The Kiss of Death<\/h1>\r\n<\/div>\r\nThe photomicrograph in Figure 17.5.1 shows a group of killer T cells (green and red) surrounding a [pb_glossary id=\"5605\"]cancer[\/pb_glossary] cell (blue, centre). When a killer T cell makes contact with the cancer cell, it attaches to and spreads over the dangerous target. The killer T cell then uses special chemicals stored in [pb_glossary id=\"5825\"]vesicles<\/span>[\/pb_glossary]<\/span>\u00a0(red) to deliver the killing blow. This event has thus been nicknamed \u201cthe kiss of death.\u201d After the target cell is killed, the killer T cells move on to find the next victim. Killer T cells like these are important players in the adaptive immune system.<\/span>\r\n\r\nWhat Is the Adaptive Immune System?<\/h1>\r\n<\/div>\r\nThe\u00a0[pb_glossary id=\"4822\"]adaptive immune system[\/pb_glossary]<\/strong>\u00a0is a subsystem of the overall [pb_glossary id=\"4242\"]immune system[\/pb_glossary]. It is composed of highly specialized cells and processes that eliminate specific [pb_glossary id=\"5799\"]pathogens<\/span>[\/pb_glossary]<\/span>\u00a0and tumor cells. An adaptive immune response is set in motion by [pb_glossary id=\"2704\"]antigen<\/span>s<\/span>[\/pb_glossary]\u00a0that the immune system recognizes as foreign. Unlike an innate immune response, an adaptive immune response is highly specific to a particular pathogen (or its antigen). An important function of the adaptive immune system that is\u00a0<\/span>not<\/em>\u00a0shared by the innate immune system is the creation of immunological memory \u2014 or [pb_glossary id=\"4884\"]immunity[\/pb_glossary] \u2014 which occurs after the initial response to a specific pathogen. It allows for a faster, stronger response on subsequent encounters with the same pathogen, usually before the pathogen can even cause symptoms of illness.<\/span>\r\n\r\n[pb_glossary id=\"4832\"]Lymphocytes<\/span>[\/pb_glossary]<\/span>\u00a0are the main cells of the adaptive immune system. They are [pb_glossary id=\"5623\"]leukocyte<\/span>s<\/span>[\/pb_glossary]<\/span>\u00a0that arise and mature in organs of the lymphatic system, including the [pb_glossary id=\"4021\"]bone marrow[\/pb_glossary] and [pb_glossary id=\"4829\"]thymus[\/pb_glossary]. The human body normally has about 2 trillion lymphocytes, which constitute about 1\/3 of all leukocytes. Most of the lymphocytes are normally sequestered within tissue fluid or organs of the lymphatic system, including the [pb_glossary id=\"4830\"]tonsil<\/span>s<\/span>[\/pb_glossary]<\/span>, [pb_glossary id=\"4497\"]spleen[\/pb_glossary], and [pb_glossary id=\"4831\"]lymph node<\/span>s<\/span>[\/pb_glossary]<\/span>. Only about 2% of the lymphocytes are normally circulating in the blood. There are two main types of lymphocytes involved in adaptive immune responses, called T cells and B cells. T cells destroy infected cells or release chemicals that regulate immune responses. B cells secrete [pb_glossary id=\"5659\"]antibodies[\/pb_glossary] that bind with [pb_glossary id=\"2704\"]antigen<\/span>s<\/span>[<\/span>\/pb_glossary]<\/span>\u00a0of [pb_glossary id=\"271\"]pathogen<\/span>s<\/span>[\/pb_glossary]<\/span>\u00a0so they can be removed by other immune cells or processes.<\/span>\r\n\r\nPathways of the Adaptive Immune Response<\/h1>\r\nThere are some general similarities in the way in which the separate adaptive immune responses occur in T cell and B cell responses.\u00a0 In both pathways, a foreign antigen is recognized by the B or T cell.\u00a0 From there, cytokines produced by helper T-cells promote clonal expansion of lymphocytes.\u00a0 From this clonal expansion, two types of B or T cells are produced- cells that directly fight the pathogen invasion and cells that remain behind to provide long-term [pb_glossary id=\"4884\"]immunity[\/pb_glossary].\u00a0 Finally, once the pathogen invasion has been eradicated, the plasma cells and killer T cells go through apoptosis (programmed cell death).\r\nT Cells<\/h1>\r\n<\/div>\r\nThere are multiple types of\u00a0[pb_glossary id=\"4892\"]T cells[\/pb_glossary],<\/strong>\u00a0or T lymphocytes. Major types are [pb_glossary id=\"4893\"]killer (or cytotoxic) T cells[\/pb_glossary] and [pb_glossary id=\"5537\"]helper T cells[\/pb_glossary]. Both types develop from immature T cells that become activated by exposure to an [pb_glossary id=\"2704\"]antigen[\/pb_glossary].\r\nT Cell Activation (or Cell-Mediated Immunity)<\/h2>\r\nT cells must be activated to become either killer T cells or helper T cells. This requires presentation of a foreign antigen by [pb_glossary id=\"4895\"]antigen-presenting cells[\/pb_glossary], as shown in Figure 17.5.2. Antigen-presenting cells may be [pb_glossary id=\"4875\"]dendritic cells<\/span>[\/pb_glossary]<\/span>, [pb_glossary id=\"4340\"]macrophage<\/span>s<\/span>[\/pb_glossary]<\/span>, or [pb_glossary id=\"5539\"]B cell<\/span>s<\/span>[\/pb_glossary]<\/span>. Activation occurs when T cells are presented with a foreign antigen coupled with an MHC self antigen. Helper T cells are more easily activated than killer T cells. Activation of killer T cells is strongly regulated and may require additional stimulation from helper T cells.<\/span>\r\n\r\n[caption id=\"attachment_4898\" align=\"aligncenter\" width=\"580\"]![\"17.5.2](\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/1000px-T_cell_activation.svg_-2.png\")
Figure 17.5.2 Exposure to a foreign antigen on an antigen-presenting cell is necessary to activate T cells to become killer T cells or helper T cells.<\/em>[\/caption]\r\nKiller T Cells<\/h2>\r\nActivated killer T cells induce the death of cells that bear a specific [pb_glossary id=\"5463\"]non-self[\/pb_glossary] antigen because they are infected with pathogens or are cancerous. The antigen targets the cell for destruction by killer T cells, which travel through the bloodstream searching for target cells to kill. Killer T cells may use various mechanisms to kill target cells. One way is by releasing toxins in granules that enter and kill infected or cancerous cells (see Figure 17.5.3).\r\n\r\n[caption id=\"attachment_4900\" align=\"aligncenter\" width=\"1024\"]![\"17.5.3](\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Cytotoxic-T-Cell-function-e1592851629363-2.png\")
Figure 17.5.3 A killer T cell releases toxins that destroy an infected body cell and the viruses it contains.<\/em>[\/caption]\r\nHelper T Cells<\/h2>\r\nActivated [pb_glossary id=\"5537\"]helper T cells<\/span>[\/pb_glossary]<\/span>\u00a0do not kill infected or cancerous cells. Instead, their role is to \u201cmanage\u201d both [pb_glossary id=\"4820\"]innate[\/pb_glossary] and [pb_glossary id=\"4822\"]adaptive immune[\/pb_glossary] responses by directing other cells to perform these tasks. They control other cells by releasing [pb_glossary id=\"4865\"]cytokine<\/span>s<\/span>[\/pb_glossary]<\/span>, which are proteins that can influence the activity of many cell types, including [pb_glossary id=\"4893\"]killer T cell<\/span>s<\/span>[\/pb_glossary]<\/span>, [pb_glossary id=\"5539\"]B cells[\/pb_glossary], and [pb_glossary id=\"4340\"]macrophages[\/pb_glossary].\u00a0Some cytokines released by helper T cells assist with the activation of killer T cells.<\/span>\r\n\r\nB Cells<\/h1>\r\n<\/div>\r\n[pb_glossary id=\"5539\"]B cell<\/strong>s<\/strong>[\/pb_glossary]<\/strong>, or B lymphocytes, are the major cells involved in the creation of [pb_glossary id=\"5659\"]antibodies[\/pb_glossary] that circulate in blood [pb_glossary id=\"4395\"]plasma[\/pb_glossary] and [pb_glossary id=\"4552\"]lymph[\/pb_glossary]. Antibodies are large, Y-shaped proteins used by the [pb_glossary id=\"4242\"]immune system[\/pb_glossary] to identify and neutralize foreign invaders. Besides producing antibodies, B cells may also function as [pb_glossary id=\"4895\"]antigen-presenting cell<\/span>s<\/span>[\/pb_glossary]<\/span>, or secrete cytokines that help control other immune cells and responses.<\/span>\r\nB Cell Activation (or Antibody-Mediated Immunity)<\/h2>\r\nBefore B cells can actively function to defend the host, they must be activated. As shown in Figure 17.5.4, B cell activation begins when a B cell engulfs and digests an antigen. The antigen may be either free floating in the lymph, or it may be presented by an antigen-presenting cell, such as a dendritic cell or macrophage. In either case, the B cell then displays antigen fragments bound to its own MHC antigens. The MHC-antigen complex on the B cell attracts helper T cells. The helper T cells, in turn, secrete cytokines that help the B cell to multiply, and the daughter cells to mature into plasma cells.\r\n\r\n[caption id=\"attachment_4901\" align=\"aligncenter\" width=\"583\"]![\"17.5.4](\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/B_cell_activation.svg_-2.png\")
