{"id":5264,"date":"2019-06-24T17:38:04","date_gmt":"2019-06-24T17:38:04","guid":{"rendered":"https:\/\/pressbooks.ccconline.org\/acchumanbio\/chapter\/19-2-introduction-to-the-immune-system-3\/"},"modified":"2023-11-30T23:14:27","modified_gmt":"2023-11-30T23:14:27","slug":"19-2-introduction-to-the-immune-system-3","status":"publish","type":"chapter","link":"https:\/\/pressbooks.ccconline.org\/acchumanbio\/chapter\/19-2-introduction-to-the-immune-system-3\/","title":{"raw":"17.2\u00a0Introduction to the Immune System","rendered":"17.2\u00a0Introduction to the Immune System"},"content":{"raw":" \r\n\r\n[caption id=\"attachment_4810\" align=\"aligncenter\" width=\"405\"]\"17.2.1 Figure 17.2.1 From your nightmares...the\u00a0Schistosoma worm.<\/em>[\/caption]\r\n\r\n
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Worm Attack!<\/h1>\r\n<\/div>\r\nDoes the organism in Figure 17.2.1 look like a space alien? A scary creature from a nightmare? In fact, it\u2019s a 1-cm long worm in the genus Schistosoma.<\/em>\u00a0It may invade and take up residence in the human body, causing a very serious illness known as [pb_glossary id=\"4811\"]schistosomiasis[\/pb_glossary]. The worm gains access to the human body while it is in a microscopic life stage. It enters through a hair follicle when the skin comes into contact with contaminated water. The worm then grows and matures inside the human organism, causing disease.\r\n
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Host vs. Pathogen<\/h1>\r\n<\/div>\r\nThe\u00a0Schistosoma<\/em>\u00a0worm has a parasitic relationship with humans. In this type of relationship, one organism, called the [pb_glossary id=\"4812\"]parasite[\/pb_glossary], lives on or in another organism, called the host. The parasite always benefits from the relationship, and the host is always harmed. The human host of the\u00a0Schistosoma<\/em>\u00a0worm is clearly harmed by the parasite when it invades the host\u2019s tissues. The urinary tract or intestines may be infected, and signs and symptoms may include abdominal pain, diarrhea, bloody stool, or blood in the urine. Those who have been infected for a long time may experience [pb_glossary id=\"2989\"]liver[\/pb_glossary] damage, [pb_glossary id=\"4782\"]kidney failure[\/pb_glossary], [pb_glossary id=\"4813\"]infertility[\/pb_glossary], or bladder [pb_glossary id=\"5605\"]cancer[\/pb_glossary]. In children,\u00a0Schistosoma<\/em>\u00a0infection may cause poor growth and difficulty learning.\r\n\r\nLike the\u00a0Schistosoma<\/em>\u00a0worm, many other organisms can make us sick if they manage to enter our body. Any such agent that can cause disease is called a\u00a0[pb_glossary id=\"5799\"]pathogen[\/pb_glossary].<\/strong>\u00a0Most pathogens are [pb_glossary id=\"5775\"]microorganisms<\/span>[\/pb_glossary]<\/span>, although some \u2014 such as the\u00a0<\/span>Schistosoma<\/em>\u00a0worm \u2014 are much larger. In addition to worms, common types of pathogens of human hosts include [pb_glossary id=\"2588\"]bacteria[\/pb_glossary], [pb_glossary id=\"4283\"]virus<\/span>es<\/span>[\/pb_glossary], fungi, and single-celled organisms called protists. You can see examples of each of these types of pathogens in Table 17.1.1. Fortunately for us, our immune system is able to keep most potential pathogens out of the body, or quickly destroy them if they do manage to get in. When you read this chapter, you\u2019ll learn how your immune system usually keeps you safe from harm \u2014 including from scary creatures like the <\/span>Schistosoma<\/em>\u00a0worm!<\/span>\r\n\r\n\r\n\r\n\r\n\r\n\r\n
Table 17.1.1: Types of Disease-Causing Pathogens<\/caption>\r\n
Type of Pathogen<\/th>\r\nDescription<\/th>\r\nDisease Caused<\/th>\r\n<\/tr>\r\n
Bacteria:<\/strong>\r\n\r\nExample shown: Escherichia coli<\/td>\r\n\"17.2a<\/td>\r\nSingle celled organisms without a nucleus<\/td>\r\nStrep throat, staph infections, tuberculosis, food poisoning, tetanus, pneumonia, syphillis<\/td>\r\n<\/tr>\r\n
Viruses:<\/strong>\r\n\r\nExample shown:\r\n\r\nHerpes simplex<\/td>\r\n\"17.2b\"<\/td>\r\nNon-living particles that reproduce by taking over living cells<\/td>\r\nCommon cold, flu, genital herpes, cold sores, measles, AIDS, genital warts, chicken pox, small pox<\/td>\r\n<\/tr>\r\n
Fungi:<\/strong>\r\n\r\nExample shown:\r\n\r\nDeath cap mushroom<\/td>\r\n\"17.2c<\/td>\r\nSimple organisms, including mushrooms and yeast, that grow as single cells or thread-like filaments<\/td>\r\nRingworm, athletes foot, tineas, candidias, histoplasmomis, mushroom poisoning<\/td>\r\n<\/tr>\r\n
Protozoa:<\/strong>\r\n\r\nExample shown:\r\n\r\nGiardia lamblia<\/td>\r\n\"17.2d<\/td>\r\nSingle celled organisms with a nucleus<\/td>\r\nMalaria, \"traveller's diarrhea\", giardiasis, typano somiasis (\"sleeping sickness\")<\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\nWhat is the Immune System?<\/span>\r\n\r\nThe\u00a0[pb_glossary id=\"4242\"]immune system[\/pb_glossary]<\/strong> is a host defense system. It comprises many biological structures \u2014ranging from individual leukocytes to entire organs \u2014 as well as many complex biological processes. The function of the immune system is to protect the host from pathogens and other causes of disease, such as tumor (cancer) cells. To function properly, the immune system must be able to detect a wide variety of pathogens. It also must be able to distinguish the cells of pathogens from the host\u2019s own cells, and also to distinguish cancerous or damaged host cells from healthy cells. In humans and most other vertebrates, the immune system consists of layered defenses that have increasing specificity for particular pathogens or tumor cells. The layered defenses of the human immune system are usually classified into two subsystems, called the innate immune system and the adaptive immune system.\r\n

