQPQG4I\r\n
Everything You Didn't Want To Know About Snot, Gross Science, 2017.<\/p>\r\nhttps:\/\/youtu.be\/dy1D3d1FBcw\r\n
Cough Grosser Than Sneeze? | Curiosity - World's Dirtiest Man, Discovery, 2011.<\/p>\r\n\r\n<\/div>\r\n<\/div>\r\n \r\n\r\n<\/div>\r\n
Attributions<\/h2>\r\nFigure 17.4.1<\/strong>\r\n\r\nOww_Papercut_14365<\/a> by Laurence Facun<\/a> on Wikimedia Commons is used under a CC BY 2.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/2.0) license.\r\n\r\nFigure 17.4.2<\/strong>\r\n\r\nhairy-nose<\/a> by Piotr Siedlecki\u00a0 on publicdomainpictures.net<\/a> is used under a CC0 1.0<\/a> Universal Public Domain Dedication (http:\/\/creativecommons.org\/publicdomain\/zero\/1.0\/) license.\r\n\r\nFigure 17.4.3<\/strong>\r\n\r\n1024px-Sneeze<\/a> by James Gathany\/ CDC Public Health Image library (PHIL) ID# 11162<\/a> on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/public_domain).\r\n\r\nFigure 17.4.4<\/strong>\r\n\r\nOSC_Microbio_17_06_Erythema<\/a> by CNX OpenStax<\/a> on Wikimedia Commons is used under a CC BY 4.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/4.0) license.\r\n\r\nFigure 17.4.5<\/strong>\r\n\r\n2212_Complement_Cascade_and_Function<\/a>\u00a0by OpenStax College<\/a> on Wikimedia Commons is used under a\u00a0CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\r\n\r\nFigure 17.4.6<\/strong>\r\n\r\n512px-Phagocytosis2<\/a> by Graham\u00a0Colm<\/span><\/a> at English Wikipedia<\/a><\/span>\u00a0on Wikimedia Commons is used under a CC BY-SA 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-sa\/3.0) license.\r\n\r\nFigure 17.4.7<\/strong>\r\n\r\nInnate_Immune_cells.svg<\/a> by Fred the Oyster<\/a> on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).\r\n\r\nFigure 17.4.8<\/strong>\r\n\r\n1024px-Gingivitis-before-and-after-3<\/a> by Onetimeuseaccount<\/a> on Wikimedia Commons is used under a CC0 1.0<\/a> Universal Public Domain Dedication (http:\/\/creativecommons.org\/publicdomain\/zero\/1.0\/) license.\r\nReferences<\/h2>\r\n
Betts, J. G., Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E., Womble, M., DeSaix, P. (2013, June 19). Figure 21.13 Complement cascade and function [digital image].\u00a0 In Anatomy and Physiology<\/em> (Section 21.2). OpenStax. https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/21-2-barrier-defenses-and-the-innate-immune-response<\/p>\r\nDiscovery. (2011, October 27). Cough grosser than sneeze? | Curiosity - World's dirtiest man. YouTube. https:\/\/www.youtube.com\/watch?v=dy1D3d1FBcw&feature=youtu.be<\/p>\r\n
Gross Science. (2017, January 31). Everything you didn't want to know about snot. YouTube. https:\/\/www.youtube.com\/watch?v=shEPwQPQG4I&feature=youtu.be<\/p>\r\n
Janux. (2015, January 10). Human physiology - Innate immune system. YouTube. https:\/\/www.youtube.com\/watch?v=sYjtMP67vyk&feature=youtu.be<\/p>\r\n
Mayo Clinic Staff. (n.d.). Anthrax [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/anthrax\/symptoms-causes\/syc-20356203<\/p>\r\n
Mayo Clinic Staff. (n.d.). Influenza (flu) [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/flu\/symptoms-causes\/syc-20351719<\/p>\r\n
Mayo Clinic Staff. (n.d.). Salmonella infection [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/salmonella\/symptoms-causes\/syc-20355329<\/p>\r\n
Mayo Clinic Staff. (n.d.). Staph infection [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/staph-infections\/multimedia\/staph-infection\/img-20008600<\/p>\r\n
Mayo Clinic Staff. (n.d.). Tuberculosis [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/tuberculosis\/symptoms-causes\/syc-20351250<\/p>\r\n
OpenStax. (2016, November 11). Figure\u00a0<\/span>17.23<\/span> A typical case of acute inflammation at the site of a skin wound - Erythema [digital image]. In OpenStax, Microbiology<\/em> (Section 17.5). https:\/\/openstax.org\/details\/books\/microbiology?Bookdetails<\/p>\r\nTED. (2014, October 14). Myriam Sidibe: The simple power of handwashing. YouTube. https:\/\/www.youtube.com\/watch?v=c64M1tZyWPM&feature=youtu.be<\/p>\r\n
TED-Ed. (2015, November 5). How mucus keeps us healthy - Katharina Ribbeck. YouTube. https:\/\/www.youtube.com\/watch?v=WW4skW6gucU&feature=youtu.be<\/p>","rendered":"
<\/p>\nFigure 17.4.1 Darn it!\u00a0 Paper cuts are the worst!<\/em><\/figcaption><\/figure>\n\nPaper Cut<\/h1>\n<\/div>\n
It\u2019s just a paper cut, but the break in your skin could provide an easy way for pathogens<\/span><\/a><\/span>\u00a0to enter your body. If\u00a0bacteria<\/a>\u00a0were to enter through the cut and infect the wound, your innate immune system would quickly respond with a dizzying array of general defenses.<\/span><\/p>\n\nWhat Is the Innate Immune System?<\/h1>\n<\/div>\n
The\u00a0innate immune system<\/a><\/strong>\u00a0is a subset of the human immune system that produces rapid, but non-specific responses to pathogens. Innate responses are generic, rather than tailored to a particular\u00a0pathogen. The innate system responds in the same general way to every pathogen\u00a0it encounters. Although the innate immune system provides immediate and rapid defenses against pathogens, it does not confer long-lasting\u00a0immunity\u00a0to them. In most organisms, the innate immune system is the dominant system of host defense. Other than most vertebrates (including humans), the innate immune system is the only system of host defense.<\/p>\nIn humans, the innate immune system includes surface barriers, inflammation, the complement system, and a variety of cellular responses. Surface barriers of various types generally keep most pathogens out of the body. If these barriers fail, then other innate defenses are triggered. The triggering event is usually the identification of pathogens by pattern-recognition receptors on\u00a0cells\u00a0of the innate immune system. These receptors recognize molecules that are broadly shared by pathogens, but distinguishable from host molecules. Alternatively, the other innate defenses may be triggered when damaged, injured, or stressed cells send out alarm signals, many of which are recognized by the same receptors as those that recognize pathogens.<\/p>\n
\nBarriers to Pathogens<\/h1>\n<\/div>\n
The body\u2019s first line of defense consists of three different types of barriers that keep most pathogens out of\u00a0body tissues. The types of barriers are mechanical, chemical, and biological barriers.<\/p>\n
Mechanical Barriers<\/h2>\nFigure 17.4.2 Nasal hairs are a mechanical barrier to larger particles in the air.<\/em><\/figcaption><\/figure>\nMechanical barriers<\/a> are the first line of defense against pathogens, and they physically block pathogens from entering the body. The skin<\/a> is the most important mechanical barrier. In fact, it is the single most important defense the body has. The outer layer of skin \u2014 the\u00a0epidermis<\/a>\u00a0\u2014 is tough, and very difficult for pathogens to penetrate. It consists of dead\u00a0cells\u00a0that are constantly shed from the body surface, a process that helps remove\u00a0bacteria\u00a0and other infectious agents that have adhered to the skin. The epidermis also lacks\u00a0blood vessels\u00a0and is usually lacking moisture, so it does not provide a suitable environment for most pathogens.\u00a0Hair<\/a> \u2014 which is an accessory organ of the skin \u2014 also helps keep out pathogens. Hairs inside the nose may trap larger pathogens and other particles in the air before they can enter the airways of the respiratory system (see Figure 17.4.2).<\/p>\nFigure 17.4.3 A sneeze can expel many pathogens from the respiratory tract, which is why you should always cover your mouth and nose and when you sneeze.<\/em><\/figcaption><\/figure>\nMucous membranes<\/span><\/a><\/span> provide a mechanical barrier to pathogens and other particles at body openings. These membranes also line the respiratory, gastrointestinal, urinary, and reproductive tracts. Mucous membranes secrete mucus, which is a slimy and somewhat sticky substance that traps pathogens. Many mucous membranes also have hair-like cilia<\/a> that sweep mucus and trapped pathogens toward body openings, where they can be removed from the body. When you sneeze or cough, mucus and pathogens are mechanically ejected from the nose and throat, as you can see in Figure 17.4.3.\u00a0 A sneeze can travel as fast as 160 Km\/hr (about 99 mi\/hour) and expel as many as 100,000 droplets into the air around you (a good reason to cover your sneezes!). Other mechanical defenses include tears, which wash pathogens from the eyes, and urine, which flushes pathogens out of the urinary tract.<\/span><\/p>\nChemical Barriers<\/h2>\n
Chemical barriers also protect against infection by pathogens. They destroy pathogens on the outer body surface, at body openings, and on inner body linings. Sweat, mucus, tears, saliva, and breastmilk all contain antimicrobial substances (such as the\u00a0enzyme\u00a0lysozyme) that kill pathogens, especially\u00a0bacteria. Sebaceous glands in the\u00a0dermis\u00a0of the skin secrete acids that form a very fine, slightly acidic film on the surface of the skin. This film acts as a barrier to bacteria, viruses, and other potential contaminants that might penetrate the skin. Urine and vaginal secretions are also too acidic for many pathogens to endure. Semen contains zinc \u2014 which most pathogens cannot tolerate \u2014 as well as defensins, which are antimicrobial\u00a0proteins\u00a0that act mainly by disrupting bacterial cell membranes. In the stomach, stomach\u00a0acid\u00a0and digestive\u00a0enzymes\u00a0called proteases (which break down proteins) kill most of the pathogens that enter the gastrointestinal tract in food or\u00a0water.<\/p>\n
Biological Barriers<\/h2>\n
Biological barriers are living organisms that help protect the body from pathogens. Trillions of harmless bacteria normally live on the human skin and in the urinary, reproductive, and gastrointestinal tracts. These bacteria use up food and surface space that help prevent pathogenic bacteria from colonizing the body. Some of these harmless bacteria also secrete substances that change the conditions of their environment, making it less hospitable to potentially harmful bacteria.\u00a0They may release toxins or change the\u00a0pH, for example. All of these effects of harmless bacteria reduce the chances that pathogenic microorganisms will be able to reach sufficient numbers and cause illness.<\/p>\n
\nInflammation<\/h1>\n<\/div>\n
If pathogens manage to breach the barriers protecting the body, one of the first active responses of the innate immune system kicks in. This response is\u00a0inflammation<\/a>.<\/strong>\u00a0The main function of inflammation is to establish a physical barrier against the spread of infection. It also eliminates the initial cause of cell injury, clears out dead\u00a0cells\u00a0and tissues damaged from the original insult and the inflammatory process, and initiates tissue repair. Inflammation is often a response to infection by pathogens, but there are other possible causes, including burns, frostbite, and exposure to toxins.<\/p>\nThe signs and symptoms of inflammation include redness, swelling, warmth, pain, and frequently some loss of function. These symptoms are caused by increased blood flow into infected tissue, and a number of other processes, illustrated in Figure 17.4.4.<\/p>\nFigure 17.4.4 This drawing shows what happens during the inflammatory response.<\/em><\/figcaption><\/figure>\nInflammation is triggered by chemicals such as\u00a0cytokines<\/a><\/strong>\u00a0and\u00a0histamines<\/a>,<\/strong>which are released by injured or infected cells, or by immune system cells such as macrophages (described below) that are already present in tissues. These chemicals cause capillaries to dilate and become leaky, increasing blood flow to the infected area and allowing blood to enter the tissues. Pathogen-destroying leukocytes and tissue-repairing proteins migrate into tissue spaces from the bloodstream to attack pathogens and repair their damage. Cytokines also promote chemotaxis<\/a>, which is migration to the site of infection by pathogen-destroying leukocytes. Some cytokines have anti-viral effects. They may shut down protein synthesis in host cells, which viruses need in order to survive and replicate.<\/p>\nSee the video “The inflammatory response” by Neural Academy to learn about inflammatory response in more detail:<\/p>\n
Inflammation<\/h1>\r\n<\/div>\r\nIf pathogens manage to breach the barriers protecting the body, one of the first active responses of the innate immune system kicks in. This response is\u00a0[pb_glossary id=\"4860\"]inflammation[\/pb_glossary].<\/strong>\u00a0The main function of inflammation is to establish a physical barrier against the spread of infection. It also eliminates the initial cause of cell injury, clears out dead\u00a0cells\u00a0and tissues damaged from the original insult and the inflammatory process, and initiates tissue repair. Inflammation is often a response to infection by pathogens, but there are other possible causes, including burns, frostbite, and exposure to toxins.\r\n\r\nThe signs and symptoms of inflammation include redness, swelling, warmth, pain, and frequently some loss of function. These symptoms are caused by increased blood flow into infected tissue, and a number of other processes, illustrated in Figure 17.4.4.\r\n\r\n[caption id=\"attachment_4864\" align=\"alignnone\" width=\"1024\"] Figure 17.4.4 This drawing shows what happens during the inflammatory response.<\/em>[\/caption]\r\n\r\nInflammation is triggered by chemicals such as\u00a0[pb_glossary id=\"4865\"]cytokines[\/pb_glossary]<\/strong>\u00a0and\u00a0[pb_glossary id=\"4866\"]histamines[\/pb_glossary],<\/strong>which are released by injured or infected cells, or by immune system cells such as macrophages (described below) that are already present in tissues. These chemicals cause capillaries to dilate and become leaky, increasing blood flow to the infected area and allowing blood to enter the tissues. Pathogen-destroying leukocytes and tissue-repairing proteins migrate into tissue spaces from the bloodstream to attack pathogens and repair their damage. Cytokines also promote [pb_glossary id=\"4867\"]chemotaxis[\/pb_glossary], which is migration to the site of infection by pathogen-destroying leukocytes. Some cytokines have anti-viral effects. They may shut down protein synthesis in host cells, which viruses need in order to survive and replicate.\r\n\r\nSee the video \"The inflammatory response\" by Neural Academy to learn about inflammatory response in more detail:\r\n\r\nhttps:\/\/youtu.be\/Fbzb75HA9M8\r\nThe inflammatory response, Neural Academy, 2019.<\/p>\r\n\r\n
\r\nComplement System<\/h1>\r\n<\/div>\r\nThe\u00a0[pb_glossary id=\"4868\"]complement system[\/pb_glossary]<\/strong>\u00a0is a complex biochemical mechanism named for its ability to \u201ccomplement\u201d the killing of pathogens by [pb_glossary id=\"5659\"]antibodies[\/pb_glossary], which are produced as part of an adaptive immune response. The complement system consists of more than two dozen\u00a0proteins\u00a0normally found in the\u00a0blood\u00a0and synthesized in the [pb_glossary id=\"2989\"]liver[\/pb_glossary]. The proteins usually circulate as non-functional precursor molecules until activated.\r\n\r\nAs shown in Figure 17.4.5, when the first protein in the complement series is activated \u2014typically by the binding of an antibody to an [pb_glossary id=\"2704\"]antigen[\/pb_glossary] on a pathogen \u2014 it sets in motion a domino effect. Each component takes its turn in a precise chain of steps known as the complement cascade. The end product is a cylinder that punctures a hole in the pathogen\u2019s cell membrane. This allows fluids and molecules to flow in and out of the cell, which swells and bursts.\r\n\r\n[caption id=\"attachment_4869\" align=\"aligncenter\" width=\"643\"] Figure 17.4.5 The complement system is a cascade of proteins that complements the killing of pathogen cells by antibodies.<\/em>[\/caption]\r\n\r\n\r\nCellular Responses<\/h1>\r\n<\/div>\r\nCellular responses of the innate immune system involve a variety of different types of [pb_glossary id=\"5623\"]leukocytes<\/span>[\/pb_glossary]<\/span>. Many of these leukocytes circulate in the blood and act like independent, single-celled organisms, searching out and destroying pathogens in the human host. These and other immune cells of the innate system identify pathogens or debris, and then help to eliminate them in some way. One way is by [pb_glossary id=\"1680\"]phagocytosis[\/pb_glossary].<\/span>\r\nPhagocytosis<\/h2>\r\n[pb_glossary id=\"1680\"]Phagocytosis[\/pb_glossary]<\/strong>\u00a0is an important feature of innate\u00a0immunity\u00a0that is performed by cells classified as phagocytes. In the process of phagocytosis, phagocytes engulf and digest pathogens or other harmful particles. Phagocytes generally patrol the body searching for pathogens, but they can also be called to specific locations by the release of [pb_glossary id=\"4865\"]cytokines<\/span>[\/pb_glossary]<\/span>\u00a0when [pb_glossary id=\"4860\"]inflammation[\/pb_glossary] occurs. Some phagocytes reside permanently in certain tissues.<\/span>\r\n\r\nAs shown in Figure 17.4.6, when a pathogen such as a bacterium is encountered by a phagocyte, the phagocyte extends a portion of its plasma membrane, wrapping the membrane around the pathogen until it is enveloped. Once inside the phagocyte, the pathogen becomes enclosed within an intracellular vesicle called a phagosome. The phagosome then fuses with another vesicle called a [pb_glossary id=\"4870\"]lysosome[\/pb_glossary], forming a phagolysosome. Digestive enzymes and acids from the lysosome kill and digest the pathogen in the phagolysosome. The final step of phagocytosis is excretion of soluble debris from the destroyed pathogen through [pb_glossary id=\"1682\"]exocytosis[\/pb_glossary].\r\n\r\n[caption id=\"attachment_4871\" align=\"aligncenter\" width=\"512\"] Figure 17.4.6 Phagocytosis is a multi-step process in which a pathogen is engulfed and digested by immune cells called phagocytes.<\/em>[\/caption]\r\n\r\nTypes of leukocytes that kill pathogens by phagocytosis include neutrophils, macrophages, and dendritic cells. You can see illustrations of these and other leukocytes involved in innate immune responses in Figure 17.4.7.\r\n\r\n[caption id=\"attachment_4872\" align=\"alignright\" width=\"292\"] Figure 17.4.7 Types of leukocytes evolved in innate immune responses are illustrated here.<\/em>[\/caption]\r\nNeutrophils<\/h2>\r\n[pb_glossary id=\"5545\"]Neutrophils[\/pb_glossary]<\/strong>\u00a0are leukocytes that travel throughout the body in the blood. They are usually the first immune cells to arrive at the site of an infection. They are the most numerous types of phagocytes, and they normally make up at least half of the total circulating leukocytes. The [pb_glossary id=\"4021\"]bone marrow[\/pb_glossary] of a normal healthy adult produces more than 100 billion neutrophils per day. During acute inflammation, more than\u00a0ten times<\/em>\u00a0that many neutrophils may be produced each day. Many neutrophils are needed to fight infections, because after a neutrophil phagocytizes just a few pathogens, it generally dies.\r\nMacrophages<\/h2>\r\n[pb_glossary id=\"4340\"]Macrophage<\/strong>s<\/strong>[\/pb_glossary]<\/strong>\u00a0are large phagocytic leukocytes that develop from monocytes. Macrophages spend much of their time within the [pb_glossary id=\"4874\"]interstitial fluid[\/pb_glossary] in\u00a0body tissues. They are the most efficient phagocytes, and they can phagocytize substantial numbers of pathogens or other cells. Macrophages are also versatile cells that produce a wide array of chemicals\u00a0\u2014\u00a0including\u00a0enzymes, complement proteins, and [pb_glossary id=\"4865\"]cytokine<\/span>s<\/span>[\/pb_glossary]<\/span>\u00a0\u2014 in addition to their phagocytic action. As phagocytes, macrophages act as scavengers that rid tissues of worn-out cells and other debris, as well as pathogens. In addition, macrophages act as antigen-presenting cells that activate the\u00a0[pb_glossary id=\"4822\"]adaptive immune system[\/pb_glossary].<\/span>\r\nDendritic Cells<\/h2>\r\nLike macrophages,\u00a0[pb_glossary id=\"4875\"]dendritic cells[\/pb_glossary]<\/strong>\u00a0develop from monocytes. They reside in tissues that have contact with the external environment, so they are located mainly in the skin, nose, lungs, stomach, and intestines. Besides engulfing and digesting pathogens, dendritic cells also act as antigen-presenting cells that trigger adaptive immune responses.\r\nEosinophils<\/h2>\r\n[pb_glossary id=\"5529\"]Eosinophils[\/pb_glossary]<\/strong> are non-phagocytic leukocytes that are related to [pb_glossary id=\"5545\"]neutrophil[\/pb_glossary]. They specialize in defending against [pb_glossary id=\"4812\"]parasites[\/pb_glossary]. They are very effective in killing large parasites (such as worms) by secreting a range of highly-toxic substances when activated. Eosinophils may become overactive and cause [pb_glossary id=\"4877\"]allergies[\/pb_glossary] or [pb_glossary id=\"4348\"]asthma[\/pb_glossary].\r\nBasophils<\/h2>\r\n[pb_glossary id=\"5573\"]Basophils[\/pb_glossary]<\/strong>\u00a0are non-phagocytic leukocytes that are also related to neutrophils. They are the least numerous of all white blood cells. Basophils secrete two types of chemicals that aid in body defenses: [pb_glossary id=\"4879\"]histamines<\/span><\/strong>[\/pb_glossary]<\/span> and heparin. <\/span>Histamines\u00a0are responsible for dilating\u00a0blood vessels\u00a0and increasing their permeability in inflammation.\u00a0<\/span>[pb_glossary id=\"4880\"]Heparin[\/pb_glossary]<\/strong>\u00a0inhibits blood clotting, and also promotes the movement of leukocytes into an area of infection.<\/span>\r\nMast Cells<\/h2>\r\n[pb_glossary id=\"4881\"]Mast cells[\/pb_glossary]<\/strong>\u00a0are non-phagocytic leukocytes that help initiate [pb_glossary id=\"4860\"]inflammation[\/pb_glossary] by secreting [pb_glossary id=\"4879\"]histamines<\/span>[\/pb_glossary]<\/span>. In some people, histamines trigger [pb_glossary id=\"4877\"]allergic reactions[\/pb_glossary], as well as inflammation. Mast cells may also secrete chemicals that help defend against parasites.<\/span>\r\nNatural Killer Cells<\/h2>\r\n[pb_glossary id=\"5515\"]Natural killer cells[\/pb_glossary]<\/strong>\u00a0are in the subset of leukocytes called [pb_glossary id=\"4832\"]lymphocytes<\/span>[\/pb_glossary]<\/span>, which are produced by the\u00a0lymphatic system. Natural killer cells destroy [pb_glossary id=\"5605\"]cancer<\/span>ous<\/span>[\/pb_glossary]<\/span>\u00a0or [pb_glossary id=\"4283\"]virus[\/pb_glossary]-infected host cells, although they do not directly attack invading pathogens. Natural killer cells recognize these host cells by a condition they exhibit called \u201cmissing self.\u201d Cells with missing self have abnormally low levels of cell-surface proteins of the [pb_glossary id=\"4825\"]major histocompatibility complex (MHC)[\/pb_glossary], which normally identify\u00a0body cells\u00a0as self.<\/span>\r\n\r\nInnate Immune Evasion<\/h1>\r\n<\/div>\r\nMany pathogens have evolved mechanisms that allow them to evade\u00a0human hosts'\u00a0innate immune systems. Some of these mechanisms include:\r\n\r\n \t- Invading host cells to replicate so they are \u201chidden\u201d from the immune system.<\/strong>\u00a0The bacterium that causes tuberculosis<\/a> uses this mechanism.<\/li>\r\n \t
- Forming a protective capsule around themselves to avoid being destroyed by immune system cells.<\/strong>\u00a0This defense occurs in bacteria, such as\u00a0Salmonella<\/em><\/a>\u00a0species.<\/li>\r\n \t
- Mimicking host cells so the immune system does not recognize them as foreign.<\/strong>\u00a0Some\u00a0species\u00a0of\u00a0Staphylococcus<\/em><\/a>\u00a0bacteria use this mechanism.<\/li>\r\n \t
- Directly killing phagocytes.<\/strong>\u00a0This ability evolved in several\u00a0species\u00a0of bacteria, including the species that causes anthrax<\/a>.<\/li>\r\n \t
- Producing molecules that prevent the formation of interferons, which are immune chemicals that fight viruses.<\/strong>\u00a0Some influenza<\/a> viruses have this capability.<\/li>\r\n \t
- Forming complex biofilms that provide protection from the cells and proteins of the immune system.<\/strong> This characterizes some species of [pb_glossary id=\"2588\"]bacteria[\/pb_glossary] and fungi. You can see an example of a bacterial biofilm on teeth in Figure 17.4.8.<\/li>\r\n<\/ul>\r\n[caption id=\"attachment_4883\" align=\"aligncenter\" width=\"400\"] Figure 17.4.8 The dental plaque on the top set of teeth is a biofilm that sticks to the teeth and consists of many species of bacteria. The plaque biofilm is difficult to remove, and it subjects the teeth and gums to high concentrations of bacterial metabolites, which result in dental disease. The same teeth after plaque removal are shown in the bottom picture.<\/em>[\/caption]\r\n\r\n\r\n\r\n \r\n\r\n<\/div>\r\n\r\n
\r\n17.4 Summary<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- The [pb_glossary id=\"4820\"]innate immune system[\/pb_glossary] is a subset of the human immune system that produces rapid, but non-specific responses to pathogens. Unlike the\u00a0[pb_glossary id=\"4822\"]adaptive immune system[\/pb_glossary], the innate system does not confer\u00a0[pb_glossary id=\"4884\"]immunity[\/pb_glossary]. The innate immune system includes surface barriers, [pb_glossary id=\"4860\"]inflammation[\/pb_glossary], the [pb_glossary id=\"4868\"]complement system[\/pb_glossary], and a variety of cellular responses.<\/li>\r\n \t
- The body\u2019s first line of defense consists of three different types of barriers that keep most pathogens out of\u00a0body tissues. The types of barriers are mechanical, chemical, and biological barriers.<\/li>\r\n \t
- Mechanical barriers\u00a0\u2014\u00a0which include the [pb_glossary id=\"3521\"]skin[\/pb_glossary], [pb_glossary id=\"3538\"]mucous membranes[\/pb_glossary], and fluids such as tears and [pb_glossary id=\"4717\"]urine[\/pb_glossary] \u2014 physically block pathogens from entering the body. Chemical barriers\u00a0\u2014\u00a0such as enzymes in [pb_glossary id=\"4711\"]sweat[\/pb_glossary], [pb_glossary id=\"4595\"]saliva[\/pb_glossary], and [pb_glossary id=\"4885\"]semen[\/pb_glossary]\u00a0\u2014\u00a0kill pathogens on body surfaces. Biological barriers are harmless bacteria that use up food and space so pathogenic bacteria cannot colonize the body.<\/li>\r\n \t
- If pathogens breach protective barriers, [pb_glossary id=\"4860\"]inflammation[\/pb_glossary] occurs. This creates a physical barrier against the spread of infection, and repairs tissue damage. Inflammation is triggered by chemicals such as [pb_glossary id=\"4865\"]cytokines[\/pb_glossary]\u00a0and [pb_glossary id=\"4879\"]histamines[\/pb_glossary], and it causes swelling, redness, and warmth.<\/li>\r\n \t
- The [pb_glossary id=\"4868\"]complement system[\/pb_glossary] is a complex biochemical mechanism that helps [pb_glossary id=\"5659\"]antibodies[\/pb_glossary] kill pathogens. Once activated, the complement system consists of more than two dozen proteins that lead to disruption of the\u00a0cell membrane\u00a0of pathogens and bursting of the cells.<\/li>\r\n \t
- Cellular responses of the innate immune system involve various types of [pb_glossary id=\"5623\"]leukocytes[\/pb_glossary]. For example, [pb_glossary id=\"5545\"]neutrophils[\/pb_glossary], [pb_glossary id=\"4340\"]macrophages[\/pb_glossary], and [pb_glossary id=\"4875\"]dendritic cells[\/pb_glossary]\u00a0phagocytize pathogens. [pb_glossary id=\"5573\"]Basophils[\/pb_glossary]\u00a0and [pb_glossary id=\"4881\"]mast cells[\/pb_glossary]\u00a0release chemicals that trigger inflammation. [pb_glossary id=\"5515\"]Natural killer cells[\/pb_glossary]\u00a0destroy cancerous or virus-infected cells, and eosinophils kill parasites.<\/li>\r\n \t
- Many pathogens have evolved mechanisms that help them evade the innate immune system. For example, some pathogens form a protective capsule around themselves, and some mimic host cells so the immune system does not recognize them as foreign.<\/li>\r\n<\/ul>\r\n<\/div>\r\n<\/div>\r\n
\r\n17.4 Review Questions<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- What is the innate immune system?<\/li>\r\n \t
- Identify the body\u2019s first line of defense.<\/li>\r\n \t
- [h5p id=\"638\"]<\/li>\r\n \t
- What are biological barriers? How do they protect the body?<\/li>\r\n \t
- State the purposes of inflammation. What triggers inflammation, and what signs and symptoms does it cause?<\/li>\r\n \t
- Define the complement system. How does it help destroy pathogens?<\/li>\r\n \t
- Describe two ways that pathogens can evade the innate immune system.<\/li>\r\n \t
- What are the ways in which phagocytes can encounter pathogens in the body?<\/li>\r\n \t
- Describe two different ways in which enzymes play a role in the innate immune response.<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n