Figure 17.5.4 B cells are activated and become antibody-producing plasma cells with the help of helper T cells.<\/em>[\/caption]\r\nPlasma Cells<\/h2>\r\n[caption id=\"attachment_4903\" align=\"alignright\" width=\"325\"]![\"17.5.5](\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Antibody.svg_-1-3.png\")
Figure 17.5.5 Each antibody fits its antigen like a lock fits a key.<\/em>[\/caption]\r\n\r\n[pb_glossary id=\"4902\"]Plasma cell<\/strong>s<\/strong>[\/pb_glossary]<\/strong> are antibody-secreting cells that form from activated B cells. Each plasma cell is like a tiny antibody factory. It may secrete millions of copies of an antibody, each of which can bind to the specific antigen that activated the original B cell. The specificity of an antibody to a specific antigen is illustrated in Figure 17.5.5. When antibodies bind with antigens, it makes the cells bearing them easier targets for phagocytes to find and destroy. Antibody-antigen complexes may also trigger the complement system of the innate immune system, which destroys the cells in a cascade of protein enzymes. In addition, the complexes are likely to clump together (agglutinate). If this occurs, they are filtered out of the blood in the spleen or liver.<\/span>\r\n\r\nImmunity<\/h1>\r\n<\/div>\r\nOnce a pathogen has been cleared from the body, most activated T cells and B cells die within a few days.\u00a0A\u00a0few of the cells, however, survive and remain in the body as memory T cells or memory B cells. These [pb_glossary id=\"4904\"]memory cell<\/strong>s[\/pb_glossary] are ready to activate an immediate response if they are exposed to the same antigen again in the future. This is the basis of\u00a0[pb_glossary id=\"4884\"]immunity[\/pb_glossary].<\/strong>\r\n\r\nThe earliest known reference to the concept of immunity relates to the bubonic plaque (see Figure 17.5.6). In 430 B.C., a Greek historian and general named Thucydides noted that people who had recovered from a previous bout of the plague could nurse people who were sick with the plague without contracting the illness a second time. We now know that this is true of many diseases, and that it occurs because of active immunity.\r\n\r\n[caption id=\"attachment_4905\" align=\"aligncenter\" width=\"332\"]![\"17.5.6](\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Acral_gangrene_due_to_plague-e1592852768241-2.jpg\")
Figure 17.5.6 Dead, blackened tissues at the finger tips and other extremities are a sign of the bubonic plague, giving rise to its other name, the black death.<\/em>[\/caption]\r\nActive Immunity<\/h2>\r\n[pb_glossary id=\"4906\"]Active immunity[\/pb_glossary]<\/strong>\u00a0is the ability of the adaptive immune system to resist a specific pathogen because it has formed an immunological memory of the pathogen. Active immunity is adaptive, because it occurs during the lifetime of an individual as an adaptation to infection with a specific pathogen, and prepares the immune system for future challenges from that pathogen. Active immunity can come about naturally or artificially.\r\nNaturally Acquired Active Immunity<\/h3>\r\nActive immunity is acquired naturally when a pathogen invades the body and activates the adaptive immune system. When the initial infection is over, memory B cells and memory T cells remain, providing immunological memory of the pathogen. As long as the memory cells are alive, the immune system is ready to mount an immediate response if the same pathogen tries to infect the body again.\r\nArtificially Acquired Active Immunity<\/h3>\r\nActive immunity can also be acquired artificially through immunization.\u00a0[pb_glossary id=\"4908\"]Immunization[\/pb_glossary]<\/strong> is the deliberate exposure of a person to a pathogen in order to provoke an adaptive immune response and the formation of memory cells specific to that pathogen. The pathogen is introduced in a vaccine \u2014 usually by injection, sometimes by nose or mouth (see Figure 17.5.7) \u2014 so immunization is also called vaccination.\r\n\r\n[caption id=\"attachment_4909\" align=\"alignleft\" width=\"511\"]![\"17.5.7](\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/immunizations-2.jpg\")
Figure 17.5.7 This young child is receiving a vaccine. Vaccines are a safe way to create immunity against life-threatening diseases.<\/em>[\/caption]\r\n\r\nTypically, only part of a pathogen, a weakened form of the pathogen, or a dead pathogen is used in a vaccine, which causes an adaptive immune response without making the immunized person sick. This is how you most likely became immune to diseases such as measles<\/a>, mumps<\/a>, and chicken pox. Immunizations may last for a lifetime, or they may require periodic booster shots to maintain immunity. While immunization generally has long-lasting effects, it usually takes several weeks to develop full immunity.\r\n\r\nImmunization is the most effective method ever discovered of preventing infectious diseases. As many as 3 million deaths are prevented each year because of vaccinations. Widespread immunity from vaccinations is largely responsible for the worldwide eradication of smallpox<\/a>, and the near elimination of several other infectious diseases from many populations, including polio<\/a> and measles. Immunization is so successful because it exploits the natural specificity and inducibility of the adaptive immune system.\r\nPassive Immunity<\/h2>\r\n[pb_glossary id=\"4910\"]Passive immunity[\/pb_glossary]<\/strong>\u00a0results when pathogen-specific antibodies or activated T cells are transferred to a person who has never been exposed to the pathogen. Passive immunity provides immediate protection from a pathogen, but the adaptive immune system does not develop immunological memory to protect the host from the same pathogen in the future. Unlike active immunity, passive immunity lasts only as long as the transferred antibodies or T cells survive in the blood \u2014 usually between a few days and a few months. However, like active immunity, passive immunity can be acquired both naturally and artificially.\r\nNaturally Acquired Passive Immunity<\/h3>\r\nPassive immunity is acquired naturally by a fetus through its mother\u2019s blood. [pb_glossary id=\"5659\"]Antibodies[\/pb_glossary] are transported from mother to fetus across the placenta, so babies have high levels of antibodies at birth. Their antibodies have the same range of antigen specificities as their mother\u2019s. Passive immunity may also be acquired by an infant through the mother\u2019s breast milk. This gives young infants protection from common pathogens in their environment while their own immune system matures.\r\nArtificially Acquired Passive Immunity<\/h3>\r\nOlder children and adults can acquire passive immunity artificially through the injection of antibodies or activated T cells, which may be done when there is a high risk of infection and insufficient time for the body to develop active immunity through vaccination. It may also be done to reduce symptoms of ongoing disease, or to compensate for immunodeficiency diseases.