Innate Immune System<\/h2>\r\nThe\u00a0[pb_glossary id=\"4820\"]innate immune system[\/pb_glossary] <\/strong>(sometimes referred to as \"non-specific defense\") provides very quick, but non-specific responses to pathogens. It responds the same way regardless of the type of pathogen that is attacking the host. It includes barriers \u2014 such as the skin and mucous membranes \u2014 that normally keep pathogens out of the body. It also includes general responses to pathogens that manage to breach these barriers, including chemicals and cells that attack the pathogens inside the human host. Certain leukocytes (white blood cells), for example, engulf and destroy pathogens they encounter in the process called [pb_glossary id=\"1680\"]phagocytosis[\/pb_glossary], which is illustrated in Figure 17.2.2. Exposure to pathogens leads to an immediate maximal response from the innate immune system.\r\n\r\n[caption id=\"attachment_4821\" align=\"aligncenter\" width=\"400\"]\"17.2.2 Figure 17.2.2 A leukocyte called a macrophage phagocytizes bacteria in the series of steps shown here: engulfing a bacterium, digesting the bacterium with enzymes, and absorbing leftover products.<\/em>[\/caption]\r\n\r\n
\r\n\r\nWatch the video below, \"Neutrophil Phagocytosis - White Blood Cells Eats Staphylococcus Aureus Bacteria\" by ImmiflexImmuneSystem, to see phagocytosis in action.\r\n\r\nhttps:\/\/youtu.be\/Z_mXDvZQ6dU\r\n

Neutrophil Phagocytosis - White Blood Cell Eats Staphylococcus Aureus Bacteria, ImmiflexImmuneSystem, 2013.<\/p>\r\n\r\n<\/div>\r\n

Adaptive Immune System<\/h2>\r\nThe\u00a0[pb_glossary id=\"4822\"]adaptive immune system[\/pb_glossary]<\/strong>\u00a0is activated if pathogens successfully enter the body and manage to evade the general defenses of the innate immune system. An adaptive response is specific to the particular type of pathogen that has invaded the body, or to cancerous cells. It takes longer to launch a specific attack, but once it is underway, its specificity makes it very effective. An adaptive response also usually leads to immunity. This is a state of resistance to a specific pathogen, due to the adaptive immune system's ability to \u201cremember\u201d the pathogen and immediately mount a strong attack tailored to that particular pathogen if it invades again in the future.\r\n
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Self vs. Non-Self<\/h1>\r\n<\/div>\r\nBoth innate and adaptive immune responses depend on the immune system's ability to distinguish between self- and non-self molecules. [pb_glossary id=\"4823\"]Self molecules[\/pb_glossary]<\/strong>\u00a0are those components of an organism\u2019s body that can be distinguished from foreign substances by the immune system. Virtually all body cells have surface proteins that are part of a complex called\u00a0[pb_glossary id=\"4825\"]major histocompatibility complex (MHC)[\/pb_glossary]<\/strong>. These proteins are one way the immune system recognizes body cells as self.\u00a0[pb_glossary id=\"5463\"]Non-self proteins[\/pb_glossary]<\/strong>, in contrast, are recognized as foreign, because they are different from self proteins.\r\n