\r\n17.4 Explore More<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\nhttps:\/\/youtu.be\/WW4skW6gucU\r\nHow mucus keeps us healthy - Katharina Ribbeck, TED-Ed, 2015.<\/p>\r\nhttps:\/\/youtu.be\/sYjtMP67vyk\r\n
Human Physiology - Innate Immune System, Janux, 2015.<\/p>\r\nhttps:\/\/youtu.be\/c64M1tZyWPM\r\n
Myriam Sidibe: The simple power of handwashing, TED, 2014.<\/p>\r\nhttps:\/\/youtu.be\/shEPwQPQG4I\r\n
Everything You Didn't Want To Know About Snot, Gross Science, 2017.<\/p>\r\nhttps:\/\/youtu.be\/dy1D3d1FBcw\r\n
Cough Grosser Than Sneeze? | Curiosity - World's Dirtiest Man, Discovery, 2011.<\/p>\r\n\r\n<\/div>\r\n<\/div>\r\n \r\n\r\n<\/div>\r\n
Attributions<\/h2>\r\nFigure 17.4.1<\/strong>\r\n\r\nOww_Papercut_14365<\/a> by Laurence Facun<\/a> on Wikimedia Commons is used under a CC BY 2.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/2.0) license.\r\n\r\nFigure 17.4.2<\/strong>\r\n\r\nhairy-nose<\/a> by Piotr Siedlecki\u00a0 on publicdomainpictures.net<\/a> is used under a CC0 1.0<\/a> Universal Public Domain Dedication (http:\/\/creativecommons.org\/publicdomain\/zero\/1.0\/) license.\r\n\r\nFigure 17.4.3<\/strong>\r\n\r\n1024px-Sneeze<\/a> by James Gathany\/ CDC Public Health Image library (PHIL) ID# 11162<\/a> on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/public_domain).\r\n\r\nFigure 17.4.4<\/strong>\r\n\r\nOSC_Microbio_17_06_Erythema<\/a> by CNX OpenStax<\/a> on Wikimedia Commons is used under a CC BY 4.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/4.0) license.\r\n\r\nFigure 17.4.5<\/strong>\r\n\r\n2212_Complement_Cascade_and_Function<\/a>\u00a0by OpenStax College<\/a> on Wikimedia Commons is used under a\u00a0CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\r\n\r\nFigure 17.4.6<\/strong>\r\n\r\n512px-Phagocytosis2<\/a> by Graham\u00a0Colm<\/span><\/a> at English Wikipedia<\/a><\/span>\u00a0on Wikimedia Commons is used under a CC BY-SA 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-sa\/3.0) license.\r\n\r\nFigure 17.4.7<\/strong>\r\n\r\nInnate_Immune_cells.svg<\/a> by Fred the Oyster<\/a> on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).\r\n\r\nFigure 17.4.8<\/strong>\r\n\r\n1024px-Gingivitis-before-and-after-3<\/a> by Onetimeuseaccount<\/a> on Wikimedia Commons is used under a CC0 1.0<\/a> Universal Public Domain Dedication (http:\/\/creativecommons.org\/publicdomain\/zero\/1.0\/) license.\r\nReferences<\/h2>\r\n
Betts, J. G., Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E., Womble, M., DeSaix, P. (2013, June 19). Figure 21.13 Complement cascade and function [digital image].\u00a0 In Anatomy and Physiology<\/em> (Section 21.2). OpenStax. https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/21-2-barrier-defenses-and-the-innate-immune-response<\/p>\r\nDiscovery. (2011, October 27). Cough grosser than sneeze? | Curiosity - World's dirtiest man. YouTube. https:\/\/www.youtube.com\/watch?v=dy1D3d1FBcw&feature=youtu.be<\/p>\r\n
Gross Science. (2017, January 31). Everything you didn't want to know about snot. YouTube. https:\/\/www.youtube.com\/watch?v=shEPwQPQG4I&feature=youtu.be<\/p>\r\n
Janux. (2015, January 10). Human physiology - Innate immune system. YouTube. https:\/\/www.youtube.com\/watch?v=sYjtMP67vyk&feature=youtu.be<\/p>\r\n
Mayo Clinic Staff. (n.d.). Anthrax [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/anthrax\/symptoms-causes\/syc-20356203<\/p>\r\n
Mayo Clinic Staff. (n.d.). Influenza (flu) [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/flu\/symptoms-causes\/syc-20351719<\/p>\r\n
Mayo Clinic Staff. (n.d.). Salmonella infection [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/salmonella\/symptoms-causes\/syc-20355329<\/p>\r\n
Mayo Clinic Staff. (n.d.). Staph infection [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/staph-infections\/multimedia\/staph-infection\/img-20008600<\/p>\r\n
Mayo Clinic Staff. (n.d.). Tuberculosis [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/tuberculosis\/symptoms-causes\/syc-20351250<\/p>\r\n
OpenStax. (2016, November 11). Figure\u00a0<\/span>17.23<\/span> A typical case of acute inflammation at the site of a skin wound - Erythema [digital image]. In OpenStax, Microbiology<\/em> (Section 17.5). https:\/\/openstax.org\/details\/books\/microbiology?Bookdetails<\/p>\r\nTED. (2014, October 14). Myriam Sidibe: The simple power of handwashing. YouTube. https:\/\/www.youtube.com\/watch?v=c64M1tZyWPM&feature=youtu.be<\/p>\r\n
TED-Ed. (2015, November 5). How mucus keeps us healthy - Katharina Ribbeck. YouTube. https:\/\/www.youtube.com\/watch?v=WW4skW6gucU&feature=youtu.be<\/p>","rendered":"
<\/p>\nFigure 17.4.1 Darn it!\u00a0 Paper cuts are the worst!<\/em><\/figcaption><\/figure>\n\nPaper Cut<\/h1>\n<\/div>\n
It\u2019s just a paper cut, but the break in your skin could provide an easy way for pathogens<\/span><\/a><\/span>\u00a0to enter your body. If\u00a0bacteria<\/a>\u00a0were to enter through the cut and infect the wound, your innate immune system would quickly respond with a dizzying array of general defenses.<\/span><\/p>\n\nWhat Is the Innate Immune System?<\/h1>\n<\/div>\n
The\u00a0innate immune system<\/a><\/strong>\u00a0is a subset of the human immune system that produces rapid, but non-specific responses to pathogens. Innate responses are generic, rather than tailored to a particular\u00a0pathogen. The innate system responds in the same general way to every pathogen\u00a0it encounters. Although the innate immune system provides immediate and rapid defenses against pathogens, it does not confer long-lasting\u00a0immunity\u00a0to them. In most organisms, the innate immune system is the dominant system of host defense. Other than most vertebrates (including humans), the innate immune system is the only system of host defense.<\/p>\nIn humans, the innate immune system includes surface barriers, inflammation, the complement system, and a variety of cellular responses. Surface barriers of various types generally keep most pathogens out of the body. If these barriers fail, then other innate defenses are triggered. The triggering event is usually the identification of pathogens by pattern-recognition receptors on\u00a0cells\u00a0of the innate immune system. These receptors recognize molecules that are broadly shared by pathogens, but distinguishable from host molecules. Alternatively, the other innate defenses may be triggered when damaged, injured, or stressed cells send out alarm signals, many of which are recognized by the same receptors as those that recognize pathogens.<\/p>\n
\nBarriers to Pathogens<\/h1>\n<\/div>\n
The body\u2019s first line of defense consists of three different types of barriers that keep most pathogens out of\u00a0body tissues. The types of barriers are mechanical, chemical, and biological barriers.<\/p>\n
Mechanical Barriers<\/h2>\nFigure 17.4.2 Nasal hairs are a mechanical barrier to larger particles in the air.<\/em><\/figcaption><\/figure>\nMechanical barriers<\/a> are the first line of defense against pathogens, and they physically block pathogens from entering the body. The skin<\/a> is the most important mechanical barrier. In fact, it is the single most important defense the body has. The outer layer of skin \u2014 the\u00a0epidermis<\/a>\u00a0\u2014 is tough, and very difficult for pathogens to penetrate. It consists of dead\u00a0cells\u00a0that are constantly shed from the body surface, a process that helps remove\u00a0bacteria\u00a0and other infectious agents that have adhered to the skin. The epidermis also lacks\u00a0blood vessels\u00a0and is usually lacking moisture, so it does not provide a suitable environment for most pathogens.\u00a0Hair<\/a> \u2014 which is an accessory organ of the skin \u2014 also helps keep out pathogens. Hairs inside the nose may trap larger pathogens and other particles in the air before they can enter the airways of the respiratory system (see Figure 17.4.2).<\/p>\nFigure 17.4.3 A sneeze can expel many pathogens from the respiratory tract, which is why you should always cover your mouth and nose and when you sneeze.<\/em><\/figcaption><\/figure>\nMucous membranes<\/span><\/a><\/span> provide a mechanical barrier to pathogens and other particles at body openings. These membranes also line the respiratory, gastrointestinal, urinary, and reproductive tracts. Mucous membranes secrete mucus, which is a slimy and somewhat sticky substance that traps pathogens. Many mucous membranes also have hair-like cilia<\/a> that sweep mucus and trapped pathogens toward body openings, where they can be removed from the body. When you sneeze or cough, mucus and pathogens are mechanically ejected from the nose and throat, as you can see in Figure 17.4.3.\u00a0 A sneeze can travel as fast as 160 Km\/hr (about 99 mi\/hour) and expel as many as 100,000 droplets into the air around you (a good reason to cover your sneezes!). Other mechanical defenses include tears, which wash pathogens from the eyes, and urine, which flushes pathogens out of the urinary tract.<\/span><\/p>\nChemical Barriers<\/h2>\n
Chemical barriers also protect against infection by pathogens. They destroy pathogens on the outer body surface, at body openings, and on inner body linings. Sweat, mucus, tears, saliva, and breastmilk all contain antimicrobial substances (such as the\u00a0enzyme\u00a0lysozyme) that kill pathogens, especially\u00a0bacteria. Sebaceous glands in the\u00a0dermis\u00a0of the skin secrete acids that form a very fine, slightly acidic film on the surface of the skin. This film acts as a barrier to bacteria, viruses, and other potential contaminants that might penetrate the skin. Urine and vaginal secretions are also too acidic for many pathogens to endure. Semen contains zinc \u2014 which most pathogens cannot tolerate \u2014 as well as defensins, which are antimicrobial\u00a0proteins\u00a0that act mainly by disrupting bacterial cell membranes. In the stomach, stomach\u00a0acid\u00a0and digestive\u00a0enzymes\u00a0called proteases (which break down proteins) kill most of the pathogens that enter the gastrointestinal tract in food or\u00a0water.<\/p>\n
Biological Barriers<\/h2>\n
Biological barriers are living organisms that help protect the body from pathogens. Trillions of harmless bacteria normally live on the human skin and in the urinary, reproductive, and gastrointestinal tracts. These bacteria use up food and surface space that help prevent pathogenic bacteria from colonizing the body. Some of these harmless bacteria also secrete substances that change the conditions of their environment, making it less hospitable to potentially harmful bacteria.\u00a0They may release toxins or change the\u00a0pH, for example. All of these effects of harmless bacteria reduce the chances that pathogenic microorganisms will be able to reach sufficient numbers and cause illness.<\/p>\n
\nInflammation<\/h1>\n<\/div>\n
If pathogens manage to breach the barriers protecting the body, one of the first active responses of the innate immune system kicks in. This response is\u00a0inflammation<\/a>.<\/strong>\u00a0The main function of inflammation is to establish a physical barrier against the spread of infection. It also eliminates the initial cause of cell injury, clears out dead\u00a0cells\u00a0and tissues damaged from the original insult and the inflammatory process, and initiates tissue repair. Inflammation is often a response to infection by pathogens, but there are other possible causes, including burns, frostbite, and exposure to toxins.<\/p>\nThe signs and symptoms of inflammation include redness, swelling, warmth, pain, and frequently some loss of function. These symptoms are caused by increased blood flow into infected tissue, and a number of other processes, illustrated in Figure 17.4.4.<\/p>\nFigure 17.4.4 This drawing shows what happens during the inflammatory response.<\/em><\/figcaption><\/figure>\nInflammation is triggered by chemicals such as\u00a0cytokines<\/a><\/strong>\u00a0and\u00a0histamines<\/a>,<\/strong>which are released by injured or infected cells, or by immune system cells such as macrophages (described below) that are already present in tissues. These chemicals cause capillaries to dilate and become leaky, increasing blood flow to the infected area and allowing blood to enter the tissues. Pathogen-destroying leukocytes and tissue-repairing proteins migrate into tissue spaces from the bloodstream to attack pathogens and repair their damage. Cytokines also promote chemotaxis<\/a>, which is migration to the site of infection by pathogen-destroying leukocytes. Some cytokines have anti-viral effects. They may shut down protein synthesis in host cells, which viruses need in order to survive and replicate.<\/p>\nSee the video “The inflammatory response” by Neural Academy to learn about inflammatory response in more detail:<\/p>\n
Complement System<\/h1>\r\n<\/div>\r\nThe\u00a0[pb_glossary id=\"4868\"]complement system[\/pb_glossary]<\/strong>\u00a0is a complex biochemical mechanism named for its ability to \u201ccomplement\u201d the killing of pathogens by [pb_glossary id=\"5659\"]antibodies[\/pb_glossary], which are produced as part of an adaptive immune response. The complement system consists of more than two dozen\u00a0proteins\u00a0normally found in the\u00a0blood\u00a0and synthesized in the [pb_glossary id=\"2989\"]liver[\/pb_glossary]. The proteins usually circulate as non-functional precursor molecules until activated.\r\n\r\nAs shown in Figure 17.4.5, when the first protein in the complement series is activated \u2014typically by the binding of an antibody to an [pb_glossary id=\"2704\"]antigen[\/pb_glossary] on a pathogen \u2014 it sets in motion a domino effect. Each component takes its turn in a precise chain of steps known as the complement cascade. The end product is a cylinder that punctures a hole in the pathogen\u2019s cell membrane. This allows fluids and molecules to flow in and out of the cell, which swells and bursts.\r\n\r\n[caption id=\"attachment_4869\" align=\"aligncenter\" width=\"643\"] Figure 17.4.5 The complement system is a cascade of proteins that complements the killing of pathogen cells by antibodies.<\/em>[\/caption]\r\n\r\n\r\nCellular Responses<\/h1>\r\n<\/div>\r\nCellular responses of the innate immune system involve a variety of different types of [pb_glossary id=\"5623\"]leukocytes<\/span>[\/pb_glossary]<\/span>. Many of these leukocytes circulate in the blood and act like independent, single-celled organisms, searching out and destroying pathogens in the human host. These and other immune cells of the innate system identify pathogens or debris, and then help to eliminate them in some way. One way is by [pb_glossary id=\"1680\"]phagocytosis[\/pb_glossary].<\/span>\r\nPhagocytosis<\/h2>\r\n[pb_glossary id=\"1680\"]Phagocytosis[\/pb_glossary]<\/strong>\u00a0is an important feature of innate\u00a0immunity\u00a0that is performed by cells classified as phagocytes. In the process of phagocytosis, phagocytes engulf and digest pathogens or other harmful particles. Phagocytes generally patrol the body searching for pathogens, but they can also be called to specific locations by the release of [pb_glossary id=\"4865\"]cytokines<\/span>[\/pb_glossary]<\/span>\u00a0when [pb_glossary id=\"4860\"]inflammation[\/pb_glossary] occurs. Some phagocytes reside permanently in certain tissues.<\/span>\r\n\r\nAs shown in Figure 17.4.6, when a pathogen such as a bacterium is encountered by a phagocyte, the phagocyte extends a portion of its plasma membrane, wrapping the membrane around the pathogen until it is enveloped. Once inside the phagocyte, the pathogen becomes enclosed within an intracellular vesicle called a phagosome. The phagosome then fuses with another vesicle called a [pb_glossary id=\"4870\"]lysosome[\/pb_glossary], forming a phagolysosome. Digestive enzymes and acids from the lysosome kill and digest the pathogen in the phagolysosome. The final step of phagocytosis is excretion of soluble debris from the destroyed pathogen through [pb_glossary id=\"1682\"]exocytosis[\/pb_glossary].\r\n\r\n[caption id=\"attachment_4871\" align=\"aligncenter\" width=\"512\"] Figure 17.4.6 Phagocytosis is a multi-step process in which a pathogen is engulfed and digested by immune cells called phagocytes.<\/em>[\/caption]\r\n\r\nTypes of leukocytes that kill pathogens by phagocytosis include neutrophils, macrophages, and dendritic cells. You can see illustrations of these and other leukocytes involved in innate immune responses in Figure 17.4.7.\r\n\r\n[caption id=\"attachment_4872\" align=\"alignright\" width=\"292\"] Figure 17.4.7 Types of leukocytes evolved in innate immune responses are illustrated here.<\/em>[\/caption]\r\nNeutrophils<\/h2>\r\n[pb_glossary id=\"5545\"]Neutrophils[\/pb_glossary]<\/strong>\u00a0are leukocytes that travel throughout the body in the blood. They are usually the first immune cells to arrive at the site of an infection. They are the most numerous types of phagocytes, and they normally make up at least half of the total circulating leukocytes. The [pb_glossary id=\"4021\"]bone marrow[\/pb_glossary] of a normal healthy adult produces more than 100 billion neutrophils per day. During acute inflammation, more than\u00a0ten times<\/em>\u00a0that many neutrophils may be produced each day. Many neutrophils are needed to fight infections, because after a neutrophil phagocytizes just a few pathogens, it generally dies.\r\nMacrophages<\/h2>\r\n[pb_glossary id=\"4340\"]Macrophage<\/strong>s<\/strong>[\/pb_glossary]<\/strong>\u00a0are large phagocytic leukocytes that develop from monocytes. Macrophages spend much of their time within the [pb_glossary id=\"4874\"]interstitial fluid[\/pb_glossary] in\u00a0body tissues. They are the most efficient phagocytes, and they can phagocytize substantial numbers of pathogens or other cells. Macrophages are also versatile cells that produce a wide array of chemicals\u00a0\u2014\u00a0including\u00a0enzymes, complement proteins, and [pb_glossary id=\"4865\"]cytokine<\/span>s<\/span>[\/pb_glossary]<\/span>\u00a0\u2014 in addition to their phagocytic action. As phagocytes, macrophages act as scavengers that rid tissues of worn-out cells and other debris, as well as pathogens. In addition, macrophages act as antigen-presenting cells that activate the\u00a0[pb_glossary id=\"4822\"]adaptive immune system[\/pb_glossary].<\/span>\r\nDendritic Cells<\/h2>\r\nLike macrophages,\u00a0[pb_glossary id=\"4875\"]dendritic cells[\/pb_glossary]<\/strong>\u00a0develop from monocytes. They reside in tissues that have contact with the external environment, so they are located mainly in the skin, nose, lungs, stomach, and intestines. Besides engulfing and digesting pathogens, dendritic cells also act as antigen-presenting cells that trigger adaptive immune responses.\r\nEosinophils<\/h2>\r\n[pb_glossary id=\"5529\"]Eosinophils[\/pb_glossary]<\/strong> are non-phagocytic leukocytes that are related to [pb_glossary id=\"5545\"]neutrophil[\/pb_glossary]. They specialize in defending against [pb_glossary id=\"4812\"]parasites[\/pb_glossary]. They are very effective in killing large parasites (such as worms) by secreting a range of highly-toxic substances when activated. Eosinophils may become overactive and cause [pb_glossary id=\"4877\"]allergies[\/pb_glossary] or [pb_glossary id=\"4348\"]asthma[\/pb_glossary].\r\nBasophils<\/h2>\r\n[pb_glossary id=\"5573\"]Basophils[\/pb_glossary]<\/strong>\u00a0are non-phagocytic leukocytes that are also related to neutrophils. They are the least numerous of all white blood cells. Basophils secrete two types of chemicals that aid in body defenses: [pb_glossary id=\"4879\"]histamines<\/span><\/strong>[\/pb_glossary]<\/span> and heparin. <\/span>Histamines\u00a0are responsible for dilating\u00a0blood vessels\u00a0and increasing their permeability in inflammation.\u00a0<\/span>[pb_glossary id=\"4880\"]Heparin[\/pb_glossary]<\/strong>\u00a0inhibits blood clotting, and also promotes the movement of leukocytes into an area of infection.<\/span>\r\nMast Cells<\/h2>\r\n[pb_glossary id=\"4881\"]Mast cells[\/pb_glossary]<\/strong>\u00a0are non-phagocytic leukocytes that help initiate [pb_glossary id=\"4860\"]inflammation[\/pb_glossary] by secreting [pb_glossary id=\"4879\"]histamines<\/span>[\/pb_glossary]<\/span>. In some people, histamines trigger [pb_glossary id=\"4877\"]allergic reactions[\/pb_glossary], as well as inflammation. Mast cells may also secrete chemicals that help defend against parasites.<\/span>\r\nNatural Killer Cells<\/h2>\r\n[pb_glossary id=\"5515\"]Natural killer cells[\/pb_glossary]<\/strong>\u00a0are in the subset of leukocytes called [pb_glossary id=\"4832\"]lymphocytes<\/span>[\/pb_glossary]<\/span>, which are produced by the\u00a0lymphatic system. Natural killer cells destroy [pb_glossary id=\"5605\"]cancer<\/span>ous<\/span>[\/pb_glossary]<\/span>\u00a0or [pb_glossary id=\"4283\"]virus[\/pb_glossary]-infected host cells, although they do not directly attack invading pathogens. Natural killer cells recognize these host cells by a condition they exhibit called \u201cmissing self.\u201d Cells with missing self have abnormally low levels of cell-surface proteins of the [pb_glossary id=\"4825\"]major histocompatibility complex (MHC)[\/pb_glossary], which normally identify\u00a0body cells\u00a0as self.<\/span>\r\n\r\nInnate Immune Evasion<\/h1>\r\n<\/div>\r\nMany pathogens have evolved mechanisms that allow them to evade\u00a0human hosts'\u00a0innate immune systems. Some of these mechanisms include:\r\n\r\n \t- Invading host cells to replicate so they are \u201chidden\u201d from the immune system.<\/strong>\u00a0The bacterium that causes tuberculosis<\/a> uses this mechanism.<\/li>\r\n \t
- Forming a protective capsule around themselves to avoid being destroyed by immune system cells.<\/strong>\u00a0This defense occurs in bacteria, such as\u00a0Salmonella<\/em><\/a>\u00a0species.<\/li>\r\n \t
- Mimicking host cells so the immune system does not recognize them as foreign.<\/strong>\u00a0Some\u00a0species\u00a0of\u00a0Staphylococcus<\/em><\/a>\u00a0bacteria use this mechanism.<\/li>\r\n \t
- Directly killing phagocytes.<\/strong>\u00a0This ability evolved in several\u00a0species\u00a0of bacteria, including the species that causes anthrax<\/a>.<\/li>\r\n \t
- Producing molecules that prevent the formation of interferons, which are immune chemicals that fight viruses.<\/strong>\u00a0Some influenza<\/a> viruses have this capability.<\/li>\r\n \t
- Forming complex biofilms that provide protection from the cells and proteins of the immune system.<\/strong> This characterizes some species of [pb_glossary id=\"2588\"]bacteria[\/pb_glossary] and fungi. You can see an example of a bacterial biofilm on teeth in Figure 17.4.8.<\/li>\r\n<\/ul>\r\n[caption id=\"attachment_4883\" align=\"aligncenter\" width=\"400\"] Figure 17.4.8 The dental plaque on the top set of teeth is a biofilm that sticks to the teeth and consists of many species of bacteria. The plaque biofilm is difficult to remove, and it subjects the teeth and gums to high concentrations of bacterial metabolites, which result in dental disease. The same teeth after plaque removal are shown in the bottom picture.<\/em>[\/caption]\r\n\r\n\r\n\r\n \r\n\r\n<\/div>\r\n\r\n
\r\n17.4 Summary<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- The [pb_glossary id=\"4820\"]innate immune system[\/pb_glossary] is a subset of the human immune system that produces rapid, but non-specific responses to pathogens. Unlike the\u00a0[pb_glossary id=\"4822\"]adaptive immune system[\/pb_glossary], the innate system does not confer\u00a0[pb_glossary id=\"4884\"]immunity[\/pb_glossary]. The innate immune system includes surface barriers, [pb_glossary id=\"4860\"]inflammation[\/pb_glossary], the [pb_glossary id=\"4868\"]complement system[\/pb_glossary], and a variety of cellular responses.<\/li>\r\n \t
- The body\u2019s first line of defense consists of three different types of barriers that keep most pathogens out of\u00a0body tissues. The types of barriers are mechanical, chemical, and biological barriers.<\/li>\r\n \t
- Mechanical barriers\u00a0\u2014\u00a0which include the [pb_glossary id=\"3521\"]skin[\/pb_glossary], [pb_glossary id=\"3538\"]mucous membranes[\/pb_glossary], and fluids such as tears and [pb_glossary id=\"4717\"]urine[\/pb_glossary] \u2014 physically block pathogens from entering the body. Chemical barriers\u00a0\u2014\u00a0such as enzymes in [pb_glossary id=\"4711\"]sweat[\/pb_glossary], [pb_glossary id=\"4595\"]saliva[\/pb_glossary], and [pb_glossary id=\"4885\"]semen[\/pb_glossary]\u00a0\u2014\u00a0kill pathogens on body surfaces. Biological barriers are harmless bacteria that use up food and space so pathogenic bacteria cannot colonize the body.<\/li>\r\n \t
- If pathogens breach protective barriers, [pb_glossary id=\"4860\"]inflammation[\/pb_glossary] occurs. This creates a physical barrier against the spread of infection, and repairs tissue damage. Inflammation is triggered by chemicals such as [pb_glossary id=\"4865\"]cytokines[\/pb_glossary]\u00a0and [pb_glossary id=\"4879\"]histamines[\/pb_glossary], and it causes swelling, redness, and warmth.<\/li>\r\n \t
- The [pb_glossary id=\"4868\"]complement system[\/pb_glossary] is a complex biochemical mechanism that helps [pb_glossary id=\"5659\"]antibodies[\/pb_glossary] kill pathogens. Once activated, the complement system consists of more than two dozen proteins that lead to disruption of the\u00a0cell membrane\u00a0of pathogens and bursting of the cells.<\/li>\r\n \t
- Cellular responses of the innate immune system involve various types of [pb_glossary id=\"5623\"]leukocytes[\/pb_glossary]. For example, [pb_glossary id=\"5545\"]neutrophils[\/pb_glossary], [pb_glossary id=\"4340\"]macrophages[\/pb_glossary], and [pb_glossary id=\"4875\"]dendritic cells[\/pb_glossary]\u00a0phagocytize pathogens. [pb_glossary id=\"5573\"]Basophils[\/pb_glossary]\u00a0and [pb_glossary id=\"4881\"]mast cells[\/pb_glossary]\u00a0release chemicals that trigger inflammation. [pb_glossary id=\"5515\"]Natural killer cells[\/pb_glossary]\u00a0destroy cancerous or virus-infected cells, and eosinophils kill parasites.<\/li>\r\n \t
- Many pathogens have evolved mechanisms that help them evade the innate immune system. For example, some pathogens form a protective capsule around themselves, and some mimic host cells so the immune system does not recognize them as foreign.<\/li>\r\n<\/ul>\r\n<\/div>\r\n<\/div>\r\n
\r\n17.4 Review Questions<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- What is the innate immune system?<\/li>\r\n \t
- Identify the body\u2019s first line of defense.<\/li>\r\n \t
- [h5p id=\"638\"]<\/li>\r\n \t
- What are biological barriers? How do they protect the body?<\/li>\r\n \t
- State the purposes of inflammation. What triggers inflammation, and what signs and symptoms does it cause?<\/li>\r\n \t
- Define the complement system. How does it help destroy pathogens?<\/li>\r\n \t
- Describe two ways that pathogens can evade the innate immune system.<\/li>\r\n \t
- What are the ways in which phagocytes can encounter pathogens in the body?<\/li>\r\n \t
- Describe two different ways in which enzymes play a role in the innate immune response.<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n
\r\n17.4 Explore More<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\nhttps:\/\/youtu.be\/WW4skW6gucU\r\nHow mucus keeps us healthy - Katharina Ribbeck, TED-Ed, 2015.<\/p>\r\nhttps:\/\/youtu.be\/sYjtMP67vyk\r\n
Human Physiology - Innate Immune System, Janux, 2015.<\/p>\r\nhttps:\/\/youtu.be\/c64M1tZyWPM\r\n
Myriam Sidibe: The simple power of handwashing, TED, 2014.<\/p>\r\nhttps:\/\/youtu.be\/shEPwQPQG4I\r\n
Everything You Didn't Want To Know About Snot, Gross Science, 2017.<\/p>\r\nhttps:\/\/youtu.be\/dy1D3d1FBcw\r\n
Cough Grosser Than Sneeze? | Curiosity - World's Dirtiest Man, Discovery, 2011.<\/p>\r\n\r\n<\/div>\r\n<\/div>\r\n \r\n\r\n<\/div>\r\n
Attributions<\/h2>\r\nFigure 17.4.1<\/strong>\r\n\r\nOww_Papercut_14365<\/a> by Laurence Facun<\/a> on Wikimedia Commons is used under a CC BY 2.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/2.0) license.\r\n\r\nFigure 17.4.2<\/strong>\r\n\r\nhairy-nose<\/a> by Piotr Siedlecki\u00a0 on publicdomainpictures.net<\/a> is used under a CC0 1.0<\/a> Universal Public Domain Dedication (http:\/\/creativecommons.org\/publicdomain\/zero\/1.0\/) license.\r\n\r\nFigure 17.4.3<\/strong>\r\n\r\n1024px-Sneeze<\/a> by James Gathany\/ CDC Public Health Image library (PHIL) ID# 11162<\/a> on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/public_domain).\r\n\r\nFigure 17.4.4<\/strong>\r\n\r\nOSC_Microbio_17_06_Erythema<\/a> by CNX OpenStax<\/a> on Wikimedia Commons is used under a CC BY 4.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/4.0) license.\r\n\r\nFigure 17.4.5<\/strong>\r\n\r\n2212_Complement_Cascade_and_Function<\/a>\u00a0by OpenStax College<\/a> on Wikimedia Commons is used under a\u00a0CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\r\n\r\nFigure 17.4.6<\/strong>\r\n\r\n512px-Phagocytosis2<\/a> by Graham\u00a0Colm<\/span><\/a> at English Wikipedia<\/a><\/span>\u00a0on Wikimedia Commons is used under a CC BY-SA 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-sa\/3.0) license.\r\n\r\nFigure 17.4.7<\/strong>\r\n\r\nInnate_Immune_cells.svg<\/a> by Fred the Oyster<\/a> on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).\r\n\r\nFigure 17.4.8<\/strong>\r\n\r\n1024px-Gingivitis-before-and-after-3<\/a> by Onetimeuseaccount<\/a> on Wikimedia Commons is used under a CC0 1.0<\/a> Universal Public Domain Dedication (http:\/\/creativecommons.org\/publicdomain\/zero\/1.0\/) license.\r\nReferences<\/h2>\r\n