\r\n\r\nAdaptive Immune Evasion<\/h1>\r\n<\/div>\r\nMany pathogens have been around for a long time, living with human populations for generations. To persist, some have evolved mechanisms to evade the adaptive immune system of human hosts. One way they have done this is by rapidly changing their non-essential antigens. This is called antigenic variation. An example of a pathogen that takes this approach is human immunodeficiency virus (HIV). It mutates rapidly so the proteins on its viral envelope are constantly changing. By the time the adaptive immune system responds, the virus\u2019s antigens have changed. Antigenic variation is the main reason that efforts to develop a vaccine against HIV have not yet been successful.\r\n\r\nAnother evasion approach that some pathogens may take is to mask pathogen antigens with host molecules so the host\u2019s immune system cannot detect the antigens. HIV takes this approach, as well. The envelope that covers the virus is formed from the outermost membrane of the host cell.\r\n\r\nFeature: My Human Body<\/h1>\r\n<\/div>\r\nIf you think that immunizations are just for kids, think again. There are several vaccines recommended by HealthLinkBC<\/a> for people over the age of 18. The tables below from HealthLinkBC show the vaccine schedules recommended for infants and children, school-aged children, and adults and senior. Additional vaccines may be recommended for certain adults based on specific travel plans, medical conditions or other indications. Are you up to date with your vaccines? You can check with your doctor to be sure.\r\n\r\n\r\n![\"17.5](\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/BC-Immunization-2020-Schedule-Infants-and-Children-2.png\")
\r\n\r\n<\/div>\r\n![\"17.5](\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/BC-Immunization-2020-Schedule-School-Aged-Children-2.png\")
\r\n\r\n \r\n\r\n![\"17.5](\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/BC-Immunization-2020-Schedule-Adults-Seniors-Individuals-at-High-Risk-2.png\")
\r\n\r\n17.5 Summary<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- The [pb_glossary id=\"4822\"]adaptive immune system[\/pb_glossary] is a subsystem of the overall immune system that recognizes and makes a tailored attack against specific [pb_glossary id=\"5799\"]pathogens[\/pb_glossary]\u00a0or tumor cells. It is a slower, but more effective response than the [pb_glossary id=\"4820\"]innate immune response[\/pb_glossary], and also leads to immunity to particular pathogens.<\/li>\r\n \t
- [pb_glossary id=\"4832\"]Lymphocytes[\/pb_glossary]\u00a0produced by the [pb_glossary id=\"2911\"]lymphatic system[\/pb_glossary] are the main cells of the adaptive immune system. There are two major types of lymphocytes: [pb_glossary id=\"4892\"]T cells[\/pb_glossary]\u00a0and [pb_glossary id=\"5539\"]B cells[\/pb_glossary]. Both types must be activated by foreign [pb_glossary id=\"2704\"]antigens[\/pb_glossary]\u00a0to become functioning immune cells.<\/li>\r\n \t
- Most activated T cells become either [pb_glossary id=\"4893\"]killer T cells[\/pb_glossary]\u00a0or [pb_glossary id=\"5537\"]helper T cells[\/pb_glossary]. Killer T cells destroy cells that are infected with pathogens or are cancerous. Helper T cells manage immune responses by releasing [pb_glossary id=\"4865\"]cytokines[\/pb_glossary]\u00a0that control other types of leukocytes.<\/li>\r\n \t
- Activated B cells form [pb_glossary id=\"4902\"]plasma cells[\/pb_glossary]\u00a0that secrete [pb_glossary id=\"5659\"]antibodies[\/pb_glossary], which bind to specific antigens on pathogens or infected cells. The antibody-antigen complexes generally lead to the destruction of the cells, for example, by attracting phagocytes or triggering the [pb_glossary id=\"4868\"]complement system[\/pb_glossary].<\/li>\r\n \t
- After an adaptive immune response occurs, long-lasting [pb_glossary id=\"4904\"]memory B cells and memory T cells[\/pb_glossary] may remain to confer immunity to the specific pathogen that caused the adaptive immune response. These memory cells are ready to activate an immediate response if they are exposed to the same antigen again in the future.<\/li>\r\n \t
- Immunity may be active or passive. Active immunity occurs when the immune system has been presented with antigens that elicit an adaptive immune response. This may occur naturally as the result of an infection, or artificially as the result of immunization. Active immunity may last for years or even for life.<\/li>\r\n \t
- Passive immunity occurs without an adaptive immune response by the transfer of antibodies or activated T cells. This may occur naturally between a mother and her fetus or her nursing infant, or it may occur artificially by injection. Passive immunity lasts only as long as the antibodies or activated T cells remain alive in the body, generally just weeks or months.<\/li>\r\n \t
- Many pathogens have evolved mechanisms to evade the adaptive immune system. For example, human immunodeficiency virus ([pb_glossary id=\"4501\"]HIV[\/pb_glossary]) evades the adaptive immune system by frequently changing its antigens and by forming its outer envelope from the host\u2019s cell membrane.<\/li>\r\n<\/ul>\r\n<\/div>\r\n<\/div>\r\n
\r\n17.5 Review Questions<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- What is the adaptive immune system?<\/li>\r\n \t
- Define immunity.<\/li>\r\n \t
- [h5p id=\"639\"]<\/li>\r\n \t
- How are lymphocytes activated?<\/li>\r\n \t
- Identify two common types of T cells and their functions.<\/li>\r\n \t
- How do activated B cells help defend against pathogens?<\/li>\r\n \t
- How does passive immunity differ from active immunity?\u00a0How may passive immunity occur?<\/li>\r\n \t
- What are two ways that active immunity may come about?<\/li>\r\n \t
- What ways of evading the human adaptive immune system evolved in human immunodeficiency virus (HIV)?<\/li>\r\n \t
- Why do vaccinations expose\u00a0a person to a version of a pathogen?<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n
\r\n17.5 Explore More<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\nhttps:\/\/youtu.be\/rb7TVW77ZCs\r\nHow do vaccines work? - Kelwalin Dhanasarnsombut, TED-Ed, 2015.<\/p>\r\nhttps:\/\/youtu.be\/yqUFy-t4MlQ\r\n
How we conquered the deadly smallpox virus - Simona Zompi, TED-Ed, 2013.<\/p>\r\nhttps:\/\/youtu.be\/5THf6gTNqO8\r\n
Why Do We Need A New Flu Shot Every Year? Seeker, 2015.<\/p>\r\nhttps:\/\/youtu.be\/X-rC78MKZvw\r\n
An HIV Vaccine: Mapping Uncharted Territory, NIAID, 2016.<\/p>\r\n\r\n<\/div>\r\n<\/div>\r\n \r\n
Attributions<\/h2>\r\nFigure 17.5.1<\/strong>\r\n\r\nKiller_T_cells_surround_a_cancer_cell<\/a> by Alex Ritter, Jennifer Lippincott Schwartz and Gillian Griffiths at the National Institutes of Health\/ Visuals Online<\/a> on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).\r\n\r\nFigure 17.5.2<\/strong>\r\n\r\nT_cell_activation.svg<\/a> by Rehua<\/a> (derivative work) on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).\u00a0(Original image: T_cell_activation.png<\/a>:\u00a0 from The Immune System - NIH Publication No. 03\u20135423<\/a>)\r\n\r\nFigure 17.5.3<\/strong>\r\n\r\nCytotoxic T Cell function<\/a>\u00a0by 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 17.5.4<\/strong>\r\n\r\nB_cell_activation.svg<\/a> by Fred the Oyster<\/a> on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain). (Original from The Immune System - NIH Publication No. 03\u20135423<\/a>)\r\n\r\nFigure 17.5.5<\/strong>\r\n\r\nAntibody.svg<\/a> by Fvasconcellos<\/a>\u00a0 on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain). (Original black and white image from the National Human Genome Research Institute's Talking Genetics Glossary<\/a>)\r\n\r\nFigure 17.5.7<\/strong>\r\n\r\nimmunizations<\/a> by U.S. Air Force photo by Airman 1st Class Destinee Dougherty from Military Health System website, Health.mil, is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).\r\n\r\n \r\nReferences<\/h2>\r\n
HealthLinkBC. (2018). B.C. immunization schedules. \u00a0Gov.BC.CA. \u00a0https:\/\/www.healthlinkbc.ca\/tools-videos\/bc-immunization-schedules<\/p>\r\n
Mayo Clinic Staff. (n.d.). Measles [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/measles\/symptoms-causes\/syc-20374857<\/p>\r\n