Antigens and Antibodies<\/h2>\r\nMany non-self molecules comprise a class of compounds called antigens.\u00a0[pb_glossary id=\"2704\"]Antigen<\/strong>s<\/strong>[\/pb_glossary]<\/strong>, which are usually proteins, bind to specific receptors on immune system cells and elicit an adaptive immune response. Some adaptive immune system cells (B cells) respond to foreign antigens by producing antibodies. An\u00a0<\/span>[pb_glossary id=\"5659\"]antibody[\/pb_glossary]<\/strong>\u00a0is a molecule that precisely matches and binds to a specific antigen. This may target the antigen (and the pathogen displaying it) for destruction by other immune cells.<\/span>\r\n\r\nAntigens on the surface of pathogens are how the [pb_glossary id=\"4822\"]adaptive immune system[\/pb_glossary] recognizes specific pathogens. Antigen specificity allows for the generation of responses tailored to the specific pathogen. It is also how the adaptive immune system \u201dremembers\u201d the same pathogen in the future.\r\n

Immune Surveillance<\/h2>\r\nAnother important role of the immune system is to identify and eliminate tumor cells. This is called\u00a0[pb_glossary id=\"4827\"]immune surveillance[\/pb_glossary].<\/strong>\u00a0The transformed cells of tumors express antigens that are not found on normal body cells. The main response of the immune system to tumor cells is to destroy them. This is carried out primarily by aptly-named killer T cells of the adaptive immune system.\r\n
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Lymphatic System<\/h1>\r\n<\/div>\r\nThe\u00a0[pb_glossary id=\"2911\"]lymphatic system[\/pb_glossary]<\/strong> is a human organ system that is a vital part of the adaptive immune system. It is also part of the [pb_glossary id=\"5927\"]cardiovascular system[\/pb_glossary] and plays a major role in the [pb_glossary id=\"5969\"]digestive system[\/pb_glossary] (see section 17.3 Lymphatic System<\/a>). The major structures of the lymphatic system are shown in Figure 17.2.3 .\r\n\r\n[caption id=\"attachment_4828\" align=\"aligncenter\" width=\"1024\"]\"17.2.3 Figure 17.2.3 The lymphatic system includes the organs and vessels illustrated here.<\/em>[\/caption]\r\n\r\nThe lymphatic system consists of several lymphatic organs and a body-wide network of lymphatic vessels that transport the fluid called lymph.\u00a0[pb_glossary id=\"4552\"]Lymph[\/pb_glossary]<\/strong>\u00a0is essentially blood plasma that has leaked from [pb_glossary id=\"5923\"]capillaries[\/pb_glossary] into tissue spaces. It includes many leukocytes, especially [pb_glossary id=\"4832\"]lymphocytes<\/span>[\/pb_glossary]<\/span>, which are the major cells of the lymphatic system. Like other leukocytes, lymphocytes defend the body. There are several different types of lymphocytes that fight pathogens or cancer cells as part of the adaptive immune system.<\/span>\r\n\r\nMajor lymphatic organs include the [pb_glossary id=\"4829\"]thymus[\/pb_glossary] and [pb_glossary id=\"4021\"]bone marrow[\/pb_glossary]. Their function is to form and\/or mature lymphocytes. Other lymphatic organs include the [pb_glossary id=\"4497\"]spleen[\/pb_glossary], [pb_glossary id=\"4830\"]tonsils[\/pb_glossary], and [pb_glossary id=\"4831\"]lymph nodes[\/pb_glossary], which are small clumps of lymphoid tissue clustered along lymphatic vessels. These other lymphatic organs harbor mature lymphocytes and filter lymph. They are sites where pathogens collect, and adaptive immune responses generally begin.\r\n
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Neuroimmune System vs. Peripheral Immune System<\/h1>\r\n<\/div>\r\nThe [pb_glossary id=\"5915\"]brain[\/pb_glossary] and [pb_glossary id=\"3010\"]spinal cord[\/pb_glossary] are normally protected from pathogens in the blood by the selectively permeable blood-brain and blood-spinal cord barriers. These barriers are part of the\u00a0[pb_glossary id=\"4833\"]neuroimmune system[\/pb_glossary].<\/strong>\u00a0The neuroimmune system has traditionally been considered distinct from the rest of the immune system, which is called the\u00a0[pb_glossary id=\"4834\"]peripheral immune system[\/pb_glossary]\u00a0<\/strong>\u2014 although that view may be changing. Unlike the peripheral system, in which leukocytes are the main cells, the main cells of the neuroimmune system are thought to be nervous system cells called [pb_glossary id=\"2985\"]neuroglia[\/pb_glossary]. These cells can recognize and respond to pathogens, debris, and other potential dangers. Types of neuroglia involved in neuroimmune responses include microglial cells and astrocytes.\r\n