Betts, J. G., Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E., Womble, M., DeSaix, P. (2013, June 19). Figure 21.13 Complement cascade and function [digital image].\u00a0 In Anatomy and Physiology<\/em> (Section 21.2). OpenStax. https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/21-2-barrier-defenses-and-the-innate-immune-response<\/p>\r\nDiscovery. (2011, October 27). Cough grosser than sneeze? | Curiosity - World's dirtiest man. YouTube. https:\/\/www.youtube.com\/watch?v=dy1D3d1FBcw&feature=youtu.be<\/p>\r\n
Gross Science. (2017, January 31). Everything you didn't want to know about snot. YouTube. https:\/\/www.youtube.com\/watch?v=shEPwQPQG4I&feature=youtu.be<\/p>\r\n
Janux. (2015, January 10). Human physiology - Innate immune system. YouTube. https:\/\/www.youtube.com\/watch?v=sYjtMP67vyk&feature=youtu.be<\/p>\r\n
Mayo Clinic Staff. (n.d.). Anthrax [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/anthrax\/symptoms-causes\/syc-20356203<\/p>\r\n
Mayo Clinic Staff. (n.d.). Influenza (flu) [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/flu\/symptoms-causes\/syc-20351719<\/p>\r\n
Mayo Clinic Staff. (n.d.). Salmonella infection [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/salmonella\/symptoms-causes\/syc-20355329<\/p>\r\n
Mayo Clinic Staff. (n.d.). Staph infection [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/staph-infections\/multimedia\/staph-infection\/img-20008600<\/p>\r\n
Mayo Clinic Staff. (n.d.). Tuberculosis [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/tuberculosis\/symptoms-causes\/syc-20351250<\/p>\r\n
OpenStax. (2016, November 11). Figure\u00a0<\/span>17.23<\/span> A typical case of acute inflammation at the site of a skin wound - Erythema [digital image]. In OpenStax, Microbiology<\/em> (Section 17.5). https:\/\/openstax.org\/details\/books\/microbiology?Bookdetails<\/p>\r\nTED. (2014, October 14). Myriam Sidibe: The simple power of handwashing. YouTube. https:\/\/www.youtube.com\/watch?v=c64M1tZyWPM&feature=youtu.be<\/p>\r\n
TED-Ed. (2015, November 5). How mucus keeps us healthy - Katharina Ribbeck. YouTube. https:\/\/www.youtube.com\/watch?v=WW4skW6gucU&feature=youtu.be<\/p>","rendered":"
<\/p>\nFigure 17.4.1 Darn it!\u00a0 Paper cuts are the worst!<\/em><\/figcaption><\/figure>\n\nPaper Cut<\/h1>\n<\/div>\n
It\u2019s just a paper cut, but the break in your skin could provide an easy way for pathogens<\/span><\/a><\/span>\u00a0to enter your body. If\u00a0bacteria<\/a>\u00a0were to enter through the cut and infect the wound, your innate immune system would quickly respond with a dizzying array of general defenses.<\/span><\/p>\n\nWhat Is the Innate Immune System?<\/h1>\n<\/div>\n
The\u00a0innate immune system<\/a><\/strong>\u00a0is a subset of the human immune system that produces rapid, but non-specific responses to pathogens. Innate responses are generic, rather than tailored to a particular\u00a0pathogen. The innate system responds in the same general way to every pathogen\u00a0it encounters. Although the innate immune system provides immediate and rapid defenses against pathogens, it does not confer long-lasting\u00a0immunity\u00a0to them. In most organisms, the innate immune system is the dominant system of host defense. Other than most vertebrates (including humans), the innate immune system is the only system of host defense.<\/p>\nIn humans, the innate immune system includes surface barriers, inflammation, the complement system, and a variety of cellular responses. Surface barriers of various types generally keep most pathogens out of the body. If these barriers fail, then other innate defenses are triggered. The triggering event is usually the identification of pathogens by pattern-recognition receptors on\u00a0cells\u00a0of the innate immune system. These receptors recognize molecules that are broadly shared by pathogens, but distinguishable from host molecules. Alternatively, the other innate defenses may be triggered when damaged, injured, or stressed cells send out alarm signals, many of which are recognized by the same receptors as those that recognize pathogens.<\/p>\n
\nBarriers to Pathogens<\/h1>\n<\/div>\n
The body\u2019s first line of defense consists of three different types of barriers that keep most pathogens out of\u00a0body tissues. The types of barriers are mechanical, chemical, and biological barriers.<\/p>\n
Mechanical Barriers<\/h2>\nFigure 17.4.2 Nasal hairs are a mechanical barrier to larger particles in the air.<\/em><\/figcaption><\/figure>\nMechanical barriers<\/a> are the first line of defense against pathogens, and they physically block pathogens from entering the body. The skin<\/a> is the most important mechanical barrier. In fact, it is the single most important defense the body has. The outer layer of skin \u2014 the\u00a0epidermis<\/a>\u00a0\u2014 is tough, and very difficult for pathogens to penetrate. It consists of dead\u00a0cells\u00a0that are constantly shed from the body surface, a process that helps remove\u00a0bacteria\u00a0and other infectious agents that have adhered to the skin. The epidermis also lacks\u00a0blood vessels\u00a0and is usually lacking moisture, so it does not provide a suitable environment for most pathogens.\u00a0Hair<\/a> \u2014 which is an accessory organ of the skin \u2014 also helps keep out pathogens. Hairs inside the nose may trap larger pathogens and other particles in the air before they can enter the airways of the respiratory system (see Figure 17.4.2).<\/p>\nFigure 17.4.3 A sneeze can expel many pathogens from the respiratory tract, which is why you should always cover your mouth and nose and when you sneeze.<\/em><\/figcaption><\/figure>\nMucous membranes<\/span><\/a><\/span> provide a mechanical barrier to pathogens and other particles at body openings. These membranes also line the respiratory, gastrointestinal, urinary, and reproductive tracts. Mucous membranes secrete mucus, which is a slimy and somewhat sticky substance that traps pathogens. Many mucous membranes also have hair-like cilia<\/a> that sweep mucus and trapped pathogens toward body openings, where they can be removed from the body. When you sneeze or cough, mucus and pathogens are mechanically ejected from the nose and throat, as you can see in Figure 17.4.3.\u00a0 A sneeze can travel as fast as 160 Km\/hr (about 99 mi\/hour) and expel as many as 100,000 droplets into the air around you (a good reason to cover your sneezes!). Other mechanical defenses include tears, which wash pathogens from the eyes, and urine, which flushes pathogens out of the urinary tract.<\/span><\/p>\nChemical Barriers<\/h2>\n
Chemical barriers also protect against infection by pathogens. They destroy pathogens on the outer body surface, at body openings, and on inner body linings. Sweat, mucus, tears, saliva, and breastmilk all contain antimicrobial substances (such as the\u00a0enzyme\u00a0lysozyme) that kill pathogens, especially\u00a0bacteria. Sebaceous glands in the\u00a0dermis\u00a0of the skin secrete acids that form a very fine, slightly acidic film on the surface of the skin. This film acts as a barrier to bacteria, viruses, and other potential contaminants that might penetrate the skin. Urine and vaginal secretions are also too acidic for many pathogens to endure. Semen contains zinc \u2014 which most pathogens cannot tolerate \u2014 as well as defensins, which are antimicrobial\u00a0proteins\u00a0that act mainly by disrupting bacterial cell membranes. In the stomach, stomach\u00a0acid\u00a0and digestive\u00a0enzymes\u00a0called proteases (which break down proteins) kill most of the pathogens that enter the gastrointestinal tract in food or\u00a0water.<\/p>\n
Biological Barriers<\/h2>\n
Biological barriers are living organisms that help protect the body from pathogens. Trillions of harmless bacteria normally live on the human skin and in the urinary, reproductive, and gastrointestinal tracts. These bacteria use up food and surface space that help prevent pathogenic bacteria from colonizing the body. Some of these harmless bacteria also secrete substances that change the conditions of their environment, making it less hospitable to potentially harmful bacteria.\u00a0They may release toxins or change the\u00a0pH, for example. All of these effects of harmless bacteria reduce the chances that pathogenic microorganisms will be able to reach sufficient numbers and cause illness.<\/p>\n
\nInflammation<\/h1>\n<\/div>\n
If pathogens manage to breach the barriers protecting the body, one of the first active responses of the innate immune system kicks in. This response is\u00a0inflammation<\/a>.<\/strong>\u00a0The main function of inflammation is to establish a physical barrier against the spread of infection. It also eliminates the initial cause of cell injury, clears out dead\u00a0cells\u00a0and tissues damaged from the original insult and the inflammatory process, and initiates tissue repair. Inflammation is often a response to infection by pathogens, but there are other possible causes, including burns, frostbite, and exposure to toxins.<\/p>\nThe signs and symptoms of inflammation include redness, swelling, warmth, pain, and frequently some loss of function. These symptoms are caused by increased blood flow into infected tissue, and a number of other processes, illustrated in Figure 17.4.4.<\/p>\nFigure 17.4.4 This drawing shows what happens during the inflammatory response.<\/em><\/figcaption><\/figure>\nInflammation is triggered by chemicals such as\u00a0cytokines<\/a><\/strong>\u00a0and\u00a0histamines<\/a>,<\/strong>which are released by injured or infected cells, or by immune system cells such as macrophages (described below) that are already present in tissues. These chemicals cause capillaries to dilate and become leaky, increasing blood flow to the infected area and allowing blood to enter the tissues. Pathogen-destroying leukocytes and tissue-repairing proteins migrate into tissue spaces from the bloodstream to attack pathogens and repair their damage. Cytokines also promote chemotaxis<\/a>, which is migration to the site of infection by pathogen-destroying leukocytes. Some cytokines have anti-viral effects. They may shut down protein synthesis in host cells, which viruses need in order to survive and replicate.<\/p>\nSee the video “The inflammatory response” by Neural Academy to learn about inflammatory response in more detail:<\/p>\n
Cellular Responses<\/h1>\r\n<\/div>\r\nCellular responses of the innate immune system involve a variety of different types of [pb_glossary id=\"5623\"]leukocytes<\/span>[\/pb_glossary]<\/span>. Many of these leukocytes circulate in the blood and act like independent, single-celled organisms, searching out and destroying pathogens in the human host. These and other immune cells of the innate system identify pathogens or debris, and then help to eliminate them in some way. One way is by [pb_glossary id=\"1680\"]phagocytosis[\/pb_glossary].<\/span>\r\nPhagocytosis<\/h2>\r\n[pb_glossary id=\"1680\"]Phagocytosis[\/pb_glossary]<\/strong>\u00a0is an important feature of innate\u00a0immunity\u00a0that is performed by cells classified as phagocytes. In the process of phagocytosis, phagocytes engulf and digest pathogens or other harmful particles. Phagocytes generally patrol the body searching for pathogens, but they can also be called to specific locations by the release of [pb_glossary id=\"4865\"]cytokines<\/span>[\/pb_glossary]<\/span>\u00a0when [pb_glossary id=\"4860\"]inflammation[\/pb_glossary] occurs. Some phagocytes reside permanently in certain tissues.<\/span>\r\n\r\nAs shown in Figure 17.4.6, when a pathogen such as a bacterium is encountered by a phagocyte, the phagocyte extends a portion of its plasma membrane, wrapping the membrane around the pathogen until it is enveloped. Once inside the phagocyte, the pathogen becomes enclosed within an intracellular vesicle called a phagosome. The phagosome then fuses with another vesicle called a [pb_glossary id=\"4870\"]lysosome[\/pb_glossary], forming a phagolysosome. Digestive enzymes and acids from the lysosome kill and digest the pathogen in the phagolysosome. The final step of phagocytosis is excretion of soluble debris from the destroyed pathogen through [pb_glossary id=\"1682\"]exocytosis[\/pb_glossary].\r\n\r\n[caption id=\"attachment_4871\" align=\"aligncenter\" width=\"512\"] Figure 17.4.6 Phagocytosis is a multi-step process in which a pathogen is engulfed and digested by immune cells called phagocytes.<\/em>[\/caption]\r\n\r\nTypes of leukocytes that kill pathogens by phagocytosis include neutrophils, macrophages, and dendritic cells. You can see illustrations of these and other leukocytes involved in innate immune responses in Figure 17.4.7.\r\n\r\n[caption id=\"attachment_4872\" align=\"alignright\" width=\"292\"] Figure 17.4.7 Types of leukocytes evolved in innate immune responses are illustrated here.<\/em>[\/caption]\r\nNeutrophils<\/h2>\r\n[pb_glossary id=\"5545\"]Neutrophils[\/pb_glossary]<\/strong>\u00a0are leukocytes that travel throughout the body in the blood. They are usually the first immune cells to arrive at the site of an infection. They are the most numerous types of phagocytes, and they normally make up at least half of the total circulating leukocytes. The [pb_glossary id=\"4021\"]bone marrow[\/pb_glossary] of a normal healthy adult produces more than 100 billion neutrophils per day. During acute inflammation, more than\u00a0ten times<\/em>\u00a0that many neutrophils may be produced each day. Many neutrophils are needed to fight infections, because after a neutrophil phagocytizes just a few pathogens, it generally dies.\r\nMacrophages<\/h2>\r\n[pb_glossary id=\"4340\"]Macrophage<\/strong>s<\/strong>[\/pb_glossary]<\/strong>\u00a0are large phagocytic leukocytes that develop from monocytes. Macrophages spend much of their time within the [pb_glossary id=\"4874\"]interstitial fluid[\/pb_glossary] in\u00a0body tissues. They are the most efficient phagocytes, and they can phagocytize substantial numbers of pathogens or other cells. Macrophages are also versatile cells that produce a wide array of chemicals\u00a0\u2014\u00a0including\u00a0enzymes, complement proteins, and [pb_glossary id=\"4865\"]cytokine<\/span>s<\/span>[\/pb_glossary]<\/span>\u00a0\u2014 in addition to their phagocytic action. As phagocytes, macrophages act as scavengers that rid tissues of worn-out cells and other debris, as well as pathogens. In addition, macrophages act as antigen-presenting cells that activate the\u00a0[pb_glossary id=\"4822\"]adaptive immune system[\/pb_glossary].<\/span>\r\nDendritic Cells<\/h2>\r\nLike macrophages,\u00a0[pb_glossary id=\"4875\"]dendritic cells[\/pb_glossary]<\/strong>\u00a0develop from monocytes. They reside in tissues that have contact with the external environment, so they are located mainly in the skin, nose, lungs, stomach, and intestines. Besides engulfing and digesting pathogens, dendritic cells also act as antigen-presenting cells that trigger adaptive immune responses.\r\nEosinophils<\/h2>\r\n[pb_glossary id=\"5529\"]Eosinophils[\/pb_glossary]<\/strong> are non-phagocytic leukocytes that are related to [pb_glossary id=\"5545\"]neutrophil[\/pb_glossary]. They specialize in defending against [pb_glossary id=\"4812\"]parasites[\/pb_glossary]. They are very effective in killing large parasites (such as worms) by secreting a range of highly-toxic substances when activated. Eosinophils may become overactive and cause [pb_glossary id=\"4877\"]allergies[\/pb_glossary] or [pb_glossary id=\"4348\"]asthma[\/pb_glossary].\r\nBasophils<\/h2>\r\n[pb_glossary id=\"5573\"]Basophils[\/pb_glossary]<\/strong>\u00a0are non-phagocytic leukocytes that are also related to neutrophils. They are the least numerous of all white blood cells. Basophils secrete two types of chemicals that aid in body defenses: [pb_glossary id=\"4879\"]histamines<\/span><\/strong>[\/pb_glossary]<\/span> and heparin. <\/span>Histamines\u00a0are responsible for dilating\u00a0blood vessels\u00a0and increasing their permeability in inflammation.\u00a0<\/span>[pb_glossary id=\"4880\"]Heparin[\/pb_glossary]<\/strong>\u00a0inhibits blood clotting, and also promotes the movement of leukocytes into an area of infection.<\/span>\r\nMast Cells<\/h2>\r\n[pb_glossary id=\"4881\"]Mast cells[\/pb_glossary]<\/strong>\u00a0are non-phagocytic leukocytes that help initiate [pb_glossary id=\"4860\"]inflammation[\/pb_glossary] by secreting [pb_glossary id=\"4879\"]histamines<\/span>[\/pb_glossary]<\/span>. In some people, histamines trigger [pb_glossary id=\"4877\"]allergic reactions[\/pb_glossary], as well as inflammation. Mast cells may also secrete chemicals that help defend against parasites.<\/span>\r\nNatural Killer Cells<\/h2>\r\n[pb_glossary id=\"5515\"]Natural killer cells[\/pb_glossary]<\/strong>\u00a0are in the subset of leukocytes called [pb_glossary id=\"4832\"]lymphocytes<\/span>[\/pb_glossary]<\/span>, which are produced by the\u00a0lymphatic system. Natural killer cells destroy [pb_glossary id=\"5605\"]cancer<\/span>ous<\/span>[\/pb_glossary]<\/span>\u00a0or [pb_glossary id=\"4283\"]virus[\/pb_glossary]-infected host cells, although they do not directly attack invading pathogens. Natural killer cells recognize these host cells by a condition they exhibit called \u201cmissing self.\u201d Cells with missing self have abnormally low levels of cell-surface proteins of the [pb_glossary id=\"4825\"]major histocompatibility complex (MHC)[\/pb_glossary], which normally identify\u00a0body cells\u00a0as self.<\/span>\r\n\r\nInnate Immune Evasion<\/h1>\r\n<\/div>\r\nMany pathogens have evolved mechanisms that allow them to evade\u00a0human hosts'\u00a0innate immune systems. Some of these mechanisms include:\r\n\r\n \t- Invading host cells to replicate so they are \u201chidden\u201d from the immune system.<\/strong>\u00a0The bacterium that causes tuberculosis<\/a> uses this mechanism.<\/li>\r\n \t
- Forming a protective capsule around themselves to avoid being destroyed by immune system cells.<\/strong>\u00a0This defense occurs in bacteria, such as\u00a0Salmonella<\/em><\/a>\u00a0species.<\/li>\r\n \t
- Mimicking host cells so the immune system does not recognize them as foreign.<\/strong>\u00a0Some\u00a0species\u00a0of\u00a0Staphylococcus<\/em><\/a>\u00a0bacteria use this mechanism.<\/li>\r\n \t
- Directly killing phagocytes.<\/strong>\u00a0This ability evolved in several\u00a0species\u00a0of bacteria, including the species that causes anthrax<\/a>.<\/li>\r\n \t
- Producing molecules that prevent the formation of interferons, which are immune chemicals that fight viruses.<\/strong>\u00a0Some influenza<\/a> viruses have this capability.<\/li>\r\n \t
- Forming complex biofilms that provide protection from the cells and proteins of the immune system.<\/strong> This characterizes some species of [pb_glossary id=\"2588\"]bacteria[\/pb_glossary] and fungi. You can see an example of a bacterial biofilm on teeth in Figure 17.4.8.<\/li>\r\n<\/ul>\r\n[caption id=\"attachment_4883\" align=\"aligncenter\" width=\"400\"] Figure 17.4.8 The dental plaque on the top set of teeth is a biofilm that sticks to the teeth and consists of many species of bacteria. The plaque biofilm is difficult to remove, and it subjects the teeth and gums to high concentrations of bacterial metabolites, which result in dental disease. The same teeth after plaque removal are shown in the bottom picture.<\/em>[\/caption]\r\n\r\n\r\n\r\n \r\n\r\n<\/div>\r\n\r\n
\r\n17.4 Summary<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- The [pb_glossary id=\"4820\"]innate immune system[\/pb_glossary] is a subset of the human immune system that produces rapid, but non-specific responses to pathogens. Unlike the\u00a0[pb_glossary id=\"4822\"]adaptive immune system[\/pb_glossary], the innate system does not confer\u00a0[pb_glossary id=\"4884\"]immunity[\/pb_glossary]. The innate immune system includes surface barriers, [pb_glossary id=\"4860\"]inflammation[\/pb_glossary], the [pb_glossary id=\"4868\"]complement system[\/pb_glossary], and a variety of cellular responses.<\/li>\r\n \t
- The body\u2019s first line of defense consists of three different types of barriers that keep most pathogens out of\u00a0body tissues. The types of barriers are mechanical, chemical, and biological barriers.<\/li>\r\n \t
- Mechanical barriers\u00a0\u2014\u00a0which include the [pb_glossary id=\"3521\"]skin[\/pb_glossary], [pb_glossary id=\"3538\"]mucous membranes[\/pb_glossary], and fluids such as tears and [pb_glossary id=\"4717\"]urine[\/pb_glossary] \u2014 physically block pathogens from entering the body. Chemical barriers\u00a0\u2014\u00a0such as enzymes in [pb_glossary id=\"4711\"]sweat[\/pb_glossary], [pb_glossary id=\"4595\"]saliva[\/pb_glossary], and [pb_glossary id=\"4885\"]semen[\/pb_glossary]\u00a0\u2014\u00a0kill pathogens on body surfaces. Biological barriers are harmless bacteria that use up food and space so pathogenic bacteria cannot colonize the body.<\/li>\r\n \t
- If pathogens breach protective barriers, [pb_glossary id=\"4860\"]inflammation[\/pb_glossary] occurs. This creates a physical barrier against the spread of infection, and repairs tissue damage. Inflammation is triggered by chemicals such as [pb_glossary id=\"4865\"]cytokines[\/pb_glossary]\u00a0and [pb_glossary id=\"4879\"]histamines[\/pb_glossary], and it causes swelling, redness, and warmth.<\/li>\r\n \t
- The [pb_glossary id=\"4868\"]complement system[\/pb_glossary] is a complex biochemical mechanism that helps [pb_glossary id=\"5659\"]antibodies[\/pb_glossary] kill pathogens. Once activated, the complement system consists of more than two dozen proteins that lead to disruption of the\u00a0cell membrane\u00a0of pathogens and bursting of the cells.<\/li>\r\n \t
- Cellular responses of the innate immune system involve various types of [pb_glossary id=\"5623\"]leukocytes[\/pb_glossary]. For example, [pb_glossary id=\"5545\"]neutrophils[\/pb_glossary], [pb_glossary id=\"4340\"]macrophages[\/pb_glossary], and [pb_glossary id=\"4875\"]dendritic cells[\/pb_glossary]\u00a0phagocytize pathogens. [pb_glossary id=\"5573\"]Basophils[\/pb_glossary]\u00a0and [pb_glossary id=\"4881\"]mast cells[\/pb_glossary]\u00a0release chemicals that trigger inflammation. [pb_glossary id=\"5515\"]Natural killer cells[\/pb_glossary]\u00a0destroy cancerous or virus-infected cells, and eosinophils kill parasites.<\/li>\r\n \t
- Many pathogens have evolved mechanisms that help them evade the innate immune system. For example, some pathogens form a protective capsule around themselves, and some mimic host cells so the immune system does not recognize them as foreign.<\/li>\r\n<\/ul>\r\n<\/div>\r\n<\/div>\r\n
\r\n17.4 Review Questions<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- What is the innate immune system?<\/li>\r\n \t
- Identify the body\u2019s first line of defense.<\/li>\r\n \t
- [h5p id=\"638\"]<\/li>\r\n \t
- What are biological barriers? How do they protect the body?<\/li>\r\n \t
- State the purposes of inflammation. What triggers inflammation, and what signs and symptoms does it cause?<\/li>\r\n \t
- Define the complement system. How does it help destroy pathogens?<\/li>\r\n \t
- Describe two ways that pathogens can evade the innate immune system.<\/li>\r\n \t
- What are the ways in which phagocytes can encounter pathogens in the body?<\/li>\r\n \t
- Describe two different ways in which enzymes play a role in the innate immune response.<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n
\r\n17.4 Explore More<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\nhttps:\/\/youtu.be\/WW4skW6gucU\r\nHow mucus keeps us healthy - Katharina Ribbeck, TED-Ed, 2015.<\/p>\r\nhttps:\/\/youtu.be\/sYjtMP67vyk\r\n
Human Physiology - Innate Immune System, Janux, 2015.<\/p>\r\nhttps:\/\/youtu.be\/c64M1tZyWPM\r\n
Myriam Sidibe: The simple power of handwashing, TED, 2014.<\/p>\r\nhttps:\/\/youtu.be\/shEPwQPQG4I\r\n
Everything You Didn't Want To Know About Snot, Gross Science, 2017.<\/p>\r\nhttps:\/\/youtu.be\/dy1D3d1FBcw\r\n
Cough Grosser Than Sneeze? | Curiosity - World's Dirtiest Man, Discovery, 2011.<\/p>\r\n\r\n<\/div>\r\n<\/div>\r\n \r\n\r\n<\/div>\r\n
Attributions<\/h2>\r\nFigure 17.4.1<\/strong>\r\n\r\nOww_Papercut_14365<\/a> by Laurence Facun<\/a> on Wikimedia Commons is used under a CC BY 2.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/2.0) license.\r\n\r\nFigure 17.4.2<\/strong>\r\n\r\nhairy-nose<\/a> by Piotr Siedlecki\u00a0 on publicdomainpictures.net<\/a> is used under a CC0 1.0<\/a> Universal Public Domain Dedication (http:\/\/creativecommons.org\/publicdomain\/zero\/1.0\/) license.\r\n\r\nFigure 17.4.3<\/strong>\r\n\r\n1024px-Sneeze<\/a> by James Gathany\/ CDC Public Health Image library (PHIL) ID# 11162<\/a> on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/public_domain).\r\n\r\nFigure 17.4.4<\/strong>\r\n\r\nOSC_Microbio_17_06_Erythema<\/a> by CNX OpenStax<\/a> on Wikimedia Commons is used under a CC BY 4.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/4.0) license.\r\n\r\nFigure 17.4.5<\/strong>\r\n\r\n2212_Complement_Cascade_and_Function<\/a>\u00a0by OpenStax College<\/a> on Wikimedia Commons is used under a\u00a0CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\r\n\r\nFigure 17.4.6<\/strong>\r\n\r\n512px-Phagocytosis2<\/a> by Graham\u00a0Colm<\/span><\/a> at English Wikipedia<\/a><\/span>\u00a0on Wikimedia Commons is used under a CC BY-SA 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-sa\/3.0) license.\r\n\r\nFigure 17.4.7<\/strong>\r\n\r\nInnate_Immune_cells.svg<\/a> by Fred the Oyster<\/a> on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).\r\n\r\nFigure 17.4.8<\/strong>\r\n\r\n1024px-Gingivitis-before-and-after-3<\/a> by Onetimeuseaccount<\/a> on Wikimedia Commons is used under a CC0 1.0<\/a> Universal Public Domain Dedication (http:\/\/creativecommons.org\/publicdomain\/zero\/1.0\/) license.\r\nReferences<\/h2>\r\n
Betts, J. G., Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E., Womble, M., DeSaix, P. (2013, June 19). Figure 21.13 Complement cascade and function [digital image].\u00a0 In Anatomy and Physiology<\/em> (Section 21.2). OpenStax. https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/21-2-barrier-defenses-and-the-innate-immune-response<\/p>\r\nDiscovery. (2011, October 27). Cough grosser than sneeze? | Curiosity - World's dirtiest man. YouTube. https:\/\/www.youtube.com\/watch?v=dy1D3d1FBcw&feature=youtu.be<\/p>\r\n
Gross Science. (2017, January 31). Everything you didn't want to know about snot. YouTube. https:\/\/www.youtube.com\/watch?v=shEPwQPQG4I&feature=youtu.be<\/p>\r\n
Janux. (2015, January 10). Human physiology - Innate immune system. YouTube. https:\/\/www.youtube.com\/watch?v=sYjtMP67vyk&feature=youtu.be<\/p>\r\n
Mayo Clinic Staff. (n.d.). Anthrax [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/anthrax\/symptoms-causes\/syc-20356203<\/p>\r\n
Mayo Clinic Staff. (n.d.). Influenza (flu) [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/flu\/symptoms-causes\/syc-20351719<\/p>\r\n
Mayo Clinic Staff. (n.d.). Salmonella infection [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/salmonella\/symptoms-causes\/syc-20355329<\/p>\r\n
Mayo Clinic Staff. (n.d.). Staph infection [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/staph-infections\/multimedia\/staph-infection\/img-20008600<\/p>\r\n
Mayo Clinic Staff. (n.d.). Tuberculosis [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/tuberculosis\/symptoms-causes\/syc-20351250<\/p>\r\n
OpenStax. (2016, November 11). Figure\u00a0<\/span>17.23<\/span> A typical case of acute inflammation at the site of a skin wound - Erythema [digital image]. In OpenStax, Microbiology<\/em> (Section 17.5). https:\/\/openstax.org\/details\/books\/microbiology?Bookdetails<\/p>\r\nTED. (2014, October 14). Myriam Sidibe: The simple power of handwashing. YouTube. https:\/\/www.youtube.com\/watch?v=c64M1tZyWPM&feature=youtu.be<\/p>\r\n
TED-Ed. (2015, November 5). How mucus keeps us healthy - Katharina Ribbeck. YouTube. https:\/\/www.youtube.com\/watch?v=WW4skW6gucU&feature=youtu.be<\/p>","rendered":"
<\/p>\nFigure 17.4.1 Darn it!\u00a0 Paper cuts are the worst!<\/em><\/figcaption><\/figure>\n\nPaper Cut<\/h1>\n<\/div>\n
It\u2019s just a paper cut, but the break in your skin could provide an easy way for pathogens<\/span><\/a><\/span>\u00a0to enter your body. If\u00a0bacteria<\/a>\u00a0were to enter through the cut and infect the wound, your innate immune system would quickly respond with a dizzying array of general defenses.<\/span><\/p>\n\nWhat Is the Innate Immune System?<\/h1>\n<\/div>\n