Mayo Clinic Staff. (n.d.). Mumps [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/mumps\/symptoms-causes\/syc-20375361<\/p>\r\n
Mayo Clinic Staff. (n.d.). Polio [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/polio\/symptoms-causes\/syc-20376512<\/p>\r\n
Mayo Clinic Staff. (n.d.). Smallpox [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/smallpox\/symptoms-causes\/syc-20353027<\/p>\r\n
NIAID. (2016, August 11). An HIV vaccine: Mapping uncharted territory. YouTube. https:\/\/www.youtube.com\/watch?v=X-rC78MKZvw&feature=youtu.be<\/p>\r\n
OpenStax. (2016, March 23). Figure 4 Na\u00efve\u00a0CD4<\/mtext>+<\/sup>\u00a0T cells engage MHC II molecules on antigen-presenting cells (APCs) and become activated. Clones of the activated helper T cell, in turn, activate B cells and CD8+<\/sup> T cells, which become cytotoxic T cells. Cytotoxic T cells kill infected cells [digital image].\u00a0In OpenStax, Biology<\/em> (Section 42.2). OpenStax CNX. https:\/\/cnx.org\/contents\/GFy_h8cu@10.53:etZobsU-@6\/Adaptive-Immune-Response<\/p>\r\nSeeker. (2015, September 2). Why do we need a new flu shot every year? YouTube. https:\/\/www.youtube.com\/watch?v=5THf6gTNqO8<\/p>\r\n
TED-Ed. (2015, January 12). How do vaccines work? - Kelwalin Dhanasarnsombut. YouTube. https:\/\/www.youtube.com\/watch?v=rb7TVW77ZCs&feature=youtu.be<\/p>\r\n
TED-Ed. (2013, October 28). How we conquered the deadly smallpox virus - Simona Zompi. YouTube. https:\/\/www.youtube.com\/watch?v=yqUFy-t4MlQ&feature=youtu.be<\/p>","rendered":"
<\/p>\n![\"Figure](\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2019\/06\/Killer_T_cells_surround_a_cancer_cell-2.png\")
Figure 17.5.1 Kill the cancer.<\/em><\/figcaption><\/figure>\n\nThe Kiss of Death<\/h1>\n<\/div>\n
The photomicrograph in Figure 17.5.1 shows a group of killer T cells (green and red) surrounding a cancer<\/a> cell (blue, centre). When a killer T cell makes contact with the cancer cell, it attaches to and spreads over the dangerous target. The killer T cell then uses special chemicals stored in vesicles<\/span><\/a><\/span>\u00a0(red) to deliver the killing blow. This event has thus been nicknamed \u201cthe kiss of death.\u201d After the target cell is killed, the killer T cells move on to find the next victim. Killer T cells like these are important players in the adaptive immune system.<\/span><\/p>\n\nWhat Is the Adaptive Immune System?<\/h1>\n<\/div>\n
The\u00a0adaptive immune system<\/a><\/strong>\u00a0is a subsystem of the overall immune system. It is composed of highly specialized cells and processes that eliminate specific pathogens<\/span><\/a><\/span>\u00a0and tumor cells. An adaptive immune response is set in motion by antigen<\/span>s<\/span><\/a>\u00a0that the immune system recognizes as foreign. Unlike an innate immune response, an adaptive immune response is highly specific to a particular pathogen (or its antigen). An important function of the adaptive immune system that is\u00a0<\/span>not<\/em>\u00a0shared by the innate immune system is the creation of immunological memory \u2014 or immunity<\/a> \u2014 which occurs after the initial response to a specific pathogen. It allows for a faster, stronger response on subsequent encounters with the same pathogen, usually before the pathogen can even cause symptoms of illness.<\/span><\/p>\nLymphocytes<\/span><\/a><\/span>\u00a0are the main cells of the adaptive immune system. They are leukocyte<\/span>s<\/span><\/a><\/span>\u00a0that arise and mature in organs of the lymphatic system, including the bone marrow<\/a> and thymus<\/a>. The human body normally has about 2 trillion lymphocytes, which constitute about 1\/3 of all leukocytes. Most of the lymphocytes are normally sequestered within tissue fluid or organs of the lymphatic system, including the tonsil<\/span>s<\/span><\/a><\/span>, spleen<\/a>, and lymph node<\/span>s<\/span><\/a><\/span>. Only about 2% of the lymphocytes are normally circulating in the blood. There are two main types of lymphocytes involved in adaptive immune responses, called T cells and B cells. T cells destroy infected cells or release chemicals that regulate immune responses. B cells secrete antibodies<\/a> that bind with antigen<\/span>s<\/span>[<\/span>\/pb_glossary]<\/span>\u00a0of [pb_glossary id=\"271\"]pathogen<\/span>s<\/span><\/a><\/span>\u00a0so they can be removed by other immune cells or processes.<\/span><\/p>\n\nPathways of the Adaptive Immune Response<\/h1>\n
There are some general similarities in the way in which the separate adaptive immune responses occur in T cell and B cell responses.\u00a0 In both pathways, a foreign antigen is recognized by the B or T cell.\u00a0 From there, cytokines produced by helper T-cells promote clonal expansion of lymphocytes.\u00a0 From this clonal expansion, two types of B or T cells are produced- cells that directly fight the pathogen invasion and cells that remain behind to provide long-term immunity<\/a>.\u00a0 Finally, once the pathogen invasion has been eradicated, the plasma cells and killer T cells go through apoptosis (programmed cell death).<\/p>\nT Cells<\/h1>\n<\/div>\n
There are multiple types of\u00a0T cells<\/a>,<\/strong>\u00a0or T lymphocytes. Major types are killer (or cytotoxic) T cells<\/a> and helper T cells<\/a>. Both types develop from immature T cells that become activated by exposure to an antigen<\/a>.<\/p>\nT Cell Activation (or Cell-Mediated Immunity)<\/h2>\n
T cells must be activated to become either killer T cells or helper T cells. This requires presentation of a foreign antigen by antigen-presenting cells<\/a>, as shown in Figure 17.5.2. Antigen-presenting cells may be dendritic cells<\/span><\/a><\/span>, macrophage<\/span>s<\/span><\/a><\/span>, or B cell<\/span>s<\/span><\/a><\/span>. Activation occurs when T cells are presented with a foreign antigen coupled with an MHC self antigen. Helper T cells are more easily activated than killer T cells. Activation of killer T cells is strongly regulated and may require additional stimulation from helper T cells.<\/span><\/p>\nFigure 17.5.2 Exposure to a foreign antigen on an antigen-presenting cell is necessary to activate T cells to become killer T cells or helper T cells.<\/em><\/figcaption><\/figure>\nKiller T Cells<\/h2>\n
Activated killer T cells induce the death of cells that bear a specific non-self<\/a> antigen because they are infected with pathogens or are cancerous. The antigen targets the cell for destruction by killer T cells, which travel through the bloodstream searching for target cells to kill. Killer T cells may use various mechanisms to kill target cells. One way is by releasing toxins in granules that enter and kill infected or cancerous cells (see Figure 17.5.3).<\/p>\nFigure 17.5.3 A killer T cell releases toxins that destroy an infected body cell and the viruses it contains.<\/em><\/figcaption><\/figure>\nHelper T Cells<\/h2>\n
Activated helper T cells<\/span><\/a><\/span>\u00a0do not kill infected or cancerous cells. Instead, their role is to \u201cmanage\u201d both innate<\/a> and adaptive immune<\/a> responses by directing other cells to perform these tasks. They control other cells by releasing cytokine<\/span>s<\/span><\/a><\/span>, which are proteins that can influence the activity of many cell types, including killer T cell<\/span>s<\/span><\/a><\/span>, B cells<\/a>, and macrophages<\/a>.\u00a0Some cytokines released by helper T cells assist with the activation of killer T cells.<\/span><\/p>\n\nB Cells<\/h1>\n<\/div>\n
B cell<\/strong>s<\/strong><\/a><\/strong>, or B lymphocytes, are the major cells involved in the creation of antibodies<\/a> that circulate in blood plasma<\/a> and lymph<\/a>. Antibodies are large, Y-shaped proteins used by the immune system to identify and neutralize foreign invaders. Besides producing antibodies, B cells may also function as antigen-presenting cell<\/span>s<\/span><\/a><\/span>, or secrete cytokines that help control other immune cells and responses.<\/span><\/p>\nB Cell Activation (or Antibody-Mediated Immunity)<\/h2>\n