The\u00a0innate immune system<\/a><\/strong>\u00a0is a subset of the human immune system that produces rapid, but non-specific responses to pathogens. Innate responses are generic, rather than tailored to a particular\u00a0pathogen. The innate system responds in the same general way to every pathogen\u00a0it encounters. Although the innate immune system provides immediate and rapid defenses against pathogens, it does not confer long-lasting\u00a0immunity\u00a0to them. In most organisms, the innate immune system is the dominant system of host defense. Other than most vertebrates (including humans), the innate immune system is the only system of host defense.<\/p>\nIn humans, the innate immune system includes surface barriers, inflammation, the complement system, and a variety of cellular responses. Surface barriers of various types generally keep most pathogens out of the body. If these barriers fail, then other innate defenses are triggered. The triggering event is usually the identification of pathogens by pattern-recognition receptors on\u00a0cells\u00a0of the innate immune system. These receptors recognize molecules that are broadly shared by pathogens, but distinguishable from host molecules. Alternatively, the other innate defenses may be triggered when damaged, injured, or stressed cells send out alarm signals, many of which are recognized by the same receptors as those that recognize pathogens.<\/p>\n
\nBarriers to Pathogens<\/h1>\n<\/div>\n
The body\u2019s first line of defense consists of three different types of barriers that keep most pathogens out of\u00a0body tissues. The types of barriers are mechanical, chemical, and biological barriers.<\/p>\n
Mechanical Barriers<\/h2>\nFigure 17.4.2 Nasal hairs are a mechanical barrier to larger particles in the air.<\/em><\/figcaption><\/figure>\nMechanical barriers<\/a> are the first line of defense against pathogens, and they physically block pathogens from entering the body. The skin<\/a> is the most important mechanical barrier. In fact, it is the single most important defense the body has. The outer layer of skin \u2014 the\u00a0epidermis<\/a>\u00a0\u2014 is tough, and very difficult for pathogens to penetrate. It consists of dead\u00a0cells\u00a0that are constantly shed from the body surface, a process that helps remove\u00a0bacteria\u00a0and other infectious agents that have adhered to the skin. The epidermis also lacks\u00a0blood vessels\u00a0and is usually lacking moisture, so it does not provide a suitable environment for most pathogens.\u00a0Hair<\/a> \u2014 which is an accessory organ of the skin \u2014 also helps keep out pathogens. Hairs inside the nose may trap larger pathogens and other particles in the air before they can enter the airways of the respiratory system (see Figure 17.4.2).<\/p>\nFigure 17.4.3 A sneeze can expel many pathogens from the respiratory tract, which is why you should always cover your mouth and nose and when you sneeze.<\/em><\/figcaption><\/figure>\nMucous membranes<\/span><\/a><\/span> provide a mechanical barrier to pathogens and other particles at body openings. These membranes also line the respiratory, gastrointestinal, urinary, and reproductive tracts. Mucous membranes secrete mucus, which is a slimy and somewhat sticky substance that traps pathogens. Many mucous membranes also have hair-like cilia<\/a> that sweep mucus and trapped pathogens toward body openings, where they can be removed from the body. When you sneeze or cough, mucus and pathogens are mechanically ejected from the nose and throat, as you can see in Figure 17.4.3.\u00a0 A sneeze can travel as fast as 160 Km\/hr (about 99 mi\/hour) and expel as many as 100,000 droplets into the air around you (a good reason to cover your sneezes!). Other mechanical defenses include tears, which wash pathogens from the eyes, and urine, which flushes pathogens out of the urinary tract.<\/span><\/p>\nChemical Barriers<\/h2>\n
Chemical barriers also protect against infection by pathogens. They destroy pathogens on the outer body surface, at body openings, and on inner body linings. Sweat, mucus, tears, saliva, and breastmilk all contain antimicrobial substances (such as the\u00a0enzyme\u00a0lysozyme) that kill pathogens, especially\u00a0bacteria. Sebaceous glands in the\u00a0dermis\u00a0of the skin secrete acids that form a very fine, slightly acidic film on the surface of the skin. This film acts as a barrier to bacteria, viruses, and other potential contaminants that might penetrate the skin. Urine and vaginal secretions are also too acidic for many pathogens to endure. Semen contains zinc \u2014 which most pathogens cannot tolerate \u2014 as well as defensins, which are antimicrobial\u00a0proteins\u00a0that act mainly by disrupting bacterial cell membranes. In the stomach, stomach\u00a0acid\u00a0and digestive\u00a0enzymes\u00a0called proteases (which break down proteins) kill most of the pathogens that enter the gastrointestinal tract in food or\u00a0water.<\/p>\n
Biological Barriers<\/h2>\n
Biological barriers are living organisms that help protect the body from pathogens. Trillions of harmless bacteria normally live on the human skin and in the urinary, reproductive, and gastrointestinal tracts. These bacteria use up food and surface space that help prevent pathogenic bacteria from colonizing the body. Some of these harmless bacteria also secrete substances that change the conditions of their environment, making it less hospitable to potentially harmful bacteria.\u00a0They may release toxins or change the\u00a0pH, for example. All of these effects of harmless bacteria reduce the chances that pathogenic microorganisms will be able to reach sufficient numbers and cause illness.<\/p>\n
\nInflammation<\/h1>\n<\/div>\n
If pathogens manage to breach the barriers protecting the body, one of the first active responses of the innate immune system kicks in. This response is\u00a0inflammation<\/a>.<\/strong>\u00a0The main function of inflammation is to establish a physical barrier against the spread of infection. It also eliminates the initial cause of cell injury, clears out dead\u00a0cells\u00a0and tissues damaged from the original insult and the inflammatory process, and initiates tissue repair. Inflammation is often a response to infection by pathogens, but there are other possible causes, including burns, frostbite, and exposure to toxins.<\/p>\nThe signs and symptoms of inflammation include redness, swelling, warmth, pain, and frequently some loss of function. These symptoms are caused by increased blood flow into infected tissue, and a number of other processes, illustrated in Figure 17.4.4.<\/p>\nFigure 17.4.4 This drawing shows what happens during the inflammatory response.<\/em><\/figcaption><\/figure>\nInflammation is triggered by chemicals such as\u00a0cytokines<\/a><\/strong>\u00a0and\u00a0histamines<\/a>,<\/strong>which are released by injured or infected cells, or by immune system cells such as macrophages (described below) that are already present in tissues. These chemicals cause capillaries to dilate and become leaky, increasing blood flow to the infected area and allowing blood to enter the tissues. Pathogen-destroying leukocytes and tissue-repairing proteins migrate into tissue spaces from the bloodstream to attack pathogens and repair their damage. Cytokines also promote chemotaxis<\/a>, which is migration to the site of infection by pathogen-destroying leukocytes. Some cytokines have anti-viral effects. They may shut down protein synthesis in host cells, which viruses need in order to survive and replicate.<\/p>\nSee the video “The inflammatory response” by Neural Academy to learn about inflammatory response in more detail:<\/p>\n
Figure 17.4.6 Phagocytosis is a multi-step process in which a pathogen is engulfed and digested by immune cells called phagocytes.<\/em>[\/caption]\r\n\r\nTypes of leukocytes that kill pathogens by phagocytosis include neutrophils, macrophages, and dendritic cells. You can see illustrations of these and other leukocytes involved in innate immune responses in Figure 17.4.7.\r\n\r\n[caption id=\"attachment_4872\" align=\"alignright\" width=\"292\"] Figure 17.4.7 Types of leukocytes evolved in innate immune responses are illustrated here.<\/em>[\/caption]\r\nNeutrophils<\/h2>\r\n[pb_glossary id=\"5545\"]Neutrophils[\/pb_glossary]<\/strong>\u00a0are leukocytes that travel throughout the body in the blood. They are usually the first immune cells to arrive at the site of an infection. They are the most numerous types of phagocytes, and they normally make up at least half of the total circulating leukocytes. The [pb_glossary id=\"4021\"]bone marrow[\/pb_glossary] of a normal healthy adult produces more than 100 billion neutrophils per day. During acute inflammation, more than\u00a0ten times<\/em>\u00a0that many neutrophils may be produced each day. Many neutrophils are needed to fight infections, because after a neutrophil phagocytizes just a few pathogens, it generally dies.\r\nMacrophages<\/h2>\r\n[pb_glossary id=\"4340\"]Macrophage<\/strong>s<\/strong>[\/pb_glossary]<\/strong>\u00a0are large phagocytic leukocytes that develop from monocytes. Macrophages spend much of their time within the [pb_glossary id=\"4874\"]interstitial fluid[\/pb_glossary] in\u00a0body tissues. They are the most efficient phagocytes, and they can phagocytize substantial numbers of pathogens or other cells. Macrophages are also versatile cells that produce a wide array of chemicals\u00a0\u2014\u00a0including\u00a0enzymes, complement proteins, and [pb_glossary id=\"4865\"]cytokine<\/span>s<\/span>[\/pb_glossary]<\/span>\u00a0\u2014 in addition to their phagocytic action. As phagocytes, macrophages act as scavengers that rid tissues of worn-out cells and other debris, as well as pathogens. In addition, macrophages act as antigen-presenting cells that activate the\u00a0[pb_glossary id=\"4822\"]adaptive immune system[\/pb_glossary].<\/span>\r\nDendritic Cells<\/h2>\r\nLike macrophages,\u00a0[pb_glossary id=\"4875\"]dendritic cells[\/pb_glossary]<\/strong>\u00a0develop from monocytes. They reside in tissues that have contact with the external environment, so they are located mainly in the skin, nose, lungs, stomach, and intestines. Besides engulfing and digesting pathogens, dendritic cells also act as antigen-presenting cells that trigger adaptive immune responses.\r\nEosinophils<\/h2>\r\n[pb_glossary id=\"5529\"]Eosinophils[\/pb_glossary]<\/strong> are non-phagocytic leukocytes that are related to [pb_glossary id=\"5545\"]neutrophil[\/pb_glossary]. They specialize in defending against [pb_glossary id=\"4812\"]parasites[\/pb_glossary]. They are very effective in killing large parasites (such as worms) by secreting a range of highly-toxic substances when activated. Eosinophils may become overactive and cause [pb_glossary id=\"4877\"]allergies[\/pb_glossary] or [pb_glossary id=\"4348\"]asthma[\/pb_glossary].\r\nBasophils<\/h2>\r\n[pb_glossary id=\"5573\"]Basophils[\/pb_glossary]<\/strong>\u00a0are non-phagocytic leukocytes that are also related to neutrophils. They are the least numerous of all white blood cells. Basophils secrete two types of chemicals that aid in body defenses: [pb_glossary id=\"4879\"]histamines<\/span><\/strong>[\/pb_glossary]<\/span> and heparin. <\/span>Histamines\u00a0are responsible for dilating\u00a0blood vessels\u00a0and increasing their permeability in inflammation.\u00a0<\/span>[pb_glossary id=\"4880\"]Heparin[\/pb_glossary]<\/strong>\u00a0inhibits blood clotting, and also promotes the movement of leukocytes into an area of infection.<\/span>\r\nMast Cells<\/h2>\r\n[pb_glossary id=\"4881\"]Mast cells[\/pb_glossary]<\/strong>\u00a0are non-phagocytic leukocytes that help initiate [pb_glossary id=\"4860\"]inflammation[\/pb_glossary] by secreting [pb_glossary id=\"4879\"]histamines<\/span>[\/pb_glossary]<\/span>. In some people, histamines trigger [pb_glossary id=\"4877\"]allergic reactions[\/pb_glossary], as well as inflammation. Mast cells may also secrete chemicals that help defend against parasites.<\/span>\r\nNatural Killer Cells<\/h2>\r\n[pb_glossary id=\"5515\"]Natural killer cells[\/pb_glossary]<\/strong>\u00a0are in the subset of leukocytes called [pb_glossary id=\"4832\"]lymphocytes<\/span>[\/pb_glossary]<\/span>, which are produced by the\u00a0lymphatic system. Natural killer cells destroy [pb_glossary id=\"5605\"]cancer<\/span>ous<\/span>[\/pb_glossary]<\/span>\u00a0or [pb_glossary id=\"4283\"]virus[\/pb_glossary]-infected host cells, although they do not directly attack invading pathogens. Natural killer cells recognize these host cells by a condition they exhibit called \u201cmissing self.\u201d Cells with missing self have abnormally low levels of cell-surface proteins of the [pb_glossary id=\"4825\"]major histocompatibility complex (MHC)[\/pb_glossary], which normally identify\u00a0body cells\u00a0as self.<\/span>\r\n\r\nInnate Immune Evasion<\/h1>\r\n<\/div>\r\nMany pathogens have evolved mechanisms that allow them to evade\u00a0human hosts'\u00a0innate immune systems. Some of these mechanisms include:\r\n\r\n \t- Invading host cells to replicate so they are \u201chidden\u201d from the immune system.<\/strong>\u00a0The bacterium that causes tuberculosis<\/a> uses this mechanism.<\/li>\r\n \t
- Forming a protective capsule around themselves to avoid being destroyed by immune system cells.<\/strong>\u00a0This defense occurs in bacteria, such as\u00a0Salmonella<\/em><\/a>\u00a0species.<\/li>\r\n \t
- Mimicking host cells so the immune system does not recognize them as foreign.<\/strong>\u00a0Some\u00a0species\u00a0of\u00a0Staphylococcus<\/em><\/a>\u00a0bacteria use this mechanism.<\/li>\r\n \t
- Directly killing phagocytes.<\/strong>\u00a0This ability evolved in several\u00a0species\u00a0of bacteria, including the species that causes anthrax<\/a>.<\/li>\r\n \t
- Producing molecules that prevent the formation of interferons, which are immune chemicals that fight viruses.<\/strong>\u00a0Some influenza<\/a> viruses have this capability.<\/li>\r\n \t
- Forming complex biofilms that provide protection from the cells and proteins of the immune system.<\/strong> This characterizes some species of [pb_glossary id=\"2588\"]bacteria[\/pb_glossary] and fungi. You can see an example of a bacterial biofilm on teeth in Figure 17.4.8.<\/li>\r\n<\/ul>\r\n[caption id=\"attachment_4883\" align=\"aligncenter\" width=\"400\"] Figure 17.4.8 The dental plaque on the top set of teeth is a biofilm that sticks to the teeth and consists of many species of bacteria. The plaque biofilm is difficult to remove, and it subjects the teeth and gums to high concentrations of bacterial metabolites, which result in dental disease. The same teeth after plaque removal are shown in the bottom picture.<\/em>[\/caption]\r\n\r\n\r\n\r\n \r\n\r\n<\/div>\r\n\r\n
\r\n17.4 Summary<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- The [pb_glossary id=\"4820\"]innate immune system[\/pb_glossary] is a subset of the human immune system that produces rapid, but non-specific responses to pathogens. Unlike the\u00a0[pb_glossary id=\"4822\"]adaptive immune system[\/pb_glossary], the innate system does not confer\u00a0[pb_glossary id=\"4884\"]immunity[\/pb_glossary]. The innate immune system includes surface barriers, [pb_glossary id=\"4860\"]inflammation[\/pb_glossary], the [pb_glossary id=\"4868\"]complement system[\/pb_glossary], and a variety of cellular responses.<\/li>\r\n \t
- The body\u2019s first line of defense consists of three different types of barriers that keep most pathogens out of\u00a0body tissues. The types of barriers are mechanical, chemical, and biological barriers.<\/li>\r\n \t
- Mechanical barriers\u00a0\u2014\u00a0which include the [pb_glossary id=\"3521\"]skin[\/pb_glossary], [pb_glossary id=\"3538\"]mucous membranes[\/pb_glossary], and fluids such as tears and [pb_glossary id=\"4717\"]urine[\/pb_glossary] \u2014 physically block pathogens from entering the body. Chemical barriers\u00a0\u2014\u00a0such as enzymes in [pb_glossary id=\"4711\"]sweat[\/pb_glossary], [pb_glossary id=\"4595\"]saliva[\/pb_glossary], and [pb_glossary id=\"4885\"]semen[\/pb_glossary]\u00a0\u2014\u00a0kill pathogens on body surfaces. Biological barriers are harmless bacteria that use up food and space so pathogenic bacteria cannot colonize the body.<\/li>\r\n \t
- If pathogens breach protective barriers, [pb_glossary id=\"4860\"]inflammation[\/pb_glossary] occurs. This creates a physical barrier against the spread of infection, and repairs tissue damage. Inflammation is triggered by chemicals such as [pb_glossary id=\"4865\"]cytokines[\/pb_glossary]\u00a0and [pb_glossary id=\"4879\"]histamines[\/pb_glossary], and it causes swelling, redness, and warmth.<\/li>\r\n \t
- The [pb_glossary id=\"4868\"]complement system[\/pb_glossary] is a complex biochemical mechanism that helps [pb_glossary id=\"5659\"]antibodies[\/pb_glossary] kill pathogens. Once activated, the complement system consists of more than two dozen proteins that lead to disruption of the\u00a0cell membrane\u00a0of pathogens and bursting of the cells.<\/li>\r\n \t
- Cellular responses of the innate immune system involve various types of [pb_glossary id=\"5623\"]leukocytes[\/pb_glossary]. For example, [pb_glossary id=\"5545\"]neutrophils[\/pb_glossary], [pb_glossary id=\"4340\"]macrophages[\/pb_glossary], and [pb_glossary id=\"4875\"]dendritic cells[\/pb_glossary]\u00a0phagocytize pathogens. [pb_glossary id=\"5573\"]Basophils[\/pb_glossary]\u00a0and [pb_glossary id=\"4881\"]mast cells[\/pb_glossary]\u00a0release chemicals that trigger inflammation. [pb_glossary id=\"5515\"]Natural killer cells[\/pb_glossary]\u00a0destroy cancerous or virus-infected cells, and eosinophils kill parasites.<\/li>\r\n \t
- Many pathogens have evolved mechanisms that help them evade the innate immune system. For example, some pathogens form a protective capsule around themselves, and some mimic host cells so the immune system does not recognize them as foreign.<\/li>\r\n<\/ul>\r\n<\/div>\r\n<\/div>\r\n
\r\n17.4 Review Questions<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- What is the innate immune system?<\/li>\r\n \t
- Identify the body\u2019s first line of defense.<\/li>\r\n \t
- [h5p id=\"638\"]<\/li>\r\n \t
- What are biological barriers? How do they protect the body?<\/li>\r\n \t
- State the purposes of inflammation. What triggers inflammation, and what signs and symptoms does it cause?<\/li>\r\n \t
- Define the complement system. How does it help destroy pathogens?<\/li>\r\n \t
- Describe two ways that pathogens can evade the innate immune system.<\/li>\r\n \t
- What are the ways in which phagocytes can encounter pathogens in the body?<\/li>\r\n \t
- Describe two different ways in which enzymes play a role in the innate immune response.<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n
\r\n17.4 Explore More<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\nhttps:\/\/youtu.be\/WW4skW6gucU\r\nHow mucus keeps us healthy - Katharina Ribbeck, TED-Ed, 2015.<\/p>\r\nhttps:\/\/youtu.be\/sYjtMP67vyk\r\n
Human Physiology - Innate Immune System, Janux, 2015.<\/p>\r\nhttps:\/\/youtu.be\/c64M1tZyWPM\r\n
Myriam Sidibe: The simple power of handwashing, TED, 2014.<\/p>\r\nhttps:\/\/youtu.be\/shEPwQPQG4I\r\n
Everything You Didn't Want To Know About Snot, Gross Science, 2017.<\/p>\r\nhttps:\/\/youtu.be\/dy1D3d1FBcw\r\n
Cough Grosser Than Sneeze? | Curiosity - World's Dirtiest Man, Discovery, 2011.<\/p>\r\n\r\n<\/div>\r\n<\/div>\r\n \r\n\r\n<\/div>\r\n
Attributions<\/h2>\r\nFigure 17.4.1<\/strong>\r\n\r\nOww_Papercut_14365<\/a> by Laurence Facun<\/a> on Wikimedia Commons is used under a CC BY 2.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/2.0) license.\r\n\r\nFigure 17.4.2<\/strong>\r\n\r\nhairy-nose<\/a> by Piotr Siedlecki\u00a0 on publicdomainpictures.net<\/a> is used under a CC0 1.0<\/a> Universal Public Domain Dedication (http:\/\/creativecommons.org\/publicdomain\/zero\/1.0\/) license.\r\n\r\nFigure 17.4.3<\/strong>\r\n\r\n1024px-Sneeze<\/a> by James Gathany\/ CDC Public Health Image library (PHIL) ID# 11162<\/a> on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/public_domain).\r\n\r\nFigure 17.4.4<\/strong>\r\n\r\nOSC_Microbio_17_06_Erythema<\/a> by CNX OpenStax<\/a> on Wikimedia Commons is used under a CC BY 4.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/4.0) license.\r\n\r\nFigure 17.4.5<\/strong>\r\n\r\n2212_Complement_Cascade_and_Function<\/a>\u00a0by OpenStax College<\/a> on Wikimedia Commons is used under a\u00a0CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\r\n\r\nFigure 17.4.6<\/strong>\r\n\r\n512px-Phagocytosis2<\/a> by Graham\u00a0Colm<\/span><\/a> at English Wikipedia<\/a><\/span>\u00a0on Wikimedia Commons is used under a CC BY-SA 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-sa\/3.0) license.\r\n\r\nFigure 17.4.7<\/strong>\r\n\r\nInnate_Immune_cells.svg<\/a> by Fred the Oyster<\/a> on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).\r\n\r\nFigure 17.4.8<\/strong>\r\n\r\n1024px-Gingivitis-before-and-after-3<\/a> by Onetimeuseaccount<\/a> on Wikimedia Commons is used under a CC0 1.0<\/a> Universal Public Domain Dedication (http:\/\/creativecommons.org\/publicdomain\/zero\/1.0\/) license.\r\nReferences<\/h2>\r\n
Betts, J. G., Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E., Womble, M., DeSaix, P. (2013, June 19). Figure 21.13 Complement cascade and function [digital image].\u00a0 In Anatomy and Physiology<\/em> (Section 21.2). OpenStax. https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/21-2-barrier-defenses-and-the-innate-immune-response<\/p>\r\nDiscovery. (2011, October 27). Cough grosser than sneeze? | Curiosity - World's dirtiest man. YouTube. https:\/\/www.youtube.com\/watch?v=dy1D3d1FBcw&feature=youtu.be<\/p>\r\n
Gross Science. (2017, January 31). Everything you didn't want to know about snot. YouTube. https:\/\/www.youtube.com\/watch?v=shEPwQPQG4I&feature=youtu.be<\/p>\r\n
Janux. (2015, January 10). Human physiology - Innate immune system. YouTube. https:\/\/www.youtube.com\/watch?v=sYjtMP67vyk&feature=youtu.be<\/p>\r\n
Mayo Clinic Staff. (n.d.). Anthrax [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/anthrax\/symptoms-causes\/syc-20356203<\/p>\r\n
Mayo Clinic Staff. (n.d.). Influenza (flu) [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/flu\/symptoms-causes\/syc-20351719<\/p>\r\n
Mayo Clinic Staff. (n.d.). Salmonella infection [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/salmonella\/symptoms-causes\/syc-20355329<\/p>\r\n
Mayo Clinic Staff. (n.d.). Staph infection [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/staph-infections\/multimedia\/staph-infection\/img-20008600<\/p>\r\n
Mayo Clinic Staff. (n.d.). Tuberculosis [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/tuberculosis\/symptoms-causes\/syc-20351250<\/p>\r\n
OpenStax. (2016, November 11). Figure\u00a0<\/span>17.23<\/span> A typical case of acute inflammation at the site of a skin wound - Erythema [digital image]. In OpenStax, Microbiology<\/em> (Section 17.5). https:\/\/openstax.org\/details\/books\/microbiology?Bookdetails<\/p>\r\nTED. (2014, October 14). Myriam Sidibe: The simple power of handwashing. YouTube. https:\/\/www.youtube.com\/watch?v=c64M1tZyWPM&feature=youtu.be<\/p>\r\n
TED-Ed. (2015, November 5). How mucus keeps us healthy - Katharina Ribbeck. YouTube. https:\/\/www.youtube.com\/watch?v=WW4skW6gucU&feature=youtu.be<\/p>","rendered":"
<\/p>\nFigure 17.4.1 Darn it!\u00a0 Paper cuts are the worst!<\/em><\/figcaption><\/figure>\n\nPaper Cut<\/h1>\n<\/div>\n
It\u2019s just a paper cut, but the break in your skin could provide an easy way for pathogens<\/span><\/a><\/span>\u00a0to enter your body. If\u00a0bacteria<\/a>\u00a0were to enter through the cut and infect the wound, your innate immune system would quickly respond with a dizzying array of general defenses.<\/span><\/p>\n\nWhat Is the Innate Immune System?<\/h1>\n<\/div>\n
The\u00a0innate immune system<\/a><\/strong>\u00a0is a subset of the human immune system that produces rapid, but non-specific responses to pathogens. Innate responses are generic, rather than tailored to a particular\u00a0pathogen. The innate system responds in the same general way to every pathogen\u00a0it encounters. Although the innate immune system provides immediate and rapid defenses against pathogens, it does not confer long-lasting\u00a0immunity\u00a0to them. In most organisms, the innate immune system is the dominant system of host defense. Other than most vertebrates (including humans), the innate immune system is the only system of host defense.<\/p>\nIn humans, the innate immune system includes surface barriers, inflammation, the complement system, and a variety of cellular responses. Surface barriers of various types generally keep most pathogens out of the body. If these barriers fail, then other innate defenses are triggered. The triggering event is usually the identification of pathogens by pattern-recognition receptors on\u00a0cells\u00a0of the innate immune system. These receptors recognize molecules that are broadly shared by pathogens, but distinguishable from host molecules. Alternatively, the other innate defenses may be triggered when damaged, injured, or stressed cells send out alarm signals, many of which are recognized by the same receptors as those that recognize pathogens.<\/p>\n
\nBarriers to Pathogens<\/h1>\n<\/div>\n
The body\u2019s first line of defense consists of three different types of barriers that keep most pathogens out of\u00a0body tissues. The types of barriers are mechanical, chemical, and biological barriers.<\/p>\n
Mechanical Barriers<\/h2>\nFigure 17.4.2 Nasal hairs are a mechanical barrier to larger particles in the air.<\/em><\/figcaption><\/figure>\nMechanical barriers<\/a> are the first line of defense against pathogens, and they physically block pathogens from entering the body. The skin<\/a> is the most important mechanical barrier. In fact, it is the single most important defense the body has. The outer layer of skin \u2014 the\u00a0epidermis<\/a>\u00a0\u2014 is tough, and very difficult for pathogens to penetrate. It consists of dead\u00a0cells\u00a0that are constantly shed from the body surface, a process that helps remove\u00a0bacteria\u00a0and other infectious agents that have adhered to the skin. The epidermis also lacks\u00a0blood vessels\u00a0and is usually lacking moisture, so it does not provide a suitable environment for most pathogens.\u00a0Hair<\/a> \u2014 which is an accessory organ of the skin \u2014 also helps keep out pathogens. Hairs inside the nose may trap larger pathogens and other particles in the air before they can enter the airways of the respiratory system (see Figure 17.4.2).<\/p>\nFigure 17.4.3 A sneeze can expel many pathogens from the respiratory tract, which is why you should always cover your mouth and nose and when you sneeze.<\/em><\/figcaption><\/figure>\nMucous membranes<\/span><\/a><\/span> provide a mechanical barrier to pathogens and other particles at body openings. These membranes also line the respiratory, gastrointestinal, urinary, and reproductive tracts. Mucous membranes secrete mucus, which is a slimy and somewhat sticky substance that traps pathogens. Many mucous membranes also have hair-like cilia<\/a> that sweep mucus and trapped pathogens toward body openings, where they can be removed from the body. When you sneeze or cough, mucus and pathogens are mechanically ejected from the nose and throat, as you can see in Figure 17.4.3.\u00a0 A sneeze can travel as fast as 160 Km\/hr (about 99 mi\/hour) and expel as many as 100,000 droplets into the air around you (a good reason to cover your sneezes!). Other mechanical defenses include tears, which wash pathogens from the eyes, and urine, which flushes pathogens out of the urinary tract.<\/span><\/p>\nChemical Barriers<\/h2>\n
Chemical barriers also protect against infection by pathogens. They destroy pathogens on the outer body surface, at body openings, and on inner body linings. Sweat, mucus, tears, saliva, and breastmilk all contain antimicrobial substances (such as the\u00a0enzyme\u00a0lysozyme) that kill pathogens, especially\u00a0bacteria. Sebaceous glands in the\u00a0dermis\u00a0of the skin secrete acids that form a very fine, slightly acidic film on the surface of the skin. This film acts as a barrier to bacteria, viruses, and other potential contaminants that might penetrate the skin. Urine and vaginal secretions are also too acidic for many pathogens to endure. Semen contains zinc \u2014 which most pathogens cannot tolerate \u2014 as well as defensins, which are antimicrobial\u00a0proteins\u00a0that act mainly by disrupting bacterial cell membranes. In the stomach, stomach\u00a0acid\u00a0and digestive\u00a0enzymes\u00a0called proteases (which break down proteins) kill most of the pathogens that enter the gastrointestinal tract in food or\u00a0water.<\/p>\n
Biological Barriers<\/h2>\n
Biological barriers are living organisms that help protect the body from pathogens. Trillions of harmless bacteria normally live on the human skin and in the urinary, reproductive, and gastrointestinal tracts. These bacteria use up food and surface space that help prevent pathogenic bacteria from colonizing the body. Some of these harmless bacteria also secrete substances that change the conditions of their environment, making it less hospitable to potentially harmful bacteria.\u00a0They may release toxins or change the\u00a0pH, for example. All of these effects of harmless bacteria reduce the chances that pathogenic microorganisms will be able to reach sufficient numbers and cause illness.<\/p>\n
\nInflammation<\/h1>\n<\/div>\n
If pathogens manage to breach the barriers protecting the body, one of the first active responses of the innate immune system kicks in. This response is\u00a0inflammation<\/a>.<\/strong>\u00a0The main function of inflammation is to establish a physical barrier against the spread of infection. It also eliminates the initial cause of cell injury, clears out dead\u00a0cells\u00a0and tissues damaged from the original insult and the inflammatory process, and initiates tissue repair. Inflammation is often a response to infection by pathogens, but there are other possible causes, including burns, frostbite, and exposure to toxins.<\/p>\nThe signs and symptoms of inflammation include redness, swelling, warmth, pain, and frequently some loss of function. These symptoms are caused by increased blood flow into infected tissue, and a number of other processes, illustrated in Figure 17.4.4.<\/p>\nFigure 17.4.4 This drawing shows what happens during the inflammatory response.<\/em><\/figcaption><\/figure>\nInflammation is triggered by chemicals such as\u00a0cytokines<\/a><\/strong>\u00a0and\u00a0histamines<\/a>,<\/strong>which are released by injured or infected cells, or by immune system cells such as macrophages (described below) that are already present in tissues. These chemicals cause capillaries to dilate and become leaky, increasing blood flow to the infected area and allowing blood to enter the tissues. Pathogen-destroying leukocytes and tissue-repairing proteins migrate into tissue spaces from the bloodstream to attack pathogens and repair their damage. Cytokines also promote chemotaxis<\/a>, which is migration to the site of infection by pathogen-destroying leukocytes. Some cytokines have anti-viral effects. They may shut down protein synthesis in host cells, which viruses need in order to survive and replicate.<\/p>\nSee the video “The inflammatory response” by Neural Academy to learn about inflammatory response in more detail:<\/p>\n
Dendritic Cells<\/h2>\r\nLike macrophages,\u00a0[pb_glossary id=\"4875\"]dendritic cells[\/pb_glossary]<\/strong>\u00a0develop from monocytes. They reside in tissues that have contact with the external environment, so they are located mainly in the skin, nose, lungs, stomach, and intestines. Besides engulfing and digesting pathogens, dendritic cells also act as antigen-presenting cells that trigger adaptive immune responses.\r\nEosinophils<\/h2>\r\n[pb_glossary id=\"5529\"]Eosinophils[\/pb_glossary]<\/strong> are non-phagocytic leukocytes that are related to [pb_glossary id=\"5545\"]neutrophil[\/pb_glossary]. They specialize in defending against [pb_glossary id=\"4812\"]parasites[\/pb_glossary]. They are very effective in killing large parasites (such as worms) by secreting a range of highly-toxic substances when activated. Eosinophils may become overactive and cause [pb_glossary id=\"4877\"]allergies[\/pb_glossary] or [pb_glossary id=\"4348\"]asthma[\/pb_glossary].\r\nBasophils<\/h2>\r\n[pb_glossary id=\"5573\"]Basophils[\/pb_glossary]<\/strong>\u00a0are non-phagocytic leukocytes that are also related to neutrophils. They are the least numerous of all white blood cells. Basophils secrete two types of chemicals that aid in body defenses: [pb_glossary id=\"4879\"]histamines<\/span><\/strong>[\/pb_glossary]<\/span> and heparin. <\/span>Histamines\u00a0are responsible for dilating\u00a0blood vessels\u00a0and increasing their permeability in inflammation.\u00a0<\/span>[pb_glossary id=\"4880\"]Heparin[\/pb_glossary]<\/strong>\u00a0inhibits blood clotting, and also promotes the movement of leukocytes into an area of infection.<\/span>\r\nMast Cells<\/h2>\r\n[pb_glossary id=\"4881\"]Mast cells[\/pb_glossary]<\/strong>\u00a0are non-phagocytic leukocytes that help initiate [pb_glossary id=\"4860\"]inflammation[\/pb_glossary] by secreting [pb_glossary id=\"4879\"]histamines<\/span>[\/pb_glossary]<\/span>. In some people, histamines trigger [pb_glossary id=\"4877\"]allergic reactions[\/pb_glossary], as well as inflammation. Mast cells may also secrete chemicals that help defend against parasites.<\/span>\r\nNatural Killer Cells<\/h2>\r\n[pb_glossary id=\"5515\"]Natural killer cells[\/pb_glossary]<\/strong>\u00a0are in the subset of leukocytes called [pb_glossary id=\"4832\"]lymphocytes<\/span>[\/pb_glossary]<\/span>, which are produced by the\u00a0lymphatic system. Natural killer cells destroy [pb_glossary id=\"5605\"]cancer<\/span>ous<\/span>[\/pb_glossary]<\/span>\u00a0or [pb_glossary id=\"4283\"]virus[\/pb_glossary]-infected host cells, although they do not directly attack invading pathogens. Natural killer cells recognize these host cells by a condition they exhibit called \u201cmissing self.\u201d Cells with missing self have abnormally low levels of cell-surface proteins of the [pb_glossary id=\"4825\"]major histocompatibility complex (MHC)[\/pb_glossary], which normally identify\u00a0body cells\u00a0as self.<\/span>\r\n\r\nInnate Immune Evasion<\/h1>\r\n<\/div>\r\nMany pathogens have evolved mechanisms that allow them to evade\u00a0human hosts'\u00a0innate immune systems. Some of these mechanisms include:\r\n\r\n \t- Invading host cells to replicate so they are \u201chidden\u201d from the immune system.<\/strong>\u00a0The bacterium that causes tuberculosis<\/a> uses this mechanism.<\/li>\r\n \t
- Forming a protective capsule around themselves to avoid being destroyed by immune system cells.<\/strong>\u00a0This defense occurs in bacteria, such as\u00a0Salmonella<\/em><\/a>\u00a0species.<\/li>\r\n \t
- Mimicking host cells so the immune system does not recognize them as foreign.<\/strong>\u00a0Some\u00a0species\u00a0of\u00a0Staphylococcus<\/em><\/a>\u00a0bacteria use this mechanism.<\/li>\r\n \t
- Directly killing phagocytes.<\/strong>\u00a0This ability evolved in several\u00a0species\u00a0of bacteria, including the species that causes anthrax<\/a>.<\/li>\r\n \t
- Producing molecules that prevent the formation of interferons, which are immune chemicals that fight viruses.<\/strong>\u00a0Some influenza<\/a> viruses have this capability.<\/li>\r\n \t
- Forming complex biofilms that provide protection from the cells and proteins of the immune system.<\/strong> This characterizes some species of [pb_glossary id=\"2588\"]bacteria[\/pb_glossary] and fungi. You can see an example of a bacterial biofilm on teeth in Figure 17.4.8.<\/li>\r\n<\/ul>\r\n[caption id=\"attachment_4883\" align=\"aligncenter\" width=\"400\"] Figure 17.4.8 The dental plaque on the top set of teeth is a biofilm that sticks to the teeth and consists of many species of bacteria. The plaque biofilm is difficult to remove, and it subjects the teeth and gums to high concentrations of bacterial metabolites, which result in dental disease. The same teeth after plaque removal are shown in the bottom picture.<\/em>[\/caption]\r\n\r\n\r\n\r\n \r\n\r\n<\/div>\r\n\r\n
\r\n17.4 Summary<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- The [pb_glossary id=\"4820\"]innate immune system[\/pb_glossary] is a subset of the human immune system that produces rapid, but non-specific responses to pathogens. Unlike the\u00a0[pb_glossary id=\"4822\"]adaptive immune system[\/pb_glossary], the innate system does not confer\u00a0[pb_glossary id=\"4884\"]immunity[\/pb_glossary]. The innate immune system includes surface barriers, [pb_glossary id=\"4860\"]inflammation[\/pb_glossary], the [pb_glossary id=\"4868\"]complement system[\/pb_glossary], and a variety of cellular responses.<\/li>\r\n \t
- The body\u2019s first line of defense consists of three different types of barriers that keep most pathogens out of\u00a0body tissues. The types of barriers are mechanical, chemical, and biological barriers.<\/li>\r\n \t
- Mechanical barriers\u00a0\u2014\u00a0which include the [pb_glossary id=\"3521\"]skin[\/pb_glossary], [pb_glossary id=\"3538\"]mucous membranes[\/pb_glossary], and fluids such as tears and [pb_glossary id=\"4717\"]urine[\/pb_glossary] \u2014 physically block pathogens from entering the body. Chemical barriers\u00a0\u2014\u00a0such as enzymes in [pb_glossary id=\"4711\"]sweat[\/pb_glossary], [pb_glossary id=\"4595\"]saliva[\/pb_glossary], and [pb_glossary id=\"4885\"]semen[\/pb_glossary]\u00a0\u2014\u00a0kill pathogens on body surfaces. Biological barriers are harmless bacteria that use up food and space so pathogenic bacteria cannot colonize the body.<\/li>\r\n \t
- If pathogens breach protective barriers, [pb_glossary id=\"4860\"]inflammation[\/pb_glossary] occurs. This creates a physical barrier against the spread of infection, and repairs tissue damage. Inflammation is triggered by chemicals such as [pb_glossary id=\"4865\"]cytokines[\/pb_glossary]\u00a0and [pb_glossary id=\"4879\"]histamines[\/pb_glossary], and it causes swelling, redness, and warmth.<\/li>\r\n \t
- The [pb_glossary id=\"4868\"]complement system[\/pb_glossary] is a complex biochemical mechanism that helps [pb_glossary id=\"5659\"]antibodies[\/pb_glossary] kill pathogens. Once activated, the complement system consists of more than two dozen proteins that lead to disruption of the\u00a0cell membrane\u00a0of pathogens and bursting of the cells.<\/li>\r\n \t
- Cellular responses of the innate immune system involve various types of [pb_glossary id=\"5623\"]leukocytes[\/pb_glossary]. For example, [pb_glossary id=\"5545\"]neutrophils[\/pb_glossary], [pb_glossary id=\"4340\"]macrophages[\/pb_glossary], and [pb_glossary id=\"4875\"]dendritic cells[\/pb_glossary]\u00a0phagocytize pathogens. [pb_glossary id=\"5573\"]Basophils[\/pb_glossary]\u00a0and [pb_glossary id=\"4881\"]mast cells[\/pb_glossary]\u00a0release chemicals that trigger inflammation. [pb_glossary id=\"5515\"]Natural killer cells[\/pb_glossary]\u00a0destroy cancerous or virus-infected cells, and eosinophils kill parasites.<\/li>\r\n \t
- Many pathogens have evolved mechanisms that help them evade the innate immune system. For example, some pathogens form a protective capsule around themselves, and some mimic host cells so the immune system does not recognize them as foreign.<\/li>\r\n<\/ul>\r\n<\/div>\r\n<\/div>\r\n
\r\n17.4 Review Questions<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- What is the innate immune system?<\/li>\r\n \t
- Identify the body\u2019s first line of defense.<\/li>\r\n \t
- [h5p id=\"638\"]<\/li>\r\n \t
- What are biological barriers? How do they protect the body?<\/li>\r\n \t
- State the purposes of inflammation. What triggers inflammation, and what signs and symptoms does it cause?<\/li>\r\n \t
- Define the complement system. How does it help destroy pathogens?<\/li>\r\n \t
- Describe two ways that pathogens can evade the innate immune system.<\/li>\r\n \t
- What are the ways in which phagocytes can encounter pathogens in the body?<\/li>\r\n \t
- Describe two different ways in which enzymes play a role in the innate immune response.<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n
\r\n17.4 Explore More<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\nhttps:\/\/youtu.be\/WW4skW6gucU\r\nHow mucus keeps us healthy - Katharina Ribbeck, TED-Ed, 2015.<\/p>\r\nhttps:\/\/youtu.be\/sYjtMP67vyk\r\n
Human Physiology - Innate Immune System, Janux, 2015.<\/p>\r\nhttps:\/\/youtu.be\/c64M1tZyWPM\r\n
Myriam Sidibe: The simple power of handwashing, TED, 2014.<\/p>\r\nhttps:\/\/youtu.be\/shEPwQPQG4I\r\n
Everything You Didn't Want To Know About Snot, Gross Science, 2017.<\/p>\r\nhttps:\/\/youtu.be\/dy1D3d1FBcw\r\n
Cough Grosser Than Sneeze? | Curiosity - World's Dirtiest Man, Discovery, 2011.<\/p>\r\n\r\n<\/div>\r\n<\/div>\r\n \r\n\r\n<\/div>\r\n
Attributions<\/h2>\r\nFigure 17.4.1<\/strong>\r\n\r\nOww_Papercut_14365<\/a> by Laurence Facun<\/a> on Wikimedia Commons is used under a CC BY 2.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/2.0) license.\r\n\r\nFigure 17.4.2<\/strong>\r\n\r\nhairy-nose<\/a> by Piotr Siedlecki\u00a0 on publicdomainpictures.net<\/a> is used under a CC0 1.0<\/a> Universal Public Domain Dedication (http:\/\/creativecommons.org\/publicdomain\/zero\/1.0\/) license.\r\n\r\nFigure 17.4.3<\/strong>\r\n\r\n1024px-Sneeze<\/a> by James Gathany\/ CDC Public Health Image library (PHIL) ID# 11162<\/a> on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/public_domain).\r\n\r\nFigure 17.4.4<\/strong>\r\n\r\nOSC_Microbio_17_06_Erythema<\/a> by CNX OpenStax<\/a> on Wikimedia Commons is used under a CC BY 4.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/4.0) license.\r\n\r\nFigure 17.4.5<\/strong>\r\n\r\n2212_Complement_Cascade_and_Function<\/a>\u00a0by OpenStax College<\/a> on Wikimedia Commons is used under a\u00a0CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\r\n\r\nFigure 17.4.6<\/strong>\r\n\r\n512px-Phagocytosis2<\/a> by Graham\u00a0Colm<\/span><\/a> at English Wikipedia<\/a><\/span>\u00a0on Wikimedia Commons is used under a CC BY-SA 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-sa\/3.0) license.\r\n\r\nFigure 17.4.7<\/strong>\r\n\r\nInnate_Immune_cells.svg<\/a> by Fred the Oyster<\/a> on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).\r\n\r\nFigure 17.4.8<\/strong>\r\n\r\n1024px-Gingivitis-before-and-after-3<\/a> by Onetimeuseaccount<\/a> on Wikimedia Commons is used under a CC0 1.0<\/a> Universal Public Domain Dedication (http:\/\/creativecommons.org\/publicdomain\/zero\/1.0\/) license.\r\nReferences<\/h2>\r\n
Betts, J. G., Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E., Womble, M., DeSaix, P. (2013, June 19). Figure 21.13 Complement cascade and function [digital image].\u00a0 In Anatomy and Physiology<\/em> (Section 21.2). OpenStax. https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/21-2-barrier-defenses-and-the-innate-immune-response<\/p>\r\nDiscovery. (2011, October 27). Cough grosser than sneeze? | Curiosity - World's dirtiest man. YouTube. https:\/\/www.youtube.com\/watch?v=dy1D3d1FBcw&feature=youtu.be<\/p>\r\n
Gross Science. (2017, January 31). Everything you didn't want to know about snot. YouTube. https:\/\/www.youtube.com\/watch?v=shEPwQPQG4I&feature=youtu.be<\/p>\r\n
Janux. (2015, January 10). Human physiology - Innate immune system. YouTube. https:\/\/www.youtube.com\/watch?v=sYjtMP67vyk&feature=youtu.be<\/p>\r\n
Mayo Clinic Staff. (n.d.). Anthrax [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/anthrax\/symptoms-causes\/syc-20356203<\/p>\r\n
Mayo Clinic Staff. (n.d.). Influenza (flu) [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/flu\/symptoms-causes\/syc-20351719<\/p>\r\n
Mayo Clinic Staff. (n.d.). Salmonella infection [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/salmonella\/symptoms-causes\/syc-20355329<\/p>\r\n
Mayo Clinic Staff. (n.d.). Staph infection [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/staph-infections\/multimedia\/staph-infection\/img-20008600<\/p>\r\n
Mayo Clinic Staff. (n.d.). Tuberculosis [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/tuberculosis\/symptoms-causes\/syc-20351250<\/p>\r\n
OpenStax. (2016, November 11). Figure\u00a0<\/span>17.23<\/span> A typical case of acute inflammation at the site of a skin wound - Erythema [digital image]. In OpenStax, Microbiology<\/em> (Section 17.5). https:\/\/openstax.org\/details\/books\/microbiology?Bookdetails<\/p>\r\nTED. (2014, October 14). Myriam Sidibe: The simple power of handwashing. YouTube. https:\/\/www.youtube.com\/watch?v=c64M1tZyWPM&feature=youtu.be<\/p>\r\n
TED-Ed. (2015, November 5). How mucus keeps us healthy - Katharina Ribbeck. YouTube. https:\/\/www.youtube.com\/watch?v=WW4skW6gucU&feature=youtu.be<\/p>","rendered":"
<\/p>\nFigure 17.4.1 Darn it!\u00a0 Paper cuts are the worst!<\/em><\/figcaption><\/figure>\n\nPaper Cut<\/h1>\n<\/div>\n
It\u2019s just a paper cut, but the break in your skin could provide an easy way for pathogens<\/span><\/a><\/span>\u00a0to enter your body. If\u00a0bacteria<\/a>\u00a0were to enter through the cut and infect the wound, your innate immune system would quickly respond with a dizzying array of general defenses.<\/span><\/p>\n\nWhat Is the Innate Immune System?<\/h1>\n<\/div>\n
The\u00a0innate immune system<\/a><\/strong>\u00a0is a subset of the human immune system that produces rapid, but non-specific responses to pathogens. Innate responses are generic, rather than tailored to a particular\u00a0pathogen. The innate system responds in the same general way to every pathogen\u00a0it encounters. Although the innate immune system provides immediate and rapid defenses against pathogens, it does not confer long-lasting\u00a0immunity\u00a0to them. In most organisms, the innate immune system is the dominant system of host defense. Other than most vertebrates (including humans), the innate immune system is the only system of host defense.<\/p>\nIn humans, the innate immune system includes surface barriers, inflammation, the complement system, and a variety of cellular responses. Surface barriers of various types generally keep most pathogens out of the body. If these barriers fail, then other innate defenses are triggered. The triggering event is usually the identification of pathogens by pattern-recognition receptors on\u00a0cells\u00a0of the innate immune system. These receptors recognize molecules that are broadly shared by pathogens, but distinguishable from host molecules. Alternatively, the other innate defenses may be triggered when damaged, injured, or stressed cells send out alarm signals, many of which are recognized by the same receptors as those that recognize pathogens.<\/p>\n
\nBarriers to Pathogens<\/h1>\n<\/div>\n
The body\u2019s first line of defense consists of three different types of barriers that keep most pathogens out of\u00a0body tissues. The types of barriers are mechanical, chemical, and biological barriers.<\/p>\n
Mechanical Barriers<\/h2>\nFigure 17.4.2 Nasal hairs are a mechanical barrier to larger particles in the air.<\/em><\/figcaption><\/figure>\nMechanical barriers<\/a> are the first line of defense against pathogens, and they physically block pathogens from entering the body. The skin<\/a> is the most important mechanical barrier. In fact, it is the single most important defense the body has. The outer layer of skin \u2014 the\u00a0epidermis<\/a>\u00a0\u2014 is tough, and very difficult for pathogens to penetrate. It consists of dead\u00a0cells\u00a0that are constantly shed from the body surface, a process that helps remove\u00a0bacteria\u00a0and other infectious agents that have adhered to the skin. The epidermis also lacks\u00a0blood vessels\u00a0and is usually lacking moisture, so it does not provide a suitable environment for most pathogens.\u00a0Hair<\/a> \u2014 which is an accessory organ of the skin \u2014 also helps keep out pathogens. Hairs inside the nose may trap larger pathogens and other particles in the air before they can enter the airways of the respiratory system (see Figure 17.4.2).<\/p>\nFigure 17.4.3 A sneeze can expel many pathogens from the respiratory tract, which is why you should always cover your mouth and nose and when you sneeze.<\/em><\/figcaption><\/figure>\nMucous membranes<\/span><\/a><\/span> provide a mechanical barrier to pathogens and other particles at body openings. These membranes also line the respiratory, gastrointestinal, urinary, and reproductive tracts. Mucous membranes secrete mucus, which is a slimy and somewhat sticky substance that traps pathogens. Many mucous membranes also have hair-like cilia<\/a> that sweep mucus and trapped pathogens toward body openings, where they can be removed from the body. When you sneeze or cough, mucus and pathogens are mechanically ejected from the nose and throat, as you can see in Figure 17.4.3.\u00a0 A sneeze can travel as fast as 160 Km\/hr (about 99 mi\/hour) and expel as many as 100,000 droplets into the air around you (a good reason to cover your sneezes!). Other mechanical defenses include tears, which wash pathogens from the eyes, and urine, which flushes pathogens out of the urinary tract.<\/span><\/p>\nChemical Barriers<\/h2>\n
Chemical barriers also protect against infection by pathogens. They destroy pathogens on the outer body surface, at body openings, and on inner body linings. Sweat, mucus, tears, saliva, and breastmilk all contain antimicrobial substances (such as the\u00a0enzyme\u00a0lysozyme) that kill pathogens, especially\u00a0bacteria. Sebaceous glands in the\u00a0dermis\u00a0of the skin secrete acids that form a very fine, slightly acidic film on the surface of the skin. This film acts as a barrier to bacteria, viruses, and other potential contaminants that might penetrate the skin. Urine and vaginal secretions are also too acidic for many pathogens to endure. Semen contains zinc \u2014 which most pathogens cannot tolerate \u2014 as well as defensins, which are antimicrobial\u00a0proteins\u00a0that act mainly by disrupting bacterial cell membranes. In the stomach, stomach\u00a0acid\u00a0and digestive\u00a0enzymes\u00a0called proteases (which break down proteins) kill most of the pathogens that enter the gastrointestinal tract in food or\u00a0water.<\/p>\n
Biological Barriers<\/h2>\n
Biological barriers are living organisms that help protect the body from pathogens. Trillions of harmless bacteria normally live on the human skin and in the urinary, reproductive, and gastrointestinal tracts. These bacteria use up food and surface space that help prevent pathogenic bacteria from colonizing the body. Some of these harmless bacteria also secrete substances that change the conditions of their environment, making it less hospitable to potentially harmful bacteria.\u00a0They may release toxins or change the\u00a0pH, for example. All of these effects of harmless bacteria reduce the chances that pathogenic microorganisms will be able to reach sufficient numbers and cause illness.<\/p>\n
\nInflammation<\/h1>\n<\/div>\n
If pathogens manage to breach the barriers protecting the body, one of the first active responses of the innate immune system kicks in. This response is\u00a0inflammation<\/a>.<\/strong>\u00a0The main function of inflammation is to establish a physical barrier against the spread of infection. It also eliminates the initial cause of cell injury, clears out dead\u00a0cells\u00a0and tissues damaged from the original insult and the inflammatory process, and initiates tissue repair. Inflammation is often a response to infection by pathogens, but there are other possible causes, including burns, frostbite, and exposure to toxins.<\/p>\nThe signs and symptoms of inflammation include redness, swelling, warmth, pain, and frequently some loss of function. These symptoms are caused by increased blood flow into infected tissue, and a number of other processes, illustrated in Figure 17.4.4.<\/p>\nFigure 17.4.4 This drawing shows what happens during the inflammatory response.<\/em><\/figcaption><\/figure>\nInflammation is triggered by chemicals such as\u00a0cytokines<\/a><\/strong>\u00a0and\u00a0histamines<\/a>,<\/strong>which are released by injured or infected cells, or by immune system cells such as macrophages (described below) that are already present in tissues. These chemicals cause capillaries to dilate and become leaky, increasing blood flow to the infected area and allowing blood to enter the tissues. Pathogen-destroying leukocytes and tissue-repairing proteins migrate into tissue spaces from the bloodstream to attack pathogens and repair their damage. Cytokines also promote chemotaxis<\/a>, which is migration to the site of infection by pathogen-destroying leukocytes. Some cytokines have anti-viral effects. They may shut down protein synthesis in host cells, which viruses need in order to survive and replicate.<\/p>\nSee the video “The inflammatory response” by Neural Academy to learn about inflammatory response in more detail:<\/p>\n
Basophils<\/h2>\r\n[pb_glossary id=\"5573\"]Basophils[\/pb_glossary]<\/strong>\u00a0are non-phagocytic leukocytes that are also related to neutrophils. They are the least numerous of all white blood cells. Basophils secrete two types of chemicals that aid in body defenses: [pb_glossary id=\"4879\"]histamines<\/span><\/strong>[\/pb_glossary]<\/span> and heparin. <\/span>Histamines\u00a0are responsible for dilating\u00a0blood vessels\u00a0and increasing their permeability in inflammation.\u00a0<\/span>[pb_glossary id=\"4880\"]Heparin[\/pb_glossary]<\/strong>\u00a0inhibits blood clotting, and also promotes the movement of leukocytes into an area of infection.<\/span>\r\nMast Cells<\/h2>\r\n[pb_glossary id=\"4881\"]Mast cells[\/pb_glossary]<\/strong>\u00a0are non-phagocytic leukocytes that help initiate [pb_glossary id=\"4860\"]inflammation[\/pb_glossary] by secreting [pb_glossary id=\"4879\"]histamines<\/span>[\/pb_glossary]<\/span>. In some people, histamines trigger [pb_glossary id=\"4877\"]allergic reactions[\/pb_glossary], as well as inflammation. Mast cells may also secrete chemicals that help defend against parasites.<\/span>\r\nNatural Killer Cells<\/h2>\r\n[pb_glossary id=\"5515\"]Natural killer cells[\/pb_glossary]<\/strong>\u00a0are in the subset of leukocytes called [pb_glossary id=\"4832\"]lymphocytes<\/span>[\/pb_glossary]<\/span>, which are produced by the\u00a0lymphatic system. Natural killer cells destroy [pb_glossary id=\"5605\"]cancer<\/span>ous<\/span>[\/pb_glossary]<\/span>\u00a0or [pb_glossary id=\"4283\"]virus[\/pb_glossary]-infected host cells, although they do not directly attack invading pathogens. Natural killer cells recognize these host cells by a condition they exhibit called \u201cmissing self.\u201d Cells with missing self have abnormally low levels of cell-surface proteins of the [pb_glossary id=\"4825\"]major histocompatibility complex (MHC)[\/pb_glossary], which normally identify\u00a0body cells\u00a0as self.<\/span>\r\n\r\nInnate Immune Evasion<\/h1>\r\n<\/div>\r\nMany pathogens have evolved mechanisms that allow them to evade\u00a0human hosts'\u00a0innate immune systems. Some of these mechanisms include:\r\n\r\n \t- Invading host cells to replicate so they are \u201chidden\u201d from the immune system.<\/strong>\u00a0The bacterium that causes tuberculosis<\/a> uses this mechanism.<\/li>\r\n \t
- Forming a protective capsule around themselves to avoid being destroyed by immune system cells.<\/strong>\u00a0This defense occurs in bacteria, such as\u00a0Salmonella<\/em><\/a>\u00a0species.<\/li>\r\n \t
- Mimicking host cells so the immune system does not recognize them as foreign.<\/strong>\u00a0Some\u00a0species\u00a0of\u00a0Staphylococcus<\/em><\/a>\u00a0bacteria use this mechanism.<\/li>\r\n \t
- Directly killing phagocytes.<\/strong>\u00a0This ability evolved in several\u00a0species\u00a0of bacteria, including the species that causes anthrax<\/a>.<\/li>\r\n \t
- Producing molecules that prevent the formation of interferons, which are immune chemicals that fight viruses.<\/strong>\u00a0Some influenza<\/a> viruses have this capability.<\/li>\r\n \t
- Forming complex biofilms that provide protection from the cells and proteins of the immune system.<\/strong> This characterizes some species of [pb_glossary id=\"2588\"]bacteria[\/pb_glossary] and fungi. You can see an example of a bacterial biofilm on teeth in Figure 17.4.8.<\/li>\r\n<\/ul>\r\n[caption id=\"attachment_4883\" align=\"aligncenter\" width=\"400\"] Figure 17.4.8 The dental plaque on the top set of teeth is a biofilm that sticks to the teeth and consists of many species of bacteria. The plaque biofilm is difficult to remove, and it subjects the teeth and gums to high concentrations of bacterial metabolites, which result in dental disease. The same teeth after plaque removal are shown in the bottom picture.<\/em>[\/caption]\r\n\r\n\r\n\r\n \r\n\r\n<\/div>\r\n\r\n
\r\n17.4 Summary<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- The [pb_glossary id=\"4820\"]innate immune system[\/pb_glossary] is a subset of the human immune system that produces rapid, but non-specific responses to pathogens. Unlike the\u00a0[pb_glossary id=\"4822\"]adaptive immune system[\/pb_glossary], the innate system does not confer\u00a0[pb_glossary id=\"4884\"]immunity[\/pb_glossary]. The innate immune system includes surface barriers, [pb_glossary id=\"4860\"]inflammation[\/pb_glossary], the [pb_glossary id=\"4868\"]complement system[\/pb_glossary], and a variety of cellular responses.<\/li>\r\n \t
- The body\u2019s first line of defense consists of three different types of barriers that keep most pathogens out of\u00a0body tissues. The types of barriers are mechanical, chemical, and biological barriers.<\/li>\r\n \t
- Mechanical barriers\u00a0\u2014\u00a0which include the [pb_glossary id=\"3521\"]skin[\/pb_glossary], [pb_glossary id=\"3538\"]mucous membranes[\/pb_glossary], and fluids such as tears and [pb_glossary id=\"4717\"]urine[\/pb_glossary] \u2014 physically block pathogens from entering the body. Chemical barriers\u00a0\u2014\u00a0such as enzymes in [pb_glossary id=\"4711\"]sweat[\/pb_glossary], [pb_glossary id=\"4595\"]saliva[\/pb_glossary], and [pb_glossary id=\"4885\"]semen[\/pb_glossary]\u00a0\u2014\u00a0kill pathogens on body surfaces. Biological barriers are harmless bacteria that use up food and space so pathogenic bacteria cannot colonize the body.<\/li>\r\n \t