Before B cells can actively function to defend the host, they must be activated. As shown in Figure 17.5.4, B cell activation begins when a B cell engulfs and digests an antigen. The antigen may be either free floating in the lymph, or it may be presented by an antigen-presenting cell, such as a dendritic cell or macrophage. In either case, the B cell then displays antigen fragments bound to its own MHC antigens. The MHC-antigen complex on the B cell attracts helper T cells. The helper T cells, in turn, secrete cytokines that help the B cell to multiply, and the daughter cells to mature into plasma cells.<\/p>\nFigure 17.5.4 B cells are activated and become antibody-producing plasma cells with the help of helper T cells.<\/em><\/figcaption><\/figure>\nPlasma Cells<\/h2>\nFigure 17.5.5 Each antibody fits its antigen like a lock fits a key.<\/em><\/figcaption><\/figure>\n
What Is the Adaptive Immune System?<\/h1>\r\n<\/div>\r\nThe\u00a0[pb_glossary id=\"4822\"]adaptive immune system[\/pb_glossary]<\/strong>\u00a0is a subsystem of the overall [pb_glossary id=\"4242\"]immune system[\/pb_glossary]. It is composed of highly specialized cells and processes that eliminate specific [pb_glossary id=\"5799\"]pathogens<\/span>[\/pb_glossary]<\/span>\u00a0and tumor cells. An adaptive immune response is set in motion by [pb_glossary id=\"2704\"]antigen<\/span>s<\/span>[\/pb_glossary]\u00a0that the immune system recognizes as foreign. Unlike an innate immune response, an adaptive immune response is highly specific to a particular pathogen (or its antigen). An important function of the adaptive immune system that is\u00a0<\/span>not<\/em>\u00a0shared by the innate immune system is the creation of immunological memory \u2014 or [pb_glossary id=\"4884\"]immunity[\/pb_glossary] \u2014 which occurs after the initial response to a specific pathogen. It allows for a faster, stronger response on subsequent encounters with the same pathogen, usually before the pathogen can even cause symptoms of illness.<\/span>\r\n\r\n[pb_glossary id=\"4832\"]Lymphocytes<\/span>[\/pb_glossary]<\/span>\u00a0are the main cells of the adaptive immune system. They are [pb_glossary id=\"5623\"]leukocyte<\/span>s<\/span>[\/pb_glossary]<\/span>\u00a0that arise and mature in organs of the lymphatic system, including the [pb_glossary id=\"4021\"]bone marrow[\/pb_glossary] and [pb_glossary id=\"4829\"]thymus[\/pb_glossary]. The human body normally has about 2 trillion lymphocytes, which constitute about 1\/3 of all leukocytes. Most of the lymphocytes are normally sequestered within tissue fluid or organs of the lymphatic system, including the [pb_glossary id=\"4830\"]tonsil<\/span>s<\/span>[\/pb_glossary]<\/span>, [pb_glossary id=\"4497\"]spleen[\/pb_glossary], and [pb_glossary id=\"4831\"]lymph node<\/span>s<\/span>[\/pb_glossary]<\/span>. Only about 2% of the lymphocytes are normally circulating in the blood. There are two main types of lymphocytes involved in adaptive immune responses, called T cells and B cells. T cells destroy infected cells or release chemicals that regulate immune responses. B cells secrete [pb_glossary id=\"5659\"]antibodies[\/pb_glossary] that bind with [pb_glossary id=\"2704\"]antigen<\/span>s<\/span>[<\/span>\/pb_glossary]<\/span>\u00a0of [pb_glossary id=\"271\"]pathogen<\/span>s<\/span>[\/pb_glossary]<\/span>\u00a0so they can be removed by other immune cells or processes.<\/span>\r\n\r\nPathways of the Adaptive Immune Response<\/h1>\r\nThere are some general similarities in the way in which the separate adaptive immune responses occur in T cell and B cell responses.\u00a0 In both pathways, a foreign antigen is recognized by the B or T cell.\u00a0 From there, cytokines produced by helper T-cells promote clonal expansion of lymphocytes.\u00a0 From this clonal expansion, two types of B or T cells are produced- cells that directly fight the pathogen invasion and cells that remain behind to provide long-term [pb_glossary id=\"4884\"]immunity[\/pb_glossary].\u00a0 Finally, once the pathogen invasion has been eradicated, the plasma cells and killer T cells go through apoptosis (programmed cell death).\r\nT Cells<\/h1>\r\n<\/div>\r\nThere are multiple types of\u00a0[pb_glossary id=\"4892\"]T cells[\/pb_glossary],<\/strong>\u00a0or T lymphocytes. Major types are [pb_glossary id=\"4893\"]killer (or cytotoxic) T cells[\/pb_glossary] and [pb_glossary id=\"5537\"]helper T cells[\/pb_glossary]. Both types develop from immature T cells that become activated by exposure to an [pb_glossary id=\"2704\"]antigen[\/pb_glossary].\r\nT Cell Activation (or Cell-Mediated Immunity)<\/h2>\r\nT cells must be activated to become either killer T cells or helper T cells. This requires presentation of a foreign antigen by [pb_glossary id=\"4895\"]antigen-presenting cells[\/pb_glossary], as shown in Figure 17.5.2. Antigen-presenting cells may be [pb_glossary id=\"4875\"]dendritic cells<\/span>[\/pb_glossary]<\/span>, [pb_glossary id=\"4340\"]macrophage<\/span>s<\/span>[\/pb_glossary]<\/span>, or [pb_glossary id=\"5539\"]B cell<\/span>s<\/span>[\/pb_glossary]<\/span>. Activation occurs when T cells are presented with a foreign antigen coupled with an MHC self antigen. Helper T cells are more easily activated than killer T cells. Activation of killer T cells is strongly regulated and may require additional stimulation from helper T cells.<\/span>\r\n\r\n[caption id=\"attachment_4898\" align=\"aligncenter\" width=\"580\"] Figure 17.5.2 Exposure to a foreign antigen on an antigen-presenting cell is necessary to activate T cells to become killer T cells or helper T cells.<\/em>[\/caption]\r\nKiller T Cells<\/h2>\r\nActivated killer T cells induce the death of cells that bear a specific [pb_glossary id=\"5463\"]non-self[\/pb_glossary] antigen because they are infected with pathogens or are cancerous. The antigen targets the cell for destruction by killer T cells, which travel through the bloodstream searching for target cells to kill. Killer T cells may use various mechanisms to kill target cells. One way is by releasing toxins in granules that enter and kill infected or cancerous cells (see Figure 17.5.3).\r\n\r\n[caption id=\"attachment_4900\" align=\"aligncenter\" width=\"1024\"] Figure 17.5.3 A killer T cell releases toxins that destroy an infected body cell and the viruses it contains.<\/em>[\/caption]\r\nHelper T Cells<\/h2>\r\nActivated [pb_glossary id=\"5537\"]helper T cells<\/span>[\/pb_glossary]<\/span>\u00a0do not kill infected or cancerous cells. Instead, their role is to \u201cmanage\u201d both [pb_glossary id=\"4820\"]innate[\/pb_glossary] and [pb_glossary id=\"4822\"]adaptive immune[\/pb_glossary] responses by directing other cells to perform these tasks. They control other cells by releasing [pb_glossary id=\"4865\"]cytokine<\/span>s<\/span>[\/pb_glossary]<\/span>, which are proteins that can influence the activity of many cell types, including [pb_glossary id=\"4893\"]killer T cell<\/span>s<\/span>[\/pb_glossary]<\/span>, [pb_glossary id=\"5539\"]B cells[\/pb_glossary], and [pb_glossary id=\"4340\"]macrophages[\/pb_glossary].\u00a0Some cytokines released by helper T cells assist with the activation of killer T cells.<\/span>\r\n\r\nB Cells<\/h1>\r\n<\/div>\r\n[pb_glossary id=\"5539\"]B cell<\/strong>s<\/strong>[\/pb_glossary]<\/strong>, or B lymphocytes, are the major cells involved in the creation of [pb_glossary id=\"5659\"]antibodies[\/pb_glossary] that circulate in blood [pb_glossary id=\"4395\"]plasma[\/pb_glossary] and [pb_glossary id=\"4552\"]lymph[\/pb_glossary]. Antibodies are large, Y-shaped proteins used by the [pb_glossary id=\"4242\"]immune system[\/pb_glossary] to identify and neutralize foreign invaders. Besides producing antibodies, B cells may also function as [pb_glossary id=\"4895\"]antigen-presenting cell<\/span>s<\/span>[\/pb_glossary]<\/span>, or secrete cytokines that help control other immune cells and responses.