- If pathogens breach protective barriers, [pb_glossary id=\"4860\"]inflammation[\/pb_glossary] occurs. This creates a physical barrier against the spread of infection, and repairs tissue damage. Inflammation is triggered by chemicals such as [pb_glossary id=\"4865\"]cytokines[\/pb_glossary]\u00a0and [pb_glossary id=\"4879\"]histamines[\/pb_glossary], and it causes swelling, redness, and warmth.<\/li>\r\n \t
- The [pb_glossary id=\"4868\"]complement system[\/pb_glossary] is a complex biochemical mechanism that helps [pb_glossary id=\"5659\"]antibodies[\/pb_glossary] kill pathogens. Once activated, the complement system consists of more than two dozen proteins that lead to disruption of the\u00a0cell membrane\u00a0of pathogens and bursting of the cells.<\/li>\r\n \t
- Cellular responses of the innate immune system involve various types of [pb_glossary id=\"5623\"]leukocytes[\/pb_glossary]. For example, [pb_glossary id=\"5545\"]neutrophils[\/pb_glossary], [pb_glossary id=\"4340\"]macrophages[\/pb_glossary], and [pb_glossary id=\"4875\"]dendritic cells[\/pb_glossary]\u00a0phagocytize pathogens. [pb_glossary id=\"5573\"]Basophils[\/pb_glossary]\u00a0and [pb_glossary id=\"4881\"]mast cells[\/pb_glossary]\u00a0release chemicals that trigger inflammation. [pb_glossary id=\"5515\"]Natural killer cells[\/pb_glossary]\u00a0destroy cancerous or virus-infected cells, and eosinophils kill parasites.<\/li>\r\n \t
- Many pathogens have evolved mechanisms that help them evade the innate immune system. For example, some pathogens form a protective capsule around themselves, and some mimic host cells so the immune system does not recognize them as foreign.<\/li>\r\n<\/ul>\r\n<\/div>\r\n<\/div>\r\n
\r\n17.4 Review Questions<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- What is the innate immune system?<\/li>\r\n \t
- Identify the body\u2019s first line of defense.<\/li>\r\n \t
- [h5p id=\"638\"]<\/li>\r\n \t
- What are biological barriers? How do they protect the body?<\/li>\r\n \t
- State the purposes of inflammation. What triggers inflammation, and what signs and symptoms does it cause?<\/li>\r\n \t
- Define the complement system. How does it help destroy pathogens?<\/li>\r\n \t
- Describe two ways that pathogens can evade the innate immune system.<\/li>\r\n \t
- What are the ways in which phagocytes can encounter pathogens in the body?<\/li>\r\n \t
- Describe two different ways in which enzymes play a role in the innate immune response.<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n
\r\n17.4 Explore More<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\nhttps:\/\/youtu.be\/WW4skW6gucU\r\nHow mucus keeps us healthy - Katharina Ribbeck, TED-Ed, 2015.<\/p>\r\nhttps:\/\/youtu.be\/sYjtMP67vyk\r\n
Human Physiology - Innate Immune System, Janux, 2015.<\/p>\r\nhttps:\/\/youtu.be\/c64M1tZyWPM\r\n
Myriam Sidibe: The simple power of handwashing, TED, 2014.<\/p>\r\nhttps:\/\/youtu.be\/shEPwQPQG4I\r\n
Everything You Didn't Want To Know About Snot, Gross Science, 2017.<\/p>\r\nhttps:\/\/youtu.be\/dy1D3d1FBcw\r\n
Cough Grosser Than Sneeze? | Curiosity - World's Dirtiest Man, Discovery, 2011.<\/p>\r\n\r\n<\/div>\r\n<\/div>\r\n \r\n\r\n<\/div>\r\n
Attributions<\/h2>\r\nFigure 17.4.1<\/strong>\r\n\r\nOww_Papercut_14365<\/a> by Laurence Facun<\/a> on Wikimedia Commons is used under a CC BY 2.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/2.0) license.\r\n\r\nFigure 17.4.2<\/strong>\r\n\r\nhairy-nose<\/a> by Piotr Siedlecki\u00a0 on publicdomainpictures.net<\/a> is used under a CC0 1.0<\/a> Universal Public Domain Dedication (http:\/\/creativecommons.org\/publicdomain\/zero\/1.0\/) license.\r\n\r\nFigure 17.4.3<\/strong>\r\n\r\n1024px-Sneeze<\/a> by James Gathany\/ CDC Public Health Image library (PHIL) ID# 11162<\/a> on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/public_domain).\r\n\r\nFigure 17.4.4<\/strong>\r\n\r\nOSC_Microbio_17_06_Erythema<\/a> by CNX OpenStax<\/a> on Wikimedia Commons is used under a CC BY 4.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/4.0) license.\r\n\r\nFigure 17.4.5<\/strong>\r\n\r\n2212_Complement_Cascade_and_Function<\/a>\u00a0by OpenStax College<\/a> on Wikimedia Commons is used under a\u00a0CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\r\n\r\nFigure 17.4.6<\/strong>\r\n\r\n512px-Phagocytosis2<\/a> by Graham\u00a0Colm<\/span><\/a> at English Wikipedia<\/a><\/span>\u00a0on Wikimedia Commons is used under a CC BY-SA 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-sa\/3.0) license.\r\n\r\nFigure 17.4.7<\/strong>\r\n\r\nInnate_Immune_cells.svg<\/a> by Fred the Oyster<\/a> on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).\r\n\r\nFigure 17.4.8<\/strong>\r\n\r\n1024px-Gingivitis-before-and-after-3<\/a> by Onetimeuseaccount<\/a> on Wikimedia Commons is used under a CC0 1.0<\/a> Universal Public Domain Dedication (http:\/\/creativecommons.org\/publicdomain\/zero\/1.0\/) license.\r\nReferences<\/h2>\r\n
Betts, J. G., Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E., Womble, M., DeSaix, P. (2013, June 19). Figure 21.13 Complement cascade and function [digital image].\u00a0 In Anatomy and Physiology<\/em> (Section 21.2). OpenStax. https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/21-2-barrier-defenses-and-the-innate-immune-response<\/p>\r\nDiscovery. (2011, October 27). Cough grosser than sneeze? | Curiosity - World's dirtiest man. YouTube. https:\/\/www.youtube.com\/watch?v=dy1D3d1FBcw&feature=youtu.be<\/p>\r\n
Gross Science. (2017, January 31). Everything you didn't want to know about snot. YouTube. https:\/\/www.youtube.com\/watch?v=shEPwQPQG4I&feature=youtu.be<\/p>\r\n
Janux. (2015, January 10). Human physiology - Innate immune system. YouTube. https:\/\/www.youtube.com\/watch?v=sYjtMP67vyk&feature=youtu.be<\/p>\r\n
Mayo Clinic Staff. (n.d.). Anthrax [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/anthrax\/symptoms-causes\/syc-20356203<\/p>\r\n
Mayo Clinic Staff. (n.d.). Influenza (flu) [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/flu\/symptoms-causes\/syc-20351719<\/p>\r\n
Mayo Clinic Staff. (n.d.). Salmonella infection [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/salmonella\/symptoms-causes\/syc-20355329<\/p>\r\n
Mayo Clinic Staff. (n.d.). Staph infection [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/staph-infections\/multimedia\/staph-infection\/img-20008600<\/p>\r\n
Mayo Clinic Staff. (n.d.). Tuberculosis [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/tuberculosis\/symptoms-causes\/syc-20351250<\/p>\r\n
OpenStax. (2016, November 11). Figure\u00a0<\/span>17.23<\/span> A typical case of acute inflammation at the site of a skin wound - Erythema [digital image]. In OpenStax, Microbiology<\/em> (Section 17.5). https:\/\/openstax.org\/details\/books\/microbiology?Bookdetails<\/p>\r\nTED. (2014, October 14). Myriam Sidibe: The simple power of handwashing. YouTube. https:\/\/www.youtube.com\/watch?v=c64M1tZyWPM&feature=youtu.be<\/p>\r\n
TED-Ed. (2015, November 5). How mucus keeps us healthy - Katharina Ribbeck. YouTube. https:\/\/www.youtube.com\/watch?v=WW4skW6gucU&feature=youtu.be<\/p>","rendered":"
<\/p>\nFigure 17.4.1 Darn it!\u00a0 Paper cuts are the worst!<\/em><\/figcaption><\/figure>\n\nPaper Cut<\/h1>\n<\/div>\n
It\u2019s just a paper cut, but the break in your skin could provide an easy way for pathogens<\/span><\/a><\/span>\u00a0to enter your body. If\u00a0bacteria<\/a>\u00a0were to enter through the cut and infect the wound, your innate immune system would quickly respond with a dizzying array of general defenses.<\/span><\/p>\n\nWhat Is the Innate Immune System?<\/h1>\n<\/div>\n
The\u00a0innate immune system<\/a><\/strong>\u00a0is a subset of the human immune system that produces rapid, but non-specific responses to pathogens. Innate responses are generic, rather than tailored to a particular\u00a0pathogen. The innate system responds in the same general way to every pathogen\u00a0it encounters. Although the innate immune system provides immediate and rapid defenses against pathogens, it does not confer long-lasting\u00a0immunity\u00a0to them. In most organisms, the innate immune system is the dominant system of host defense. Other than most vertebrates (including humans), the innate immune system is the only system of host defense.<\/p>\nIn humans, the innate immune system includes surface barriers, inflammation, the complement system, and a variety of cellular responses. Surface barriers of various types generally keep most pathogens out of the body. If these barriers fail, then other innate defenses are triggered. The triggering event is usually the identification of pathogens by pattern-recognition receptors on\u00a0cells\u00a0of the innate immune system. These receptors recognize molecules that are broadly shared by pathogens, but distinguishable from host molecules. Alternatively, the other innate defenses may be triggered when damaged, injured, or stressed cells send out alarm signals, many of which are recognized by the same receptors as those that recognize pathogens.<\/p>\n
\nBarriers to Pathogens<\/h1>\n<\/div>\n
The body\u2019s first line of defense consists of three different types of barriers that keep most pathogens out of\u00a0body tissues. The types of barriers are mechanical, chemical, and biological barriers.<\/p>\n
Mechanical Barriers<\/h2>\nFigure 17.4.2 Nasal hairs are a mechanical barrier to larger particles in the air.<\/em><\/figcaption><\/figure>\nMechanical barriers<\/a> are the first line of defense against pathogens, and they physically block pathogens from entering the body. The skin<\/a> is the most important mechanical barrier. In fact, it is the single most important defense the body has. The outer layer of skin \u2014 the\u00a0epidermis<\/a>\u00a0\u2014 is tough, and very difficult for pathogens to penetrate. It consists of dead\u00a0cells\u00a0that are constantly shed from the body surface, a process that helps remove\u00a0bacteria\u00a0and other infectious agents that have adhered to the skin. The epidermis also lacks\u00a0blood vessels\u00a0and is usually lacking moisture, so it does not provide a suitable environment for most pathogens.\u00a0Hair<\/a> \u2014 which is an accessory organ of the skin \u2014 also helps keep out pathogens. Hairs inside the nose may trap larger pathogens and other particles in the air before they can enter the airways of the respiratory system (see Figure 17.4.2).<\/p>\nFigure 17.4.3 A sneeze can expel many pathogens from the respiratory tract, which is why you should always cover your mouth and nose and when you sneeze.<\/em><\/figcaption><\/figure>\nMucous membranes<\/span><\/a><\/span> provide a mechanical barrier to pathogens and other particles at body openings. These membranes also line the respiratory, gastrointestinal, urinary, and reproductive tracts. Mucous membranes secrete mucus, which is a slimy and somewhat sticky substance that traps pathogens. Many mucous membranes also have hair-like cilia<\/a> that sweep mucus and trapped pathogens toward body openings, where they can be removed from the body. When you sneeze or cough, mucus and pathogens are mechanically ejected from the nose and throat, as you can see in Figure 17.4.3.\u00a0 A sneeze can travel as fast as 160 Km\/hr (about 99 mi\/hour) and expel as many as 100,000 droplets into the air around you (a good reason to cover your sneezes!). Other mechanical defenses include tears, which wash pathogens from the eyes, and urine, which flushes pathogens out of the urinary tract.<\/span><\/p>\nChemical Barriers<\/h2>\n
Chemical barriers also protect against infection by pathogens. They destroy pathogens on the outer body surface, at body openings, and on inner body linings. Sweat, mucus, tears, saliva, and breastmilk all contain antimicrobial substances (such as the\u00a0enzyme\u00a0lysozyme) that kill pathogens, especially\u00a0bacteria. Sebaceous glands in the\u00a0dermis\u00a0of the skin secrete acids that form a very fine, slightly acidic film on the surface of the skin. This film acts as a barrier to bacteria, viruses, and other potential contaminants that might penetrate the skin. Urine and vaginal secretions are also too acidic for many pathogens to endure. Semen contains zinc \u2014 which most pathogens cannot tolerate \u2014 as well as defensins, which are antimicrobial\u00a0proteins\u00a0that act mainly by disrupting bacterial cell membranes. In the stomach, stomach\u00a0acid\u00a0and digestive\u00a0enzymes\u00a0called proteases (which break down proteins) kill most of the pathogens that enter the gastrointestinal tract in food or\u00a0water.<\/p>\n
Biological Barriers<\/h2>\n
Biological barriers are living organisms that help protect the body from pathogens. Trillions of harmless bacteria normally live on the human skin and in the urinary, reproductive, and gastrointestinal tracts. These bacteria use up food and surface space that help prevent pathogenic bacteria from colonizing the body. Some of these harmless bacteria also secrete substances that change the conditions of their environment, making it less hospitable to potentially harmful bacteria.\u00a0They may release toxins or change the\u00a0pH, for example. All of these effects of harmless bacteria reduce the chances that pathogenic microorganisms will be able to reach sufficient numbers and cause illness.<\/p>\n
\nInflammation<\/h1>\n<\/div>\n
If pathogens manage to breach the barriers protecting the body, one of the first active responses of the innate immune system kicks in. This response is\u00a0inflammation<\/a>.<\/strong>\u00a0The main function of inflammation is to establish a physical barrier against the spread of infection. It also eliminates the initial cause of cell injury, clears out dead\u00a0cells\u00a0and tissues damaged from the original insult and the inflammatory process, and initiates tissue repair. Inflammation is often a response to infection by pathogens, but there are other possible causes, including burns, frostbite, and exposure to toxins.<\/p>\nThe signs and symptoms of inflammation include redness, swelling, warmth, pain, and frequently some loss of function. These symptoms are caused by increased blood flow into infected tissue, and a number of other processes, illustrated in Figure 17.4.4.<\/p>\nFigure 17.4.4 This drawing shows what happens during the inflammatory response.<\/em><\/figcaption><\/figure>\nInflammation is triggered by chemicals such as\u00a0cytokines<\/a><\/strong>\u00a0and\u00a0histamines<\/a>,<\/strong>which are released by injured or infected cells, or by immune system cells such as macrophages (described below) that are already present in tissues. These chemicals cause capillaries to dilate and become leaky, increasing blood flow to the infected area and allowing blood to enter the tissues. Pathogen-destroying leukocytes and tissue-repairing proteins migrate into tissue spaces from the bloodstream to attack pathogens and repair their damage. Cytokines also promote chemotaxis<\/a>, which is migration to the site of infection by pathogen-destroying leukocytes. Some cytokines have anti-viral effects. They may shut down protein synthesis in host cells, which viruses need in order to survive and replicate.<\/p>\nSee the video “The inflammatory response” by Neural Academy to learn about inflammatory response in more detail:<\/p>\n
Natural Killer Cells<\/h2>\r\n[pb_glossary id=\"5515\"]Natural killer cells[\/pb_glossary]<\/strong>\u00a0are in the subset of leukocytes called [pb_glossary id=\"4832\"]lymphocytes<\/span>[\/pb_glossary]<\/span>, which are produced by the\u00a0lymphatic system. Natural killer cells destroy [pb_glossary id=\"5605\"]cancer<\/span>ous<\/span>[\/pb_glossary]<\/span>\u00a0or [pb_glossary id=\"4283\"]virus[\/pb_glossary]-infected host cells, although they do not directly attack invading pathogens. Natural killer cells recognize these host cells by a condition they exhibit called \u201cmissing self.\u201d Cells with missing self have abnormally low levels of cell-surface proteins of the [pb_glossary id=\"4825\"]major histocompatibility complex (MHC)[\/pb_glossary], which normally identify\u00a0body cells\u00a0as self.<\/span>\r\n\r\nInnate Immune Evasion<\/h1>\r\n<\/div>\r\nMany pathogens have evolved mechanisms that allow them to evade\u00a0human hosts'\u00a0innate immune systems. Some of these mechanisms include:\r\n\r\n \t- Invading host cells to replicate so they are \u201chidden\u201d from the immune system.<\/strong>\u00a0The bacterium that causes tuberculosis<\/a> uses this mechanism.<\/li>\r\n \t
- Forming a protective capsule around themselves to avoid being destroyed by immune system cells.<\/strong>\u00a0This defense occurs in bacteria, such as\u00a0Salmonella<\/em><\/a>\u00a0species.<\/li>\r\n \t
- Mimicking host cells so the immune system does not recognize them as foreign.<\/strong>\u00a0Some\u00a0species\u00a0of\u00a0Staphylococcus<\/em><\/a>\u00a0bacteria use this mechanism.<\/li>\r\n \t
- Directly killing phagocytes.<\/strong>\u00a0This ability evolved in several\u00a0species\u00a0of bacteria, including the species that causes anthrax<\/a>.<\/li>\r\n \t
- Producing molecules that prevent the formation of interferons, which are immune chemicals that fight viruses.<\/strong>\u00a0Some influenza<\/a> viruses have this capability.<\/li>\r\n \t
- Forming complex biofilms that provide protection from the cells and proteins of the immune system.<\/strong> This characterizes some species of [pb_glossary id=\"2588\"]bacteria[\/pb_glossary] and fungi. You can see an example of a bacterial biofilm on teeth in Figure 17.4.8.<\/li>\r\n<\/ul>\r\n[caption id=\"attachment_4883\" align=\"aligncenter\" width=\"400\"] Figure 17.4.8 The dental plaque on the top set of teeth is a biofilm that sticks to the teeth and consists of many species of bacteria. The plaque biofilm is difficult to remove, and it subjects the teeth and gums to high concentrations of bacterial metabolites, which result in dental disease. The same teeth after plaque removal are shown in the bottom picture.<\/em>[\/caption]\r\n\r\n\r\n\r\n \r\n\r\n<\/div>\r\n\r\n
\r\n17.4 Summary<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- The [pb_glossary id=\"4820\"]innate immune system[\/pb_glossary] is a subset of the human immune system that produces rapid, but non-specific responses to pathogens. Unlike the\u00a0[pb_glossary id=\"4822\"]adaptive immune system[\/pb_glossary], the innate system does not confer\u00a0[pb_glossary id=\"4884\"]immunity[\/pb_glossary]. The innate immune system includes surface barriers, [pb_glossary id=\"4860\"]inflammation[\/pb_glossary], the [pb_glossary id=\"4868\"]complement system[\/pb_glossary], and a variety of cellular responses.<\/li>\r\n \t
- The body\u2019s first line of defense consists of three different types of barriers that keep most pathogens out of\u00a0body tissues. The types of barriers are mechanical, chemical, and biological barriers.<\/li>\r\n \t
- Mechanical barriers\u00a0\u2014\u00a0which include the [pb_glossary id=\"3521\"]skin[\/pb_glossary], [pb_glossary id=\"3538\"]mucous membranes[\/pb_glossary], and fluids such as tears and [pb_glossary id=\"4717\"]urine[\/pb_glossary] \u2014 physically block pathogens from entering the body. Chemical barriers\u00a0\u2014\u00a0such as enzymes in [pb_glossary id=\"4711\"]sweat[\/pb_glossary], [pb_glossary id=\"4595\"]saliva[\/pb_glossary], and [pb_glossary id=\"4885\"]semen[\/pb_glossary]\u00a0\u2014\u00a0kill pathogens on body surfaces. Biological barriers are harmless bacteria that use up food and space so pathogenic bacteria cannot colonize the body.<\/li>\r\n \t
- If pathogens breach protective barriers, [pb_glossary id=\"4860\"]inflammation[\/pb_glossary] occurs. This creates a physical barrier against the spread of infection, and repairs tissue damage. Inflammation is triggered by chemicals such as [pb_glossary id=\"4865\"]cytokines[\/pb_glossary]\u00a0and [pb_glossary id=\"4879\"]histamines[\/pb_glossary], and it causes swelling, redness, and warmth.<\/li>\r\n \t
- The [pb_glossary id=\"4868\"]complement system[\/pb_glossary] is a complex biochemical mechanism that helps [pb_glossary id=\"5659\"]antibodies[\/pb_glossary] kill pathogens. Once activated, the complement system consists of more than two dozen proteins that lead to disruption of the\u00a0cell membrane\u00a0of pathogens and bursting of the cells.<\/li>\r\n \t
- Cellular responses of the innate immune system involve various types of [pb_glossary id=\"5623\"]leukocytes[\/pb_glossary]. For example, [pb_glossary id=\"5545\"]neutrophils[\/pb_glossary], [pb_glossary id=\"4340\"]macrophages[\/pb_glossary], and [pb_glossary id=\"4875\"]dendritic cells[\/pb_glossary]\u00a0phagocytize pathogens. [pb_glossary id=\"5573\"]Basophils[\/pb_glossary]\u00a0and [pb_glossary id=\"4881\"]mast cells[\/pb_glossary]\u00a0release chemicals that trigger inflammation. [pb_glossary id=\"5515\"]Natural killer cells[\/pb_glossary]\u00a0destroy cancerous or virus-infected cells, and eosinophils kill parasites.<\/li>\r\n \t
- Many pathogens have evolved mechanisms that help them evade the innate immune system. For example, some pathogens form a protective capsule around themselves, and some mimic host cells so the immune system does not recognize them as foreign.<\/li>\r\n<\/ul>\r\n<\/div>\r\n<\/div>\r\n
\r\n17.4 Review Questions<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- What is the innate immune system?<\/li>\r\n \t
- Identify the body\u2019s first line of defense.<\/li>\r\n \t
- [h5p id=\"638\"]<\/li>\r\n \t
- What are biological barriers? How do they protect the body?<\/li>\r\n \t
- State the purposes of inflammation. What triggers inflammation, and what signs and symptoms does it cause?<\/li>\r\n \t
- Define the complement system. How does it help destroy pathogens?<\/li>\r\n \t
- Describe two ways that pathogens can evade the innate immune system.<\/li>\r\n \t
- What are the ways in which phagocytes can encounter pathogens in the body?<\/li>\r\n \t
- Describe two different ways in which enzymes play a role in the innate immune response.<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n
\r\n17.4 Explore More<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\nhttps:\/\/youtu.be\/WW4skW6gucU\r\nHow mucus keeps us healthy - Katharina Ribbeck, TED-Ed, 2015.<\/p>\r\nhttps:\/\/youtu.be\/sYjtMP67vyk\r\n
Human Physiology - Innate Immune System, Janux, 2015.<\/p>\r\nhttps:\/\/youtu.be\/c64M1tZyWPM\r\n
Myriam Sidibe: The simple power of handwashing, TED, 2014.<\/p>\r\nhttps:\/\/youtu.be\/shEPwQPQG4I\r\n
Everything You Didn't Want To Know About Snot, Gross Science, 2017.<\/p>\r\nhttps:\/\/youtu.be\/dy1D3d1FBcw\r\n
Cough Grosser Than Sneeze? | Curiosity - World's Dirtiest Man, Discovery, 2011.<\/p>\r\n\r\n<\/div>\r\n<\/div>\r\n \r\n\r\n<\/div>\r\n
Attributions<\/h2>\r\nFigure 17.4.1<\/strong>\r\n\r\nOww_Papercut_14365<\/a> by Laurence Facun<\/a> on Wikimedia Commons is used under a CC BY 2.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/2.0) license.\r\n\r\nFigure 17.4.2<\/strong>\r\n\r\nhairy-nose<\/a> by Piotr Siedlecki\u00a0 on publicdomainpictures.net<\/a> is used under a CC0 1.0<\/a> Universal Public Domain Dedication (http:\/\/creativecommons.org\/publicdomain\/zero\/1.0\/) license.\r\n\r\nFigure 17.4.3<\/strong>\r\n\r\n1024px-Sneeze<\/a> by James Gathany\/ CDC Public Health Image library (PHIL) ID# 11162<\/a> on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/public_domain).\r\n\r\nFigure 17.4.4<\/strong>\r\n\r\nOSC_Microbio_17_06_Erythema<\/a> by CNX OpenStax<\/a> on Wikimedia Commons is used under a CC BY 4.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/4.0) license.\r\n\r\nFigure 17.4.5<\/strong>\r\n\r\n2212_Complement_Cascade_and_Function<\/a>\u00a0by OpenStax College<\/a> on Wikimedia Commons is used under a\u00a0CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\r\n\r\nFigure 17.4.6<\/strong>\r\n\r\n512px-Phagocytosis2<\/a> by Graham\u00a0Colm<\/span><\/a> at English Wikipedia<\/a><\/span>\u00a0on Wikimedia Commons is used under a CC BY-SA 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-sa\/3.0) license.\r\n\r\nFigure 17.4.7<\/strong>\r\n\r\nInnate_Immune_cells.svg<\/a> by Fred the Oyster<\/a> on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).\r\n\r\nFigure 17.4.8<\/strong>\r\n\r\n1024px-Gingivitis-before-and-after-3<\/a> by Onetimeuseaccount<\/a> on Wikimedia Commons is used under a CC0 1.0<\/a> Universal Public Domain Dedication (http:\/\/creativecommons.org\/publicdomain\/zero\/1.0\/) license.\r\nReferences<\/h2>\r\n
Betts, J. G., Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E., Womble, M., DeSaix, P. (2013, June 19). Figure 21.13 Complement cascade and function [digital image].\u00a0 In Anatomy and Physiology<\/em> (Section 21.2). OpenStax. https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/21-2-barrier-defenses-and-the-innate-immune-response<\/p>\r\nDiscovery. (2011, October 27). Cough grosser than sneeze? | Curiosity - World's dirtiest man. YouTube. https:\/\/www.youtube.com\/watch?v=dy1D3d1FBcw&feature=youtu.be<\/p>\r\n
Gross Science. (2017, January 31). Everything you didn't want to know about snot. YouTube. https:\/\/www.youtube.com\/watch?v=shEPwQPQG4I&feature=youtu.be<\/p>\r\n
Janux. (2015, January 10). Human physiology - Innate immune system. YouTube. https:\/\/www.youtube.com\/watch?v=sYjtMP67vyk&feature=youtu.be<\/p>\r\n
Mayo Clinic Staff. (n.d.). Anthrax [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/anthrax\/symptoms-causes\/syc-20356203<\/p>\r\n
Mayo Clinic Staff. (n.d.). Influenza (flu) [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/flu\/symptoms-causes\/syc-20351719<\/p>\r\n
Mayo Clinic Staff. (n.d.). Salmonella infection [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/salmonella\/symptoms-causes\/syc-20355329<\/p>\r\n
Mayo Clinic Staff. (n.d.). Staph infection [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/staph-infections\/multimedia\/staph-infection\/img-20008600<\/p>\r\n
Mayo Clinic Staff. (n.d.). Tuberculosis [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/tuberculosis\/symptoms-causes\/syc-20351250<\/p>\r\n
OpenStax. (2016, November 11). Figure\u00a0<\/span>17.23<\/span> A typical case of acute inflammation at the site of a skin wound - Erythema [digital image]. In OpenStax, Microbiology<\/em> (Section 17.5). https:\/\/openstax.org\/details\/books\/microbiology?Bookdetails<\/p>\r\nTED. (2014, October 14). Myriam Sidibe: The simple power of handwashing. YouTube. https:\/\/www.youtube.com\/watch?v=c64M1tZyWPM&feature=youtu.be<\/p>\r\n
TED-Ed. (2015, November 5). How mucus keeps us healthy - Katharina Ribbeck. YouTube. https:\/\/www.youtube.com\/watch?v=WW4skW6gucU&feature=youtu.be<\/p>","rendered":"
<\/p>\nFigure 17.4.1 Darn it!\u00a0 Paper cuts are the worst!<\/em><\/figcaption><\/figure>\n\nPaper Cut<\/h1>\n<\/div>\n
It\u2019s just a paper cut, but the break in your skin could provide an easy way for pathogens<\/span><\/a><\/span>\u00a0to enter your body. If\u00a0bacteria<\/a>\u00a0were to enter through the cut and infect the wound, your innate immune system would quickly respond with a dizzying array of general defenses.<\/span><\/p>\n\nWhat Is the Innate Immune System?<\/h1>\n<\/div>\n
The\u00a0innate immune system<\/a><\/strong>\u00a0is a subset of the human immune system that produces rapid, but non-specific responses to pathogens. Innate responses are generic, rather than tailored to a particular\u00a0pathogen. The innate system responds in the same general way to every pathogen\u00a0it encounters. Although the innate immune system provides immediate and rapid defenses against pathogens, it does not confer long-lasting\u00a0immunity\u00a0to them. In most organisms, the innate immune system is the dominant system of host defense. Other than most vertebrates (including humans), the innate immune system is the only system of host defense.<\/p>\nIn humans, the innate immune system includes surface barriers, inflammation, the complement system, and a variety of cellular responses. Surface barriers of various types generally keep most pathogens out of the body. If these barriers fail, then other innate defenses are triggered. The triggering event is usually the identification of pathogens by pattern-recognition receptors on\u00a0cells\u00a0of the innate immune system. These receptors recognize molecules that are broadly shared by pathogens, but distinguishable from host molecules. Alternatively, the other innate defenses may be triggered when damaged, injured, or stressed cells send out alarm signals, many of which are recognized by the same receptors as those that recognize pathogens.<\/p>\n
\nBarriers to Pathogens<\/h1>\n<\/div>\n
The body\u2019s first line of defense consists of three different types of barriers that keep most pathogens out of\u00a0body tissues. The types of barriers are mechanical, chemical, and biological barriers.<\/p>\n
Mechanical Barriers<\/h2>\nFigure 17.4.2 Nasal hairs are a mechanical barrier to larger particles in the air.<\/em><\/figcaption><\/figure>\nMechanical barriers<\/a> are the first line of defense against pathogens, and they physically block pathogens from entering the body. The skin<\/a> is the most important mechanical barrier. In fact, it is the single most important defense the body has. The outer layer of skin \u2014 the\u00a0epidermis<\/a>\u00a0\u2014 is tough, and very difficult for pathogens to penetrate. It consists of dead\u00a0cells\u00a0that are constantly shed from the body surface, a process that helps remove\u00a0bacteria\u00a0and other infectious agents that have adhered to the skin. The epidermis also lacks\u00a0blood vessels\u00a0and is usually lacking moisture, so it does not provide a suitable environment for most pathogens.\u00a0Hair<\/a> \u2014 which is an accessory organ of the skin \u2014 also helps keep out pathogens. Hairs inside the nose may trap larger pathogens and other particles in the air before they can enter the airways of the respiratory system (see Figure 17.4.2).<\/p>\nFigure 17.4.3 A sneeze can expel many pathogens from the respiratory tract, which is why you should always cover your mouth and nose and when you sneeze.<\/em><\/figcaption><\/figure>\nMucous membranes<\/span><\/a><\/span> provide a mechanical barrier to pathogens and other particles at body openings. These membranes also line the respiratory, gastrointestinal, urinary, and reproductive tracts. Mucous membranes secrete mucus, which is a slimy and somewhat sticky substance that traps pathogens. Many mucous membranes also have hair-like cilia<\/a> that sweep mucus and trapped pathogens toward body openings, where they can be removed from the body. When you sneeze or cough, mucus and pathogens are mechanically ejected from the nose and throat, as you can see in Figure 17.4.3.\u00a0 A sneeze can travel as fast as 160 Km\/hr (about 99 mi\/hour) and expel as many as 100,000 droplets into the air around you (a good reason to cover your sneezes!). Other mechanical defenses include tears, which wash pathogens from the eyes, and urine, which flushes pathogens out of the urinary tract.<\/span><\/p>\nChemical Barriers<\/h2>\n
Chemical barriers also protect against infection by pathogens. They destroy pathogens on the outer body surface, at body openings, and on inner body linings. Sweat, mucus, tears, saliva, and breastmilk all contain antimicrobial substances (such as the\u00a0enzyme\u00a0lysozyme) that kill pathogens, especially\u00a0bacteria. Sebaceous glands in the\u00a0dermis\u00a0of the skin secrete acids that form a very fine, slightly acidic film on the surface of the skin. This film acts as a barrier to bacteria, viruses, and other potential contaminants that might penetrate the skin. Urine and vaginal secretions are also too acidic for many pathogens to endure. Semen contains zinc \u2014 which most pathogens cannot tolerate \u2014 as well as defensins, which are antimicrobial\u00a0proteins\u00a0that act mainly by disrupting bacterial cell membranes. In the stomach, stomach\u00a0acid\u00a0and digestive\u00a0enzymes\u00a0called proteases (which break down proteins) kill most of the pathogens that enter the gastrointestinal tract in food or\u00a0water.<\/p>\n
Biological Barriers<\/h2>\n
Biological barriers are living organisms that help protect the body from pathogens. Trillions of harmless bacteria normally live on the human skin and in the urinary, reproductive, and gastrointestinal tracts. These bacteria use up food and surface space that help prevent pathogenic bacteria from colonizing the body. Some of these harmless bacteria also secrete substances that change the conditions of their environment, making it less hospitable to potentially harmful bacteria.\u00a0They may release toxins or change the\u00a0pH, for example. All of these effects of harmless bacteria reduce the chances that pathogenic microorganisms will be able to reach sufficient numbers and cause illness.<\/p>\n
\nInflammation<\/h1>\n<\/div>\n
If pathogens manage to breach the barriers protecting the body, one of the first active responses of the innate immune system kicks in. This response is\u00a0inflammation<\/a>.<\/strong>\u00a0The main function of inflammation is to establish a physical barrier against the spread of infection. It also eliminates the initial cause of cell injury, clears out dead\u00a0cells\u00a0and tissues damaged from the original insult and the inflammatory process, and initiates tissue repair. Inflammation is often a response to infection by pathogens, but there are other possible causes, including burns, frostbite, and exposure to toxins.<\/p>\nThe signs and symptoms of inflammation include redness, swelling, warmth, pain, and frequently some loss of function. These symptoms are caused by increased blood flow into infected tissue, and a number of other processes, illustrated in Figure 17.4.4.<\/p>\nFigure 17.4.4 This drawing shows what happens during the inflammatory response.<\/em><\/figcaption><\/figure>\nInflammation is triggered by chemicals such as\u00a0cytokines<\/a><\/strong>\u00a0and\u00a0histamines<\/a>,<\/strong>which are released by injured or infected cells, or by immune system cells such as macrophages (described below) that are already present in tissues. These chemicals cause capillaries to dilate and become leaky, increasing blood flow to the infected area and allowing blood to enter the tissues. Pathogen-destroying leukocytes and tissue-repairing proteins migrate into tissue spaces from the bloodstream to attack pathogens and repair their damage. Cytokines also promote chemotaxis<\/a>, which is migration to the site of infection by pathogen-destroying leukocytes. Some cytokines have anti-viral effects. They may shut down protein synthesis in host cells, which viruses need in order to survive and replicate.<\/p>\nSee the video “The inflammatory response” by Neural Academy to learn about inflammatory response in more detail:<\/p>\n
Innate Immune Evasion<\/h1>\r\n<\/div>\r\nMany pathogens have evolved mechanisms that allow them to evade\u00a0human hosts'\u00a0innate immune systems. Some of these mechanisms include:\r\n\r\n \t- Invading host cells to replicate so they are \u201chidden\u201d from the immune system.<\/strong>\u00a0The bacterium that causes tuberculosis<\/a> uses this mechanism.<\/li>\r\n \t
- Forming a protective capsule around themselves to avoid being destroyed by immune system cells.<\/strong>\u00a0This defense occurs in bacteria, such as\u00a0Salmonella<\/em><\/a>\u00a0species.<\/li>\r\n \t
- Mimicking host cells so the immune system does not recognize them as foreign.<\/strong>\u00a0Some\u00a0species\u00a0of\u00a0Staphylococcus<\/em><\/a>\u00a0bacteria use this mechanism.<\/li>\r\n \t
- Directly killing phagocytes.<\/strong>\u00a0This ability evolved in several\u00a0species\u00a0of bacteria, including the species that causes anthrax<\/a>.<\/li>\r\n \t
- Producing molecules that prevent the formation of interferons, which are immune chemicals that fight viruses.<\/strong>\u00a0Some influenza<\/a> viruses have this capability.<\/li>\r\n \t
- Forming complex biofilms that provide protection from the cells and proteins of the immune system.<\/strong> This characterizes some species of [pb_glossary id=\"2588\"]bacteria[\/pb_glossary] and fungi. You can see an example of a bacterial biofilm on teeth in Figure 17.4.8.<\/li>\r\n<\/ul>\r\n[caption id=\"attachment_4883\" align=\"aligncenter\" width=\"400\"] Figure 17.4.8 The dental plaque on the top set of teeth is a biofilm that sticks to the teeth and consists of many species of bacteria. The plaque biofilm is difficult to remove, and it subjects the teeth and gums to high concentrations of bacterial metabolites, which result in dental disease. The same teeth after plaque removal are shown in the bottom picture.<\/em>[\/caption]\r\n\r\n\r\n\r\n \r\n\r\n<\/div>\r\n\r\n
\r\n17.4 Summary<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- The [pb_glossary id=\"4820\"]innate immune system[\/pb_glossary] is a subset of the human immune system that produces rapid, but non-specific responses to pathogens. Unlike the\u00a0[pb_glossary id=\"4822\"]adaptive immune system[\/pb_glossary], the innate system does not confer\u00a0[pb_glossary id=\"4884\"]immunity[\/pb_glossary]. The innate immune system includes surface barriers, [pb_glossary id=\"4860\"]inflammation[\/pb_glossary], the [pb_glossary id=\"4868\"]complement system[\/pb_glossary], and a variety of cellular responses.<\/li>\r\n \t
- The body\u2019s first line of defense consists of three different types of barriers that keep most pathogens out of\u00a0body tissues. The types of barriers are mechanical, chemical, and biological barriers.<\/li>\r\n \t
- Mechanical barriers\u00a0\u2014\u00a0which include the [pb_glossary id=\"3521\"]skin[\/pb_glossary], [pb_glossary id=\"3538\"]mucous membranes[\/pb_glossary], and fluids such as tears and [pb_glossary id=\"4717\"]urine[\/pb_glossary] \u2014 physically block pathogens from entering the body. Chemical barriers\u00a0\u2014\u00a0such as enzymes in [pb_glossary id=\"4711\"]sweat[\/pb_glossary], [pb_glossary id=\"4595\"]saliva[\/pb_glossary], and [pb_glossary id=\"4885\"]semen[\/pb_glossary]\u00a0\u2014\u00a0kill pathogens on body surfaces. Biological barriers are harmless bacteria that use up food and space so pathogenic bacteria cannot colonize the body.<\/li>\r\n \t
- If pathogens breach protective barriers, [pb_glossary id=\"4860\"]inflammation[\/pb_glossary] occurs. This creates a physical barrier against the spread of infection, and repairs tissue damage. Inflammation is triggered by chemicals such as [pb_glossary id=\"4865\"]cytokines[\/pb_glossary]\u00a0and [pb_glossary id=\"4879\"]histamines[\/pb_glossary], and it causes swelling, redness, and warmth.<\/li>\r\n \t
- The [pb_glossary id=\"4868\"]complement system[\/pb_glossary] is a complex biochemical mechanism that helps [pb_glossary id=\"5659\"]antibodies[\/pb_glossary] kill pathogens. Once activated, the complement system consists of more than two dozen proteins that lead to disruption of the\u00a0cell membrane\u00a0of pathogens and bursting of the cells.<\/li>\r\n \t
- Cellular responses of the innate immune system involve various types of [pb_glossary id=\"5623\"]leukocytes[\/pb_glossary]. For example, [pb_glossary id=\"5545\"]neutrophils[\/pb_glossary], [pb_glossary id=\"4340\"]macrophages[\/pb_glossary], and [pb_glossary id=\"4875\"]dendritic cells[\/pb_glossary]\u00a0phagocytize pathogens. [pb_glossary id=\"5573\"]Basophils[\/pb_glossary]\u00a0and [pb_glossary id=\"4881\"]mast cells[\/pb_glossary]\u00a0release chemicals that trigger inflammation. [pb_glossary id=\"5515\"]Natural killer cells[\/pb_glossary]\u00a0destroy cancerous or virus-infected cells, and eosinophils kill parasites.<\/li>\r\n \t
- Many pathogens have evolved mechanisms that help them evade the innate immune system. For example, some pathogens form a protective capsule around themselves, and some mimic host cells so the immune system does not recognize them as foreign.<\/li>\r\n<\/ul>\r\n<\/div>\r\n<\/div>\r\n
\r\n17.4 Review Questions<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- What is the innate immune system?<\/li>\r\n \t
- Identify the body\u2019s first line of defense.<\/li>\r\n \t
- [h5p id=\"638\"]<\/li>\r\n \t
- What are biological barriers? How do they protect the body?<\/li>\r\n \t
- State the purposes of inflammation. What triggers inflammation, and what signs and symptoms does it cause?<\/li>\r\n \t
- Define the complement system. How does it help destroy pathogens?<\/li>\r\n \t
- Describe two ways that pathogens can evade the innate immune system.<\/li>\r\n \t
- What are the ways in which phagocytes can encounter pathogens in the body?<\/li>\r\n \t
- Describe two different ways in which enzymes play a role in the innate immune response.<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n
\r\n17.4 Explore More<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\nhttps:\/\/youtu.be\/WW4skW6gucU\r\nHow mucus keeps us healthy - Katharina Ribbeck, TED-Ed, 2015.<\/p>\r\nhttps:\/\/youtu.be\/sYjtMP67vyk\r\n
Human Physiology - Innate Immune System, Janux, 2015.<\/p>\r\nhttps:\/\/youtu.be\/c64M1tZyWPM\r\n
Myriam Sidibe: The simple power of handwashing, TED, 2014.<\/p>\r\nhttps:\/\/youtu.be\/shEPwQPQG4I\r\n
Everything You Didn't Want To Know About Snot, Gross Science, 2017.<\/p>\r\nhttps:\/\/youtu.be\/dy1D3d1FBcw\r\n
Cough Grosser Than Sneeze? | Curiosity - World's Dirtiest Man, Discovery, 2011.<\/p>\r\n\r\n<\/div>\r\n<\/div>\r\n \r\n\r\n<\/div>\r\n
Attributions<\/h2>\r\nFigure 17.4.1<\/strong>\r\n\r\nOww_Papercut_14365<\/a> by Laurence Facun<\/a> on Wikimedia Commons is used under a CC BY 2.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/2.0) license.\r\n\r\nFigure 17.4.2<\/strong>\r\n\r\nhairy-nose<\/a> by Piotr Siedlecki\u00a0 on publicdomainpictures.net<\/a> is used under a CC0 1.0<\/a> Universal Public Domain Dedication (http:\/\/creativecommons.org\/publicdomain\/zero\/1.0\/) license.\r\n\r\nFigure 17.4.3<\/strong>\r\n\r\n1024px-Sneeze<\/a> by James Gathany\/ CDC Public Health Image library (PHIL) ID# 11162<\/a> on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/public_domain).\r\n\r\nFigure 17.4.4<\/strong>\r\n\r\nOSC_Microbio_17_06_Erythema<\/a> by CNX OpenStax<\/a> on Wikimedia Commons is used under a CC BY 4.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/4.0) license.\r\n\r\nFigure 17.4.5<\/strong>\r\n\r\n2212_Complement_Cascade_and_Function<\/a>\u00a0by OpenStax College<\/a> on Wikimedia Commons is used under a\u00a0CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\r\n\r\nFigure 17.4.6<\/strong>\r\n\r\n512px-Phagocytosis2<\/a> by Graham\u00a0Colm<\/span><\/a> at English Wikipedia<\/a><\/span>\u00a0on Wikimedia Commons is used under a CC BY-SA 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-sa\/3.0) license.\r\n\r\nFigure 17.4.7<\/strong>\r\n\r\nInnate_Immune_cells.svg<\/a> by Fred the Oyster<\/a> on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).\r\n\r\nFigure 17.4.8<\/strong>\r\n\r\n1024px-Gingivitis-before-and-after-3<\/a> by Onetimeuseaccount<\/a> on Wikimedia Commons is used under a CC0 1.0<\/a> Universal Public Domain Dedication (http:\/\/creativecommons.org\/publicdomain\/zero\/1.0\/) license.\r\nReferences<\/h2>\r\n
Betts, J. G., Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E., Womble, M., DeSaix, P. (2013, June 19). Figure 21.13 Complement cascade and function [digital image].\u00a0 In Anatomy and Physiology<\/em> (Section 21.2). OpenStax. https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/21-2-barrier-defenses-and-the-innate-immune-response<\/p>\r\nDiscovery. (2011, October 27). Cough grosser than sneeze? | Curiosity - World's dirtiest man. YouTube. https:\/\/www.youtube.com\/watch?v=dy1D3d1FBcw&feature=youtu.be<\/p>\r\n
Gross Science. (2017, January 31). Everything you didn't want to know about snot. YouTube. https:\/\/www.youtube.com\/watch?v=shEPwQPQG4I&feature=youtu.be<\/p>\r\n
Janux. (2015, January 10). Human physiology - Innate immune system. YouTube. https:\/\/www.youtube.com\/watch?v=sYjtMP67vyk&feature=youtu.be<\/p>\r\n
Mayo Clinic Staff. (n.d.). Anthrax [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/anthrax\/symptoms-causes\/syc-20356203<\/p>\r\n
Mayo Clinic Staff. (n.d.). Influenza (flu) [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/flu\/symptoms-causes\/syc-20351719<\/p>\r\n
Mayo Clinic Staff. (n.d.). Salmonella infection [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/salmonella\/symptoms-causes\/syc-20355329<\/p>\r\n
Mayo Clinic Staff. (n.d.). Staph infection [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/staph-infections\/multimedia\/staph-infection\/img-20008600<\/p>\r\n
Mayo Clinic Staff. (n.d.). Tuberculosis [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/tuberculosis\/symptoms-causes\/syc-20351250<\/p>\r\n
OpenStax. (2016, November 11). Figure\u00a0<\/span>17.23<\/span> A typical case of acute inflammation at the site of a skin wound - Erythema [digital image]. In OpenStax, Microbiology<\/em> (Section 17.5). https:\/\/openstax.org\/details\/books\/microbiology?Bookdetails<\/p>\r\nTED. (2014, October 14). Myriam Sidibe: The simple power of handwashing. YouTube. https:\/\/www.youtube.com\/watch?v=c64M1tZyWPM&feature=youtu.be<\/p>\r\n
TED-Ed. (2015, November 5). How mucus keeps us healthy - Katharina Ribbeck. YouTube. https:\/\/www.youtube.com\/watch?v=WW4skW6gucU&feature=youtu.be<\/p>","rendered":"
<\/p>\nFigure 17.4.1 Darn it!\u00a0 Paper cuts are the worst!<\/em><\/figcaption><\/figure>\n\nPaper Cut<\/h1>\n<\/div>\n
It\u2019s just a paper cut, but the break in your skin could provide an easy way for pathogens<\/span><\/a><\/span>\u00a0to enter your body. If\u00a0bacteria<\/a>\u00a0were to enter through the cut and infect the wound, your innate immune system would quickly respond with a dizzying array of general defenses.<\/span><\/p>\n\nWhat Is the Innate Immune System?<\/h1>\n<\/div>\n
The\u00a0innate immune system<\/a><\/strong>\u00a0is a subset of the human immune system that produces rapid, but non-specific responses to pathogens. Innate responses are generic, rather than tailored to a particular\u00a0pathogen. The innate system responds in the same general way to every pathogen\u00a0it encounters. Although the innate immune system provides immediate and rapid defenses against pathogens, it does not confer long-lasting\u00a0immunity\u00a0to them. In most organisms, the innate immune system is the dominant system of host defense. Other than most vertebrates (including humans), the innate immune system is the only system of host defense.<\/p>\nIn humans, the innate immune system includes surface barriers, inflammation, the complement system, and a variety of cellular responses. Surface barriers of various types generally keep most pathogens out of the body. If these barriers fail, then other innate defenses are triggered. The triggering event is usually the identification of pathogens by pattern-recognition receptors on\u00a0cells\u00a0of the innate immune system. These receptors recognize molecules that are broadly shared by pathogens, but distinguishable from host molecules. Alternatively, the other innate defenses may be triggered when damaged, injured, or stressed cells send out alarm signals, many of which are recognized by the same receptors as those that recognize pathogens.<\/p>\n
\nBarriers to Pathogens<\/h1>\n<\/div>\n
The body\u2019s first line of defense consists of three different types of barriers that keep most pathogens out of\u00a0body tissues. The types of barriers are mechanical, chemical, and biological barriers.<\/p>\n
Mechanical Barriers<\/h2>\nFigure 17.4.2 Nasal hairs are a mechanical barrier to larger particles in the air.<\/em><\/figcaption><\/figure>\nMechanical barriers<\/a> are the first line of defense against pathogens, and they physically block pathogens from entering the body. The skin<\/a> is the most important mechanical barrier. In fact, it is the single most important defense the body has. The outer layer of skin \u2014 the\u00a0epidermis<\/a>\u00a0\u2014 is tough, and very difficult for pathogens to penetrate. It consists of dead\u00a0cells\u00a0that are constantly shed from the body surface, a process that helps remove\u00a0bacteria\u00a0and other infectious agents that have adhered to the skin. The epidermis also lacks\u00a0blood vessels\u00a0and is usually lacking moisture, so it does not provide a suitable environment for most pathogens.\u00a0Hair<\/a> \u2014 which is an accessory organ of the skin \u2014 also helps keep out pathogens. Hairs inside the nose may trap larger pathogens and other particles in the air before they can enter the airways of the respiratory system (see Figure 17.4.2).<\/p>\nFigure 17.4.3 A sneeze can expel many pathogens from the respiratory tract, which is why you should always cover your mouth and nose and when you sneeze.<\/em><\/figcaption><\/figure>\nMucous membranes<\/span><\/a><\/span> provide a mechanical barrier to pathogens and other particles at body openings. These membranes also line the respiratory, gastrointestinal, urinary, and reproductive tracts. Mucous membranes secrete mucus, which is a slimy and somewhat sticky substance that traps pathogens. Many mucous membranes also have hair-like cilia<\/a> that sweep mucus and trapped pathogens toward body openings, where they can be removed from the body. When you sneeze or cough, mucus and pathogens are mechanically ejected from the nose and throat, as you can see in Figure 17.4.3.\u00a0 A sneeze can travel as fast as 160 Km\/hr (about 99 mi\/hour) and expel as many as 100,000 droplets into the air around you (a good reason to cover your sneezes!). Other mechanical defenses include tears, which wash pathogens from the eyes, and urine, which flushes pathogens out of the urinary tract.<\/span><\/p>\nChemical Barriers<\/h2>\n
Chemical barriers also protect against infection by pathogens. They destroy pathogens on the outer body surface, at body openings, and on inner body linings. Sweat, mucus, tears, saliva, and breastmilk all contain antimicrobial substances (such as the\u00a0enzyme\u00a0lysozyme) that kill pathogens, especially\u00a0bacteria. Sebaceous glands in the\u00a0dermis\u00a0of the skin secrete acids that form a very fine, slightly acidic film on the surface of the skin. This film acts as a barrier to bacteria, viruses, and other potential contaminants that might penetrate the skin. Urine and vaginal secretions are also too acidic for many pathogens to endure. Semen contains zinc \u2014 which most pathogens cannot tolerate \u2014 as well as defensins, which are antimicrobial\u00a0proteins\u00a0that act mainly by disrupting bacterial cell membranes. In the stomach, stomach\u00a0acid\u00a0and digestive\u00a0enzymes\u00a0called proteases (which break down proteins) kill most of the pathogens that enter the gastrointestinal tract in food or\u00a0water.<\/p>\n
Biological Barriers<\/h2>\n
Biological barriers are living organisms that help protect the body from pathogens. Trillions of harmless bacteria normally live on the human skin and in the urinary, reproductive, and gastrointestinal tracts. These bacteria use up food and surface space that help prevent pathogenic bacteria from colonizing the body. Some of these harmless bacteria also secrete substances that change the conditions of their environment, making it less hospitable to potentially harmful bacteria.\u00a0They may release toxins or change the\u00a0pH, for example. All of these effects of harmless bacteria reduce the chances that pathogenic microorganisms will be able to reach sufficient numbers and cause illness.<\/p>\n
\nInflammation<\/h1>\n<\/div>\n
If pathogens manage to breach the barriers protecting the body, one of the first active responses of the innate immune system kicks in. This response is\u00a0inflammation<\/a>.<\/strong>\u00a0The main function of inflammation is to establish a physical barrier against the spread of infection. It also eliminates the initial cause of cell injury, clears out dead\u00a0cells\u00a0and tissues damaged from the original insult and the inflammatory process, and initiates tissue repair. Inflammation is often a response to infection by pathogens, but there are other possible causes, including burns, frostbite, and exposure to toxins.<\/p>\nThe signs and symptoms of inflammation include redness, swelling, warmth, pain, and frequently some loss of function. These symptoms are caused by increased blood flow into infected tissue, and a number of other processes, illustrated in Figure 17.4.4.<\/p>\nFigure 17.4.4 This drawing shows what happens during the inflammatory response.<\/em><\/figcaption><\/figure>\nInflammation is triggered by chemicals such as\u00a0cytokines<\/a><\/strong>\u00a0and\u00a0histamines<\/a>,<\/strong>which are released by injured or infected cells, or by immune system cells such as macrophages (described below) that are already present in tissues. These chemicals cause capillaries to dilate and become leaky, increasing blood flow to the infected area and allowing blood to enter the tissues. Pathogen-destroying leukocytes and tissue-repairing proteins migrate into tissue spaces from the bloodstream to attack pathogens and repair their damage. Cytokines also promote chemotaxis<\/a>, which is migration to the site of infection by pathogen-destroying leukocytes. Some cytokines have anti-viral effects. They may shut down protein synthesis in host cells, which viruses need in order to survive and replicate.<\/p>\nSee the video “The inflammatory response” by Neural Academy to learn about inflammatory response in more detail:<\/p>\n
Figure 17.4.8 The dental plaque on the top set of teeth is a biofilm that sticks to the teeth and consists of many species of bacteria. The plaque biofilm is difficult to remove, and it subjects the teeth and gums to high concentrations of bacterial metabolites, which result in dental disease. The same teeth after plaque removal are shown in the bottom picture.<\/em>[\/caption]\r\n\r\n17.4 Summary<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- The [pb_glossary id=\"4820\"]innate immune system[\/pb_glossary] is a subset of the human immune system that produces rapid, but non-specific responses to pathogens. Unlike the\u00a0[pb_glossary id=\"4822\"]adaptive immune system[\/pb_glossary], the innate system does not confer\u00a0[pb_glossary id=\"4884\"]immunity[\/pb_glossary]. The innate immune system includes surface barriers, [pb_glossary id=\"4860\"]inflammation[\/pb_glossary], the [pb_glossary id=\"4868\"]complement system[\/pb_glossary], and a variety of cellular responses.<\/li>\r\n \t
- The body\u2019s first line of defense consists of three different types of barriers that keep most pathogens out of\u00a0body tissues. The types of barriers are mechanical, chemical, and biological barriers.<\/li>\r\n \t
- Mechanical barriers\u00a0\u2014\u00a0which include the [pb_glossary id=\"3521\"]skin[\/pb_glossary], [pb_glossary id=\"3538\"]mucous membranes[\/pb_glossary], and fluids such as tears and [pb_glossary id=\"4717\"]urine[\/pb_glossary] \u2014 physically block pathogens from entering the body. Chemical barriers\u00a0\u2014\u00a0such as enzymes in [pb_glossary id=\"4711\"]sweat[\/pb_glossary], [pb_glossary id=\"4595\"]saliva[\/pb_glossary], and [pb_glossary id=\"4885\"]semen[\/pb_glossary]\u00a0\u2014\u00a0kill pathogens on body surfaces. Biological barriers are harmless bacteria that use up food and space so pathogenic bacteria cannot colonize the body.<\/li>\r\n \t
- If pathogens breach protective barriers, [pb_glossary id=\"4860\"]inflammation[\/pb_glossary] occurs. This creates a physical barrier against the spread of infection, and repairs tissue damage. Inflammation is triggered by chemicals such as [pb_glossary id=\"4865\"]cytokines[\/pb_glossary]\u00a0and [pb_glossary id=\"4879\"]histamines[\/pb_glossary], and it causes swelling, redness, and warmth.<\/li>\r\n \t
- The [pb_glossary id=\"4868\"]complement system[\/pb_glossary] is a complex biochemical mechanism that helps [pb_glossary id=\"5659\"]antibodies[\/pb_glossary] kill pathogens. Once activated, the complement system consists of more than two dozen proteins that lead to disruption of the\u00a0cell membrane\u00a0of pathogens and bursting of the cells.<\/li>\r\n \t
- Cellular responses of the innate immune system involve various types of [pb_glossary id=\"5623\"]leukocytes[\/pb_glossary]. For example, [pb_glossary id=\"5545\"]neutrophils[\/pb_glossary], [pb_glossary id=\"4340\"]macrophages[\/pb_glossary], and [pb_glossary id=\"4875\"]dendritic cells[\/pb_glossary]\u00a0phagocytize pathogens. [pb_glossary id=\"5573\"]Basophils[\/pb_glossary]\u00a0and [pb_glossary id=\"4881\"]mast cells[\/pb_glossary]\u00a0release chemicals that trigger inflammation. [pb_glossary id=\"5515\"]Natural killer cells[\/pb_glossary]\u00a0destroy cancerous or virus-infected cells, and eosinophils kill parasites.<\/li>\r\n \t
- Many pathogens have evolved mechanisms that help them evade the innate immune system. For example, some pathogens form a protective capsule around themselves, and some mimic host cells so the immune system does not recognize them as foreign.<\/li>\r\n<\/ul>\r\n<\/div>\r\n<\/div>\r\n
\r\n17.4 Review Questions<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- What is the innate immune system?<\/li>\r\n \t
- Identify the body\u2019s first line of defense.<\/li>\r\n \t
- [h5p id=\"638\"]<\/li>\r\n \t
- What are biological barriers? How do they protect the body?<\/li>\r\n \t
- State the purposes of inflammation. What triggers inflammation, and what signs and symptoms does it cause?<\/li>\r\n \t
- Define the complement system. How does it help destroy pathogens?<\/li>\r\n \t
- Describe two ways that pathogens can evade the innate immune system.<\/li>\r\n \t
- What are the ways in which phagocytes can encounter pathogens in the body?<\/li>\r\n \t
- Describe two different ways in which enzymes play a role in the innate immune response.<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n
\r\n17.4 Explore More<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\nhttps:\/\/youtu.be\/WW4skW6gucU\r\nHow mucus keeps us healthy - Katharina Ribbeck, TED-Ed, 2015.<\/p>\r\nhttps:\/\/youtu.be\/sYjtMP67vyk\r\n
Human Physiology - Innate Immune System, Janux, 2015.<\/p>\r\nhttps:\/\/youtu.be\/c64M1tZyWPM\r\n
Myriam Sidibe: The simple power of handwashing, TED, 2014.<\/p>\r\nhttps:\/\/youtu.be\/shEPwQPQG4I\r\n