<\/span>\r\nB Cell Activation (or Antibody-Mediated Immunity)<\/h2>\r\nBefore B cells can actively function to defend the host, they must be activated. As shown in Figure 17.5.4, B cell activation begins when a B cell engulfs and digests an antigen. The antigen may be either free floating in the lymph, or it may be presented by an antigen-presenting cell, such as a dendritic cell or macrophage. In either case, the B cell then displays antigen fragments bound to its own MHC antigens. The MHC-antigen complex on the B cell attracts helper T cells. The helper T cells, in turn, secrete cytokines that help the B cell to multiply, and the daughter cells to mature into plasma cells.\r\n\r\n[caption id=\"attachment_4901\" align=\"aligncenter\" width=\"583\"] Figure 17.5.4 B cells are activated and become antibody-producing plasma cells with the help of helper T cells.<\/em>[\/caption]\r\nPlasma Cells<\/h2>\r\n[caption id=\"attachment_4903\" align=\"alignright\" width=\"325\"] Figure 17.5.5 Each antibody fits its antigen like a lock fits a key.<\/em>[\/caption]\r\n\r\n[pb_glossary id=\"4902\"]Plasma cell<\/strong>s<\/strong>[\/pb_glossary]<\/strong> are antibody-secreting cells that form from activated B cells. Each plasma cell is like a tiny antibody factory. It may secrete millions of copies of an antibody, each of which can bind to the specific antigen that activated the original B cell. The specificity of an antibody to a specific antigen is illustrated in Figure 17.5.5. When antibodies bind with antigens, it makes the cells bearing them easier targets for phagocytes to find and destroy. Antibody-antigen complexes may also trigger the complement system of the innate immune system, which destroys the cells in a cascade of protein enzymes. In addition, the complexes are likely to clump together (agglutinate). If this occurs, they are filtered out of the blood in the spleen or liver.<\/span>\r\n\r\nImmunity<\/h1>\r\n<\/div>\r\nOnce a pathogen has been cleared from the body, most activated T cells and B cells die within a few days.\u00a0A\u00a0few of the cells, however, survive and remain in the body as memory T cells or memory B cells. These [pb_glossary id=\"4904\"]memory cell<\/strong>s[\/pb_glossary] are ready to activate an immediate response if they are exposed to the same antigen again in the future. This is the basis of\u00a0[pb_glossary id=\"4884\"]immunity[\/pb_glossary].<\/strong>\r\n\r\nThe earliest known reference to the concept of immunity relates to the bubonic plaque (see Figure 17.5.6). In 430 B.C., a Greek historian and general named Thucydides noted that people who had recovered from a previous bout of the plague could nurse people who were sick with the plague without contracting the illness a second time. We now know that this is true of many diseases, and that it occurs because of active immunity.\r\n\r\n[caption id=\"attachment_4905\" align=\"aligncenter\" width=\"332\"] Figure 17.5.6 Dead, blackened tissues at the finger tips and other extremities are a sign of the bubonic plague, giving rise to its other name, the black death.<\/em>[\/caption]\r\nActive Immunity<\/h2>\r\n[pb_glossary id=\"4906\"]Active immunity[\/pb_glossary]<\/strong>\u00a0is the ability of the adaptive immune system to resist a specific pathogen because it has formed an immunological memory of the pathogen. Active immunity is adaptive, because it occurs during the lifetime of an individual as an adaptation to infection with a specific pathogen, and prepares the immune system for future challenges from that pathogen. Active immunity can come about naturally or artificially.\r\nNaturally Acquired Active Immunity<\/h3>\r\nActive immunity is acquired naturally when a pathogen invades the body and activates the adaptive immune system. When the initial infection is over, memory B cells and memory T cells remain, providing immunological memory of the pathogen. As long as the memory cells are alive, the immune system is ready to mount an immediate response if the same pathogen tries to infect the body again.\r\nArtificially Acquired Active Immunity<\/h3>\r\nActive immunity can also be acquired artificially through immunization.\u00a0[pb_glossary id=\"4908\"]Immunization[\/pb_glossary]<\/strong> is the deliberate exposure of a person to a pathogen in order to provoke an adaptive immune response and the formation of memory cells specific to that pathogen. The pathogen is introduced in a vaccine \u2014 usually by injection, sometimes by nose or mouth (see Figure 17.5.7) \u2014 so immunization is also called vaccination.\r\n\r\n[caption id=\"attachment_4909\" align=\"alignleft\" width=\"511\"] Figure 17.5.7 This young child is receiving a vaccine. Vaccines are a safe way to create immunity against life-threatening diseases.<\/em>[\/caption]\r\n\r\nTypically, only part of a pathogen, a weakened form of the pathogen, or a dead pathogen is used in a vaccine, which causes an adaptive immune response without making the immunized person sick. This is how you most likely became immune to diseases such as measles<\/a>, mumps<\/a>, and chicken pox. Immunizations may last for a lifetime, or they may require periodic booster shots to maintain immunity. While immunization generally has long-lasting effects, it usually takes several weeks to develop full immunity.\r\n\r\nImmunization is the most effective method ever discovered of preventing infectious diseases. As many as 3 million deaths are prevented each year because of vaccinations. Widespread immunity from vaccinations is largely responsible for the worldwide eradication of smallpox<\/a>, and the near elimination of several other infectious diseases from many populations, including polio<\/a> and measles. Immunization is so successful because it exploits the natural specificity and inducibility of the adaptive immune system.\r\nPassive Immunity<\/h2>\r\n[pb_glossary id=\"4910\"]Passive immunity[\/pb_glossary]<\/strong>\u00a0results when pathogen-specific antibodies or activated T cells are transferred to a person who has never been exposed to the pathogen. Passive immunity provides immediate protection from a pathogen, but the adaptive immune system does not develop immunological memory to protect the host from the same pathogen in the future. Unlike active immunity, passive immunity lasts only as long as the transferred antibodies or T cells survive in the blood \u2014 usually between a few days and a few months. However, like active immunity, passive immunity can be acquired both naturally and artificially.\r\nNaturally Acquired Passive Immunity<\/h3>\r\nPassive immunity is acquired naturally by a fetus through its mother\u2019s blood. [pb_glossary id=\"5659\"]Antibodies[\/pb_glossary] are transported from mother to fetus across the placenta, so babies have high levels of antibodies at birth. Their antibodies have the same range of antigen specificities as their mother\u2019s. Passive immunity may also be acquired by an infant through the mother\u2019s breast milk. This gives young infants protection from common pathogens in their environment while their own immune system matures.\r\nArtificially Acquired Passive Immunity<\/h3>\r\nOlder children and adults can acquire passive immunity artificially through the injection of antibodies or activated T cells, which may be done when there is a high risk of infection and insufficient time for the body to develop active immunity through vaccination. It may also be done to reduce symptoms of ongoing disease, or to compensate for immunodeficiency diseases.\r\n\r\nAdaptive Immune Evasion<\/h1>\r\n<\/div>\r\nMany pathogens have been around for a long time, living with human populations for generations. To persist, some have evolved mechanisms to evade the adaptive immune system of human hosts. One way they have done this is by rapidly changing their non-essential antigens. This is called antigenic variation. An example of a pathogen that takes this approach is human immunodeficiency virus (HIV). It mutates rapidly so the proteins on its viral envelope are constantly changing. By the time the adaptive immune system responds, the virus\u2019s antigens have changed. Antigenic variation is the main reason that efforts to develop a vaccine against HIV have not yet been successful.\r\n\r\nAnother evasion approach that some pathogens may take is to mask pathogen antigens with host molecules so the host\u2019s immune system cannot detect the antigens. HIV takes this approach, as well. The envelope that covers the virus is formed from the outermost membrane of the host cell.\r\n\r\nFeature: My Human Body<\/h1>\r\n<\/div>\r\nIf you think that immunizations are just for kids, think again. There are several vaccines recommended by HealthLinkBC<\/a> for people over the age of 18. The tables below from HealthLinkBC show the vaccine schedules recommended for infants and children, school-aged children, and adults and senior. Additional vaccines may be recommended for certain adults based on specific travel plans, medical conditions or other indications. Are you up to date with your vaccines? You can check with your doctor to be sure.\r\n\r\n\r\n\r\n\r\n<\/div>\r\n\r\n\r\n \r\n\r\n\r\n\r\n17.5 Summary<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- The [pb_glossary id=\"4822\"]adaptive immune system[\/pb_glossary] is a subsystem of the overall immune system that recognizes and makes a tailored attack against specific [pb_glossary id=\"5799\"]pathogens[\/pb_glossary]\u00a0or tumor cells. It is a slower, but more effective response than the [pb_glossary id=\"4820\"]innate immune response[\/pb_glossary], and also leads to immunity to particular pathogens.<\/li>\r\n \t
- [pb_glossary id=\"4832\"]Lymphocytes[\/pb_glossary]\u00a0produced by the [pb_glossary id=\"2911\"]lymphatic system[\/pb_glossary] are the main cells of the adaptive immune system. There are two major types of lymphocytes: [pb_glossary id=\"4892\"]T cells[\/pb_glossary]\u00a0and [pb_glossary id=\"5539\"]B cells[\/pb_glossary]. Both types must be activated by foreign [pb_glossary id=\"2704\"]antigens[\/pb_glossary]\u00a0to become functioning immune cells.<\/li>\r\n \t
- Most activated T cells become either [pb_glossary id=\"4893\"]killer T cells[\/pb_glossary]\u00a0or [pb_glossary id=\"5537\"]helper T cells[\/pb_glossary]. Killer T cells destroy cells that are infected with pathogens or are cancerous. Helper T cells manage immune responses by releasing [pb_glossary id=\"4865\"]cytokines[\/pb_glossary]\u00a0that control other types of leukocytes.<\/li>\r\n \t
- Activated B cells form [pb_glossary id=\"4902\"]plasma cells[\/pb_glossary]\u00a0that secrete [pb_glossary id=\"5659\"]antibodies[\/pb_glossary], which bind to specific antigens on pathogens or infected cells. The antibody-antigen complexes generally lead to the destruction of the cells, for example, by attracting phagocytes or triggering the [pb_glossary id=\"4868\"]complement system[\/pb_glossary].<\/li>\r\n \t
- After an adaptive immune response occurs, long-lasting [pb_glossary id=\"4904\"]memory B cells and memory T cells[\/pb_glossary] may remain to confer immunity to the specific pathogen that caused the adaptive immune response. These memory cells are ready to activate an immediate response if they are exposed to the same antigen again in the future.<\/li>\r\n \t
- Immunity may be active or passive. Active immunity occurs when the immune system has been presented with antigens that elicit an adaptive immune response. This may occur naturally as the result of an infection, or artificially as the result of immunization. Active immunity may last for years or even for life.<\/li>\r\n \t
- Passive immunity occurs without an adaptive immune response by the transfer of antibodies or activated T cells. This may occur naturally between a mother and her fetus or her nursing infant, or it may occur artificially by injection. Passive immunity lasts only as long as the antibodies or activated T cells remain alive in the body, generally just weeks or months.<\/li>\r\n \t
- Many pathogens have evolved mechanisms to evade the adaptive immune system. For example, human immunodeficiency virus ([pb_glossary id=\"4501\"]HIV[\/pb_glossary]) evades the adaptive immune system by frequently changing its antigens and by forming its outer envelope from the host\u2019s cell membrane.<\/li>\r\n<\/ul>\r\n<\/div>\r\n<\/div>\r\n
\r\n17.5 Review Questions<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- What is the adaptive immune system?<\/li>\r\n \t
- Define immunity.<\/li>\r\n \t
- [h5p id=\"639\"]<\/li>\r\n \t
- How are lymphocytes activated?<\/li>\r\n \t
- Identify two common types of T cells and their functions.<\/li>\r\n \t
- How do activated B cells help defend against pathogens?<\/li>\r\n \t
- How does passive immunity differ from active immunity?\u00a0How may passive immunity occur?<\/li>\r\n \t
- What are two ways that active immunity may come about?<\/li>\r\n \t
- What ways of evading the human adaptive immune system evolved in human immunodeficiency virus (HIV)?<\/li>\r\n \t
- Why do vaccinations expose\u00a0a person to a version of a pathogen?<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n
\r\n17.5 Explore More<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\nhttps:\/\/youtu.be\/rb7TVW77ZCs\r\nHow do vaccines work? - Kelwalin Dhanasarnsombut, TED-Ed, 2015.<\/p>\r\nhttps:\/\/youtu.be\/yqUFy-t4MlQ\r\n
How we conquered the deadly smallpox virus - Simona Zompi, TED-Ed, 2013.<\/p>\r\nhttps:\/\/youtu.be\/5THf6gTNqO8\r\n
Why Do We Need A New Flu Shot Every Year? Seeker, 2015.<\/p>\r\nhttps:\/\/youtu.be\/X-rC78MKZvw\r\n
An HIV Vaccine: Mapping Uncharted Territory, NIAID, 2016.<\/p>\r\n\r\n<\/div>\r\n<\/div>\r\n \r\n
Attributions<\/h2>\r\nFigure 17.5.1<\/strong>\r\n\r\nKiller_T_cells_surround_a_cancer_cell<\/a> by Alex Ritter, Jennifer Lippincott Schwartz and Gillian Griffiths at the National Institutes of Health\/ Visuals Online<\/a> on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).\r\n\r\nFigure 17.5.2<\/strong>\r\n\r\nT_cell_activation.svg<\/a> by Rehua<\/a> (derivative work) on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).\u00a0(Original image: T_cell_activation.png<\/a>:\u00a0 from The Immune System - NIH Publication No. 03\u20135423<\/a>)\r\n\r\nFigure 17.5.3<\/strong>\r\n\r\nCytotoxic T Cell function<\/a>\u00a0by 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 17.5.4<\/strong>\r\n\r\nB_cell_activation.svg<\/a> by Fred the Oyster<\/a> on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain). (Original from The Immune System - NIH Publication No. 03\u20135423<\/a>)\r\n\r\nFigure 17.5.5<\/strong>\r\n\r\nAntibody.svg<\/a> by Fvasconcellos<\/a>\u00a0 on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain). (Original black and white image from the National Human Genome Research Institute's Talking Genetics Glossary<\/a>)\r\n\r\nFigure 17.5.7<\/strong>\r\n\r\nimmunizations<\/a> by U.S. Air Force photo by Airman 1st Class Destinee Dougherty from Military Health System website, Health.mil, is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).\r\n\r\n \r\nReferences<\/h2>\r\n
HealthLinkBC. (2018). B.C. immunization schedules. \u00a0Gov.BC.CA. \u00a0https:\/\/www.healthlinkbc.ca\/tools-videos\/bc-immunization-schedules<\/p>\r\n
Mayo Clinic Staff. (n.d.). Measles [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/measles\/symptoms-causes\/syc-20374857<\/p>\r\n
Mayo Clinic Staff. (n.d.). Mumps [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/mumps\/symptoms-causes\/syc-20375361<\/p>\r\n
Mayo Clinic Staff. (n.d.). Polio [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/polio\/symptoms-causes\/syc-20376512<\/p>\r\n
Mayo Clinic Staff. (n.d.). Smallpox [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/smallpox\/symptoms-causes\/syc-20353027<\/p>\r\n
NIAID. (2016, August 11). An HIV vaccine: Mapping uncharted territory. YouTube. https:\/\/www.youtube.com\/watch?v=X-rC78MKZvw&feature=youtu.be<\/p>\r\n
OpenStax. (2016, March 23). Figure 4 Na\u00efve\u00a0CD4<\/mtext>+<\/sup>\u00a0T cells engage MHC II molecules on antigen-presenting cells (APCs) and become activated. Clones of the activated helper T cell, in turn, activate B cells and CD8+<\/sup> T cells, which become cytotoxic T cells. Cytotoxic T cells kill infected cells [digital image].\u00a0In OpenStax, Biology<\/em> (Section 42.2). OpenStax CNX. https:\/\/cnx.org\/contents\/GFy_h8cu@10.53:etZobsU-@6\/Adaptive-Immune-Response<\/p>\r\nSeeker. (2015, September 2). Why do we need a new flu shot every year? YouTube. https:\/\/www.youtube.com\/watch?v=5THf6gTNqO8<\/p>\r\n