Everything You Didn't Want To Know About Snot, Gross Science, 2017.<\/p>\r\nhttps:\/\/youtu.be\/dy1D3d1FBcw\r\n
Cough Grosser Than Sneeze? | Curiosity - World's Dirtiest Man, Discovery, 2011.<\/p>\r\n\r\n<\/div>\r\n<\/div>\r\n \r\n\r\n<\/div>\r\n
Attributions<\/h2>\r\nFigure 17.4.1<\/strong>\r\n\r\nOww_Papercut_14365<\/a> by Laurence Facun<\/a> on Wikimedia Commons is used under a CC BY 2.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/2.0) license.\r\n\r\nFigure 17.4.2<\/strong>\r\n\r\nhairy-nose<\/a> by Piotr Siedlecki\u00a0 on publicdomainpictures.net<\/a> is used under a CC0 1.0<\/a> Universal Public Domain Dedication (http:\/\/creativecommons.org\/publicdomain\/zero\/1.0\/) license.\r\n\r\nFigure 17.4.3<\/strong>\r\n\r\n1024px-Sneeze<\/a> by James Gathany\/ CDC Public Health Image library (PHIL) ID# 11162<\/a> on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/public_domain).\r\n\r\nFigure 17.4.4<\/strong>\r\n\r\nOSC_Microbio_17_06_Erythema<\/a> by CNX OpenStax<\/a> on Wikimedia Commons is used under a CC BY 4.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/4.0) license.\r\n\r\nFigure 17.4.5<\/strong>\r\n\r\n2212_Complement_Cascade_and_Function<\/a>\u00a0by OpenStax College<\/a> on Wikimedia Commons is used under a\u00a0CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\r\n\r\nFigure 17.4.6<\/strong>\r\n\r\n512px-Phagocytosis2<\/a> by Graham\u00a0Colm<\/span><\/a> at English Wikipedia<\/a><\/span>\u00a0on Wikimedia Commons is used under a CC BY-SA 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-sa\/3.0) license.\r\n\r\nFigure 17.4.7<\/strong>\r\n\r\nInnate_Immune_cells.svg<\/a> by Fred the Oyster<\/a> on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).\r\n\r\nFigure 17.4.8<\/strong>\r\n\r\n1024px-Gingivitis-before-and-after-3<\/a> by Onetimeuseaccount<\/a> on Wikimedia Commons is used under a CC0 1.0<\/a> Universal Public Domain Dedication (http:\/\/creativecommons.org\/publicdomain\/zero\/1.0\/) license.\r\nReferences<\/h2>\r\n
Betts, J. G., Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E., Womble, M., DeSaix, P. (2013, June 19). Figure 21.13 Complement cascade and function [digital image].\u00a0 In Anatomy and Physiology<\/em> (Section 21.2). OpenStax. https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/21-2-barrier-defenses-and-the-innate-immune-response<\/p>\r\nDiscovery. (2011, October 27). Cough grosser than sneeze? | Curiosity - World's dirtiest man. YouTube. https:\/\/www.youtube.com\/watch?v=dy1D3d1FBcw&feature=youtu.be<\/p>\r\n
Gross Science. (2017, January 31). Everything you didn't want to know about snot. YouTube. https:\/\/www.youtube.com\/watch?v=shEPwQPQG4I&feature=youtu.be<\/p>\r\n
Janux. (2015, January 10). Human physiology - Innate immune system. YouTube. https:\/\/www.youtube.com\/watch?v=sYjtMP67vyk&feature=youtu.be<\/p>\r\n
Mayo Clinic Staff. (n.d.). Anthrax [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/anthrax\/symptoms-causes\/syc-20356203<\/p>\r\n
Mayo Clinic Staff. (n.d.). Influenza (flu) [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/flu\/symptoms-causes\/syc-20351719<\/p>\r\n
Mayo Clinic Staff. (n.d.). Salmonella infection [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/salmonella\/symptoms-causes\/syc-20355329<\/p>\r\n
Mayo Clinic Staff. (n.d.). Staph infection [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/staph-infections\/multimedia\/staph-infection\/img-20008600<\/p>\r\n
Mayo Clinic Staff. (n.d.). Tuberculosis [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/tuberculosis\/symptoms-causes\/syc-20351250<\/p>\r\n
OpenStax. (2016, November 11). Figure\u00a0<\/span>17.23<\/span> A typical case of acute inflammation at the site of a skin wound - Erythema [digital image]. In OpenStax, Microbiology<\/em> (Section 17.5). https:\/\/openstax.org\/details\/books\/microbiology?Bookdetails<\/p>\r\nTED. (2014, October 14). Myriam Sidibe: The simple power of handwashing. YouTube. https:\/\/www.youtube.com\/watch?v=c64M1tZyWPM&feature=youtu.be<\/p>\r\n
TED-Ed. (2015, November 5). How mucus keeps us healthy - Katharina Ribbeck. YouTube. https:\/\/www.youtube.com\/watch?v=WW4skW6gucU&feature=youtu.be<\/p>","rendered":"
<\/p>\nFigure 17.4.1 Darn it!\u00a0 Paper cuts are the worst!<\/em><\/figcaption><\/figure>\n\nPaper Cut<\/h1>\n<\/div>\n
It\u2019s just a paper cut, but the break in your skin could provide an easy way for pathogens<\/span><\/a><\/span>\u00a0to enter your body. If\u00a0bacteria<\/a>\u00a0were to enter through the cut and infect the wound, your innate immune system would quickly respond with a dizzying array of general defenses.<\/span><\/p>\n\nWhat Is the Innate Immune System?<\/h1>\n<\/div>\n
The\u00a0innate immune system<\/a><\/strong>\u00a0is a subset of the human immune system that produces rapid, but non-specific responses to pathogens. Innate responses are generic, rather than tailored to a particular\u00a0pathogen. The innate system responds in the same general way to every pathogen\u00a0it encounters. Although the innate immune system provides immediate and rapid defenses against pathogens, it does not confer long-lasting\u00a0immunity\u00a0to them. In most organisms, the innate immune system is the dominant system of host defense. Other than most vertebrates (including humans), the innate immune system is the only system of host defense.<\/p>\nIn humans, the innate immune system includes surface barriers, inflammation, the complement system, and a variety of cellular responses. Surface barriers of various types generally keep most pathogens out of the body. If these barriers fail, then other innate defenses are triggered. The triggering event is usually the identification of pathogens by pattern-recognition receptors on\u00a0cells\u00a0of the innate immune system. These receptors recognize molecules that are broadly shared by pathogens, but distinguishable from host molecules. Alternatively, the other innate defenses may be triggered when damaged, injured, or stressed cells send out alarm signals, many of which are recognized by the same receptors as those that recognize pathogens.<\/p>\n
\nBarriers to Pathogens<\/h1>\n<\/div>\n
The body\u2019s first line of defense consists of three different types of barriers that keep most pathogens out of\u00a0body tissues. The types of barriers are mechanical, chemical, and biological barriers.<\/p>\n
Mechanical Barriers<\/h2>\nFigure 17.4.2 Nasal hairs are a mechanical barrier to larger particles in the air.<\/em><\/figcaption><\/figure>\nMechanical barriers<\/a> are the first line of defense against pathogens, and they physically block pathogens from entering the body. The skin<\/a> is the most important mechanical barrier. In fact, it is the single most important defense the body has. The outer layer of skin \u2014 the\u00a0epidermis<\/a>\u00a0\u2014 is tough, and very difficult for pathogens to penetrate. It consists of dead\u00a0cells\u00a0that are constantly shed from the body surface, a process that helps remove\u00a0bacteria\u00a0and other infectious agents that have adhered to the skin. The epidermis also lacks\u00a0blood vessels\u00a0and is usually lacking moisture, so it does not provide a suitable environment for most pathogens.\u00a0Hair<\/a> \u2014 which is an accessory organ of the skin \u2014 also helps keep out pathogens. Hairs inside the nose may trap larger pathogens and other particles in the air before they can enter the airways of the respiratory system (see Figure 17.4.2).<\/p>\nFigure 17.4.3 A sneeze can expel many pathogens from the respiratory tract, which is why you should always cover your mouth and nose and when you sneeze.<\/em><\/figcaption><\/figure>\nMucous membranes<\/span><\/a><\/span> provide a mechanical barrier to pathogens and other particles at body openings. These membranes also line the respiratory, gastrointestinal, urinary, and reproductive tracts. Mucous membranes secrete mucus, which is a slimy and somewhat sticky substance that traps pathogens. Many mucous membranes also have hair-like cilia<\/a> that sweep mucus and trapped pathogens toward body openings, where they can be removed from the body. When you sneeze or cough, mucus and pathogens are mechanically ejected from the nose and throat, as you can see in Figure 17.4.3.\u00a0 A sneeze can travel as fast as 160 Km\/hr (about 99 mi\/hour) and expel as many as 100,000 droplets into the air around you (a good reason to cover your sneezes!). Other mechanical defenses include tears, which wash pathogens from the eyes, and urine, which flushes pathogens out of the urinary tract.<\/span><\/p>\nChemical Barriers<\/h2>\n
Chemical barriers also protect against infection by pathogens. They destroy pathogens on the outer body surface, at body openings, and on inner body linings. Sweat, mucus, tears, saliva, and breastmilk all contain antimicrobial substances (such as the\u00a0enzyme\u00a0lysozyme) that kill pathogens, especially\u00a0bacteria. Sebaceous glands in the\u00a0dermis\u00a0of the skin secrete acids that form a very fine, slightly acidic film on the surface of the skin. This film acts as a barrier to bacteria, viruses, and other potential contaminants that might penetrate the skin. Urine and vaginal secretions are also too acidic for many pathogens to endure. Semen contains zinc \u2014 which most pathogens cannot tolerate \u2014 as well as defensins, which are antimicrobial\u00a0proteins\u00a0that act mainly by disrupting bacterial cell membranes. In the stomach, stomach\u00a0acid\u00a0and digestive\u00a0enzymes\u00a0called proteases (which break down proteins) kill most of the pathogens that enter the gastrointestinal tract in food or\u00a0water.<\/p>\n
Biological Barriers<\/h2>\n
Biological barriers are living organisms that help protect the body from pathogens. Trillions of harmless bacteria normally live on the human skin and in the urinary, reproductive, and gastrointestinal tracts. These bacteria use up food and surface space that help prevent pathogenic bacteria from colonizing the body. Some of these harmless bacteria also secrete substances that change the conditions of their environment, making it less hospitable to potentially harmful bacteria.\u00a0They may release toxins or change the\u00a0pH, for example. All of these effects of harmless bacteria reduce the chances that pathogenic microorganisms will be able to reach sufficient numbers and cause illness.<\/p>\n
\nInflammation<\/h1>\n<\/div>\n
If pathogens manage to breach the barriers protecting the body, one of the first active responses of the innate immune system kicks in. This response is\u00a0inflammation<\/a>.<\/strong>\u00a0The main function of inflammation is to establish a physical barrier against the spread of infection. It also eliminates the initial cause of cell injury, clears out dead\u00a0cells\u00a0and tissues damaged from the original insult and the inflammatory process, and initiates tissue repair. Inflammation is often a response to infection by pathogens, but there are other possible causes, including burns, frostbite, and exposure to toxins.<\/p>\nThe signs and symptoms of inflammation include redness, swelling, warmth, pain, and frequently some loss of function. These symptoms are caused by increased blood flow into infected tissue, and a number of other processes, illustrated in Figure 17.4.4.<\/p>\nFigure 17.4.4 This drawing shows what happens during the inflammatory response.<\/em><\/figcaption><\/figure>\nInflammation is triggered by chemicals such as\u00a0cytokines<\/a><\/strong>\u00a0and\u00a0histamines<\/a>,<\/strong>which are released by injured or infected cells, or by immune system cells such as macrophages (described below) that are already present in tissues. These chemicals cause capillaries to dilate and become leaky, increasing blood flow to the infected area and allowing blood to enter the tissues. Pathogen-destroying leukocytes and tissue-repairing proteins migrate into tissue spaces from the bloodstream to attack pathogens and repair their damage. Cytokines also promote chemotaxis<\/a>, which is migration to the site of infection by pathogen-destroying leukocytes. Some cytokines have anti-viral effects. They may shut down protein synthesis in host cells, which viruses need in order to survive and replicate.<\/p>\nSee the video “The inflammatory response” by Neural Academy to learn about inflammatory response in more detail:<\/p>\n
- \r\n \t
- The [pb_glossary id=\"4820\"]innate immune system[\/pb_glossary] is a subset of the human immune system that produces rapid, but non-specific responses to pathogens. Unlike the\u00a0[pb_glossary id=\"4822\"]adaptive immune system[\/pb_glossary], the innate system does not confer\u00a0[pb_glossary id=\"4884\"]immunity[\/pb_glossary]. The innate immune system includes surface barriers, [pb_glossary id=\"4860\"]inflammation[\/pb_glossary], the [pb_glossary id=\"4868\"]complement system[\/pb_glossary], and a variety of cellular responses.<\/li>\r\n \t
- The body\u2019s first line of defense consists of three different types of barriers that keep most pathogens out of\u00a0body tissues. The types of barriers are mechanical, chemical, and biological barriers.<\/li>\r\n \t
- Mechanical barriers\u00a0\u2014\u00a0which include the [pb_glossary id=\"3521\"]skin[\/pb_glossary], [pb_glossary id=\"3538\"]mucous membranes[\/pb_glossary], and fluids such as tears and [pb_glossary id=\"4717\"]urine[\/pb_glossary] \u2014 physically block pathogens from entering the body. Chemical barriers\u00a0\u2014\u00a0such as enzymes in [pb_glossary id=\"4711\"]sweat[\/pb_glossary], [pb_glossary id=\"4595\"]saliva[\/pb_glossary], and [pb_glossary id=\"4885\"]semen[\/pb_glossary]\u00a0\u2014\u00a0kill pathogens on body surfaces. Biological barriers are harmless bacteria that use up food and space so pathogenic bacteria cannot colonize the body.<\/li>\r\n \t
- If pathogens breach protective barriers, [pb_glossary id=\"4860\"]inflammation[\/pb_glossary] occurs. This creates a physical barrier against the spread of infection, and repairs tissue damage. Inflammation is triggered by chemicals such as [pb_glossary id=\"4865\"]cytokines[\/pb_glossary]\u00a0and [pb_glossary id=\"4879\"]histamines[\/pb_glossary], and it causes swelling, redness, and warmth.<\/li>\r\n \t
- The [pb_glossary id=\"4868\"]complement system[\/pb_glossary] is a complex biochemical mechanism that helps [pb_glossary id=\"5659\"]antibodies[\/pb_glossary] kill pathogens. Once activated, the complement system consists of more than two dozen proteins that lead to disruption of the\u00a0cell membrane\u00a0of pathogens and bursting of the cells.<\/li>\r\n \t
- Cellular responses of the innate immune system involve various types of [pb_glossary id=\"5623\"]leukocytes[\/pb_glossary]. For example, [pb_glossary id=\"5545\"]neutrophils[\/pb_glossary], [pb_glossary id=\"4340\"]macrophages[\/pb_glossary], and [pb_glossary id=\"4875\"]dendritic cells[\/pb_glossary]\u00a0phagocytize pathogens. [pb_glossary id=\"5573\"]Basophils[\/pb_glossary]\u00a0and [pb_glossary id=\"4881\"]mast cells[\/pb_glossary]\u00a0release chemicals that trigger inflammation. [pb_glossary id=\"5515\"]Natural killer cells[\/pb_glossary]\u00a0destroy cancerous or virus-infected cells, and eosinophils kill parasites.<\/li>\r\n \t
- Many pathogens have evolved mechanisms that help them evade the innate immune system. For example, some pathogens form a protective capsule around themselves, and some mimic host cells so the immune system does not recognize them as foreign.<\/li>\r\n<\/ul>\r\n<\/div>\r\n<\/div>\r\n
\r\n 17.4 Review Questions<\/span><\/h1>\r\n<\/header>\r\n
\r\n- \r\n \t
- What is the innate immune system?<\/li>\r\n \t
- Identify the body\u2019s first line of defense.<\/li>\r\n \t
- [h5p id=\"638\"]<\/li>\r\n \t
- What are biological barriers? How do they protect the body?<\/li>\r\n \t
- State the purposes of inflammation. What triggers inflammation, and what signs and symptoms does it cause?<\/li>\r\n \t
- Define the complement system. How does it help destroy pathogens?<\/li>\r\n \t
- Describe two ways that pathogens can evade the innate immune system.<\/li>\r\n \t
- What are the ways in which phagocytes can encounter pathogens in the body?<\/li>\r\n \t
- Describe two different ways in which enzymes play a role in the innate immune response.<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n
\r\n 17.4 Explore More<\/span><\/h1>\r\n<\/header>\r\n
\r\n\r\nhttps:\/\/youtu.be\/WW4skW6gucU\r\nHow mucus keeps us healthy - Katharina Ribbeck, TED-Ed, 2015.<\/p>\r\nhttps:\/\/youtu.be\/sYjtMP67vyk\r\n
Human Physiology - Innate Immune System, Janux, 2015.<\/p>\r\nhttps:\/\/youtu.be\/c64M1tZyWPM\r\n
Myriam Sidibe: The simple power of handwashing, TED, 2014.<\/p>\r\nhttps:\/\/youtu.be\/shEPwQPQG4I\r\n
Everything You Didn't Want To Know About Snot, Gross Science, 2017.<\/p>\r\nhttps:\/\/youtu.be\/dy1D3d1FBcw\r\n
Cough Grosser Than Sneeze? | Curiosity - World's Dirtiest Man, Discovery, 2011.<\/p>\r\n\r\n<\/div>\r\n<\/div>\r\n \r\n\r\n<\/div>\r\n
Attributions<\/h2>\r\nFigure 17.4.1<\/strong>\r\n\r\nOww_Papercut_14365<\/a> by Laurence Facun<\/a> on Wikimedia Commons is used under a CC BY 2.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/2.0) license.\r\n\r\nFigure 17.4.2<\/strong>\r\n\r\nhairy-nose<\/a> by Piotr Siedlecki\u00a0 on publicdomainpictures.net<\/a> is used under a CC0 1.0<\/a> Universal Public Domain Dedication (http:\/\/creativecommons.org\/publicdomain\/zero\/1.0\/) license.\r\n\r\nFigure 17.4.3<\/strong>\r\n\r\n1024px-Sneeze<\/a> by James Gathany\/ CDC Public Health Image library (PHIL) ID# 11162<\/a> on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/public_domain).\r\n\r\nFigure 17.4.4<\/strong>\r\n\r\nOSC_Microbio_17_06_Erythema<\/a> by CNX OpenStax<\/a> on Wikimedia Commons is used under a CC BY 4.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/4.0) license.\r\n\r\nFigure 17.4.5<\/strong>\r\n\r\n2212_Complement_Cascade_and_Function<\/a>\u00a0by OpenStax College<\/a> on Wikimedia Commons is used under a\u00a0CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\r\n\r\nFigure 17.4.6<\/strong>\r\n\r\n512px-Phagocytosis2<\/a> by Graham\u00a0Colm<\/span><\/a> at English Wikipedia<\/a><\/span>\u00a0on Wikimedia Commons is used under a CC BY-SA 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-sa\/3.0) license.\r\n\r\nFigure 17.4.7<\/strong>\r\n\r\nInnate_Immune_cells.svg<\/a> by Fred the Oyster<\/a> on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).\r\n\r\nFigure 17.4.8<\/strong>\r\n\r\n1024px-Gingivitis-before-and-after-3<\/a> by Onetimeuseaccount<\/a> on Wikimedia Commons is used under a CC0 1.0<\/a> Universal Public Domain Dedication (http:\/\/creativecommons.org\/publicdomain\/zero\/1.0\/) license.\r\n
References<\/h2>\r\n
Betts, J. G., Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E., Womble, M., DeSaix, P. (2013, June 19). Figure 21.13 Complement cascade and function [digital image].\u00a0 In Anatomy and Physiology<\/em> (Section 21.2). OpenStax. https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/21-2-barrier-defenses-and-the-innate-immune-response<\/p>\r\n
Discovery. (2011, October 27). Cough grosser than sneeze? | Curiosity - World's dirtiest man. YouTube. https:\/\/www.youtube.com\/watch?v=dy1D3d1FBcw&feature=youtu.be<\/p>\r\n
Gross Science. (2017, January 31). Everything you didn't want to know about snot. YouTube. https:\/\/www.youtube.com\/watch?v=shEPwQPQG4I&feature=youtu.be<\/p>\r\n
Janux. (2015, January 10). Human physiology - Innate immune system. YouTube. https:\/\/www.youtube.com\/watch?v=sYjtMP67vyk&feature=youtu.be<\/p>\r\n
Mayo Clinic Staff. (n.d.). Anthrax [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/anthrax\/symptoms-causes\/syc-20356203<\/p>\r\n
Mayo Clinic Staff. (n.d.). Influenza (flu) [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/flu\/symptoms-causes\/syc-20351719<\/p>\r\n
Mayo Clinic Staff. (n.d.). Salmonella infection [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/salmonella\/symptoms-causes\/syc-20355329<\/p>\r\n
Mayo Clinic Staff. (n.d.). Staph infection [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/staph-infections\/multimedia\/staph-infection\/img-20008600<\/p>\r\n
Mayo Clinic Staff. (n.d.). Tuberculosis [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/tuberculosis\/symptoms-causes\/syc-20351250<\/p>\r\n
OpenStax. (2016, November 11). Figure\u00a0<\/span>17.23<\/span> A typical case of acute inflammation at the site of a skin wound - Erythema [digital image]. In OpenStax, Microbiology<\/em> (Section 17.5). https:\/\/openstax.org\/details\/books\/microbiology?Bookdetails<\/p>\r\n
TED. (2014, October 14). Myriam Sidibe: The simple power of handwashing. YouTube. https:\/\/www.youtube.com\/watch?v=c64M1tZyWPM&feature=youtu.be<\/p>\r\n
TED-Ed. (2015, November 5). How mucus keeps us healthy - Katharina Ribbeck. YouTube. https:\/\/www.youtube.com\/watch?v=WW4skW6gucU&feature=youtu.be<\/p>","rendered":"
<\/p>\n
Figure 17.4.1 Darn it!\u00a0 Paper cuts are the worst!<\/em><\/figcaption><\/figure>\n \nPaper Cut<\/h1>\n<\/div>\n
It\u2019s just a paper cut, but the break in your skin could provide an easy way for pathogens<\/span><\/a><\/span>\u00a0to enter your body. If\u00a0bacteria<\/a>\u00a0were to enter through the cut and infect the wound, your innate immune system would quickly respond with a dizzying array of general defenses.<\/span><\/p>\n
\nWhat Is the Innate Immune System?<\/h1>\n<\/div>\n
The\u00a0innate immune system<\/a><\/strong>\u00a0is a subset of the human immune system that produces rapid, but non-specific responses to pathogens. Innate responses are generic, rather than tailored to a particular\u00a0pathogen. The innate system responds in the same general way to every pathogen\u00a0it encounters. Although the innate immune system provides immediate and rapid defenses against pathogens, it does not confer long-lasting\u00a0immunity\u00a0to them. In most organisms, the innate immune system is the dominant system of host defense. Other than most vertebrates (including humans), the innate immune system is the only system of host defense.<\/p>\n
In humans, the innate immune system includes surface barriers, inflammation, the complement system, and a variety of cellular responses. Surface barriers of various types generally keep most pathogens out of the body. If these barriers fail, then other innate defenses are triggered. The triggering event is usually the identification of pathogens by pattern-recognition receptors on\u00a0cells\u00a0of the innate immune system. These receptors recognize molecules that are broadly shared by pathogens, but distinguishable from host molecules. Alternatively, the other innate defenses may be triggered when damaged, injured, or stressed cells send out alarm signals, many of which are recognized by the same receptors as those that recognize pathogens.<\/p>\n
\nBarriers to Pathogens<\/h1>\n<\/div>\n
The body\u2019s first line of defense consists of three different types of barriers that keep most pathogens out of\u00a0body tissues. The types of barriers are mechanical, chemical, and biological barriers.<\/p>\n
Mechanical Barriers<\/h2>\n
Figure 17.4.2 Nasal hairs are a mechanical barrier to larger particles in the air.<\/em><\/figcaption><\/figure>\n Mechanical barriers<\/a> are the first line of defense against pathogens, and they physically block pathogens from entering the body. The skin<\/a> is the most important mechanical barrier. In fact, it is the single most important defense the body has. The outer layer of skin \u2014 the\u00a0epidermis<\/a>\u00a0\u2014 is tough, and very difficult for pathogens to penetrate. It consists of dead\u00a0cells\u00a0that are constantly shed from the body surface, a process that helps remove\u00a0bacteria\u00a0and other infectious agents that have adhered to the skin. The epidermis also lacks\u00a0blood vessels\u00a0and is usually lacking moisture, so it does not provide a suitable environment for most pathogens.\u00a0Hair<\/a> \u2014 which is an accessory organ of the skin \u2014 also helps keep out pathogens. Hairs inside the nose may trap larger pathogens and other particles in the air before they can enter the airways of the respiratory system (see Figure 17.4.2).<\/p>\n
Figure 17.4.3 A sneeze can expel many pathogens from the respiratory tract, which is why you should always cover your mouth and nose and when you sneeze.<\/em><\/figcaption><\/figure>\n Mucous membranes<\/span><\/a><\/span> provide a mechanical barrier to pathogens and other particles at body openings. These membranes also line the respiratory, gastrointestinal, urinary, and reproductive tracts. Mucous membranes secrete mucus, which is a slimy and somewhat sticky substance that traps pathogens. Many mucous membranes also have hair-like cilia<\/a> that sweep mucus and trapped pathogens toward body openings, where they can be removed from the body. When you sneeze or cough, mucus and pathogens are mechanically ejected from the nose and throat, as you can see in Figure 17.4.3.\u00a0 A sneeze can travel as fast as 160 Km\/hr (about 99 mi\/hour) and expel as many as 100,000 droplets into the air around you (a good reason to cover your sneezes!). Other mechanical defenses include tears, which wash pathogens from the eyes, and urine, which flushes pathogens out of the urinary tract.<\/span><\/p>\n
Chemical Barriers<\/h2>\n
Chemical barriers also protect against infection by pathogens. They destroy pathogens on the outer body surface, at body openings, and on inner body linings. Sweat, mucus, tears, saliva, and breastmilk all contain antimicrobial substances (such as the\u00a0enzyme\u00a0lysozyme) that kill pathogens, especially\u00a0bacteria. Sebaceous glands in the\u00a0dermis\u00a0of the skin secrete acids that form a very fine, slightly acidic film on the surface of the skin. This film acts as a barrier to bacteria, viruses, and other potential contaminants that might penetrate the skin. Urine and vaginal secretions are also too acidic for many pathogens to endure. Semen contains zinc \u2014 which most pathogens cannot tolerate \u2014 as well as defensins, which are antimicrobial\u00a0proteins\u00a0that act mainly by disrupting bacterial cell membranes. In the stomach, stomach\u00a0acid\u00a0and digestive\u00a0enzymes\u00a0called proteases (which break down proteins) kill most of the pathogens that enter the gastrointestinal tract in food or\u00a0water.<\/p>\n
Biological Barriers<\/h2>\n
Biological barriers are living organisms that help protect the body from pathogens. Trillions of harmless bacteria normally live on the human skin and in the urinary, reproductive, and gastrointestinal tracts. These bacteria use up food and surface space that help prevent pathogenic bacteria from colonizing the body. Some of these harmless bacteria also secrete substances that change the conditions of their environment, making it less hospitable to potentially harmful bacteria.\u00a0They may release toxins or change the\u00a0pH, for example. All of these effects of harmless bacteria reduce the chances that pathogenic microorganisms will be able to reach sufficient numbers and cause illness.<\/p>\n
\nInflammation<\/h1>\n<\/div>\n
If pathogens manage to breach the barriers protecting the body, one of the first active responses of the innate immune system kicks in. This response is\u00a0inflammation<\/a>.<\/strong>\u00a0The main function of inflammation is to establish a physical barrier against the spread of infection. It also eliminates the initial cause of cell injury, clears out dead\u00a0cells\u00a0and tissues damaged from the original insult and the inflammatory process, and initiates tissue repair. Inflammation is often a response to infection by pathogens, but there are other possible causes, including burns, frostbite, and exposure to toxins.<\/p>\n
The signs and symptoms of inflammation include redness, swelling, warmth, pain, and frequently some loss of function. These symptoms are caused by increased blood flow into infected tissue, and a number of other processes, illustrated in Figure 17.4.4.<\/p>\n
Figure 17.4.4 This drawing shows what happens during the inflammatory response.<\/em><\/figcaption><\/figure>\n Inflammation is triggered by chemicals such as\u00a0cytokines<\/a><\/strong>\u00a0and\u00a0histamines<\/a>,<\/strong>which are released by injured or infected cells, or by immune system cells such as macrophages (described below) that are already present in tissues. These chemicals cause capillaries to dilate and become leaky, increasing blood flow to the infected area and allowing blood to enter the tissues. Pathogen-destroying leukocytes and tissue-repairing proteins migrate into tissue spaces from the bloodstream to attack pathogens and repair their damage. Cytokines also promote chemotaxis<\/a>, which is migration to the site of infection by pathogen-destroying leukocytes. Some cytokines have anti-viral effects. They may shut down protein synthesis in host cells, which viruses need in order to survive and replicate.<\/p>\n
See the video “The inflammatory response” by Neural Academy to learn about inflammatory response in more detail:<\/p>\n
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