TED-Ed. (2015, January 12). How do vaccines work? - Kelwalin Dhanasarnsombut. YouTube. https:\/\/www.youtube.com\/watch?v=rb7TVW77ZCs&feature=youtu.be<\/p>\r\n
TED-Ed. (2013, October 28). How we conquered the deadly smallpox virus - Simona Zompi. YouTube. https:\/\/www.youtube.com\/watch?v=yqUFy-t4MlQ&feature=youtu.be<\/p>","rendered":"
<\/p>\nFigure 17.5.1 Kill the cancer.<\/em><\/figcaption><\/figure>\n\nThe Kiss of Death<\/h1>\n<\/div>\n
The photomicrograph in Figure 17.5.1 shows a group of killer T cells (green and red) surrounding a cancer<\/a> cell (blue, centre). When a killer T cell makes contact with the cancer cell, it attaches to and spreads over the dangerous target. The killer T cell then uses special chemicals stored in vesicles<\/span><\/a><\/span>\u00a0(red) to deliver the killing blow. This event has thus been nicknamed \u201cthe kiss of death.\u201d After the target cell is killed, the killer T cells move on to find the next victim. Killer T cells like these are important players in the adaptive immune system.<\/span><\/p>\n\nWhat Is the Adaptive Immune System?<\/h1>\n<\/div>\n
The\u00a0adaptive immune system<\/a><\/strong>\u00a0is a subsystem of the overall immune system. It is composed of highly specialized cells and processes that eliminate specific pathogens<\/span><\/a><\/span>\u00a0and tumor cells. An adaptive immune response is set in motion by antigen<\/span>s<\/span><\/a>\u00a0that the immune system recognizes as foreign. Unlike an innate immune response, an adaptive immune response is highly specific to a particular pathogen (or its antigen). An important function of the adaptive immune system that is\u00a0<\/span>not<\/em>\u00a0shared by the innate immune system is the creation of immunological memory \u2014 or immunity<\/a> \u2014 which occurs after the initial response to a specific pathogen. It allows for a faster, stronger response on subsequent encounters with the same pathogen, usually before the pathogen can even cause symptoms of illness.<\/span><\/p>\nLymphocytes<\/span><\/a><\/span>\u00a0are the main cells of the adaptive immune system. They are leukocyte<\/span>s<\/span><\/a><\/span>\u00a0that arise and mature in organs of the lymphatic system, including the bone marrow<\/a> and thymus<\/a>. The human body normally has about 2 trillion lymphocytes, which constitute about 1\/3 of all leukocytes. Most of the lymphocytes are normally sequestered within tissue fluid or organs of the lymphatic system, including the tonsil<\/span>s<\/span><\/a><\/span>, spleen<\/a>, and lymph node<\/span>s<\/span><\/a><\/span>. Only about 2% of the lymphocytes are normally circulating in the blood. There are two main types of lymphocytes involved in adaptive immune responses, called T cells and B cells. T cells destroy infected cells or release chemicals that regulate immune responses. B cells secrete antibodies<\/a> that bind with antigen<\/span>s<\/span>[<\/span>\/pb_glossary]<\/span>\u00a0of [pb_glossary id=\"271\"]pathogen<\/span>s<\/span><\/a><\/span>\u00a0so they can be removed by other immune cells or processes.<\/span><\/p>\n\nPathways of the Adaptive Immune Response<\/h1>\n
There are some general similarities in the way in which the separate adaptive immune responses occur in T cell and B cell responses.\u00a0 In both pathways, a foreign antigen is recognized by the B or T cell.\u00a0 From there, cytokines produced by helper T-cells promote clonal expansion of lymphocytes.\u00a0 From this clonal expansion, two types of B or T cells are produced- cells that directly fight the pathogen invasion and cells that remain behind to provide long-term immunity<\/a>.\u00a0 Finally, once the pathogen invasion has been eradicated, the plasma cells and killer T cells go through apoptosis (programmed cell death).<\/p>\nT Cells<\/h1>\n<\/div>\n
There are multiple types of\u00a0T cells<\/a>,<\/strong>\u00a0or T lymphocytes. Major types are killer (or cytotoxic) T cells<\/a> and helper T cells<\/a>. Both types develop from immature T cells that become activated by exposure to an antigen<\/a>.<\/p>\nT Cell Activation (or Cell-Mediated Immunity)<\/h2>\n
T cells must be activated to become either killer T cells or helper T cells. This requires presentation of a foreign antigen by antigen-presenting cells<\/a>, as shown in Figure 17.5.2. Antigen-presenting cells may be dendritic cells<\/span><\/a><\/span>, macrophage<\/span>s<\/span><\/a><\/span>, or B cell<\/span>s<\/span><\/a><\/span>. Activation occurs when T cells are presented with a foreign antigen coupled with an MHC self antigen. Helper T cells are more easily activated than killer T cells. Activation of killer T cells is strongly regulated and may require additional stimulation from helper T cells.<\/span><\/p>\nFigure 17.5.2 Exposure to a foreign antigen on an antigen-presenting cell is necessary to activate T cells to become killer T cells or helper T cells.<\/em><\/figcaption><\/figure>\nKiller T Cells<\/h2>\n
Activated killer T cells induce the death of cells that bear a specific non-self<\/a> antigen because they are infected with pathogens or are cancerous. The antigen targets the cell for destruction by killer T cells, which travel through the bloodstream searching for target cells to kill. Killer T cells may use various mechanisms to kill target cells. One way is by releasing toxins in granules that enter and kill infected or cancerous cells (see Figure 17.5.3).<\/p>\nFigure 17.5.3 A killer T cell releases toxins that destroy an infected body cell and the viruses it contains.<\/em><\/figcaption><\/figure>\nHelper T Cells<\/h2>\n
Activated helper T cells<\/span><\/a><\/span>\u00a0do not kill infected or cancerous cells. Instead, their role is to \u201cmanage\u201d both innate<\/a> and adaptive immune<\/a> responses by directing other cells to perform these tasks. They control other cells by releasing cytokine<\/span>s<\/span><\/a><\/span>, which are proteins that can influence the activity of many cell types, including killer T cell<\/span>s<\/span><\/a><\/span>, B cells<\/a>, and macrophages<\/a>.\u00a0Some cytokines released by helper T cells assist with the activation of killer T cells.<\/span><\/p>\n\nB Cells<\/h1>\n<\/div>\n
B cell<\/strong>s<\/strong><\/a><\/strong>, or B lymphocytes, are the major cells involved in the creation of antibodies<\/a> that circulate in blood plasma<\/a> and lymph<\/a>. Antibodies are large, Y-shaped proteins used by the immune system to identify and neutralize foreign invaders. Besides producing antibodies, B cells may also function as antigen-presenting cell<\/span>s<\/span><\/a><\/span>, or secrete cytokines that help control other immune cells and responses.<\/span><\/p>\nB Cell Activation (or Antibody-Mediated Immunity)<\/h2>\n
Before B cells can actively function to defend the host, they must be activated. As shown in Figure 17.5.4, B cell activation begins when a B cell engulfs and digests an antigen. The antigen may be either free floating in the lymph, or it may be presented by an antigen-presenting cell, such as a dendritic cell or macrophage. In either case, the B cell then displays antigen fragments bound to its own MHC antigens. The MHC-antigen complex on the B cell attracts helper T cells. The helper T cells, in turn, secrete cytokines that help the B cell to multiply, and the daughter cells to mature into plasma cells.<\/p>\nFigure 17.5.4 B cells are activated and become antibody-producing plasma cells with the help of helper T cells.<\/em><\/figcaption><\/figure>\nPlasma Cells<\/h2>\nFigure 17.5.5 Each antibody fits its antigen like a lock fits a key.<\/em><\/figcaption><\/figure>\n
Pathways of the Adaptive Immune Response<\/h1>\r\nThere are some general similarities in the way in which the separate adaptive immune responses occur in T cell and B cell responses.\u00a0 In both pathways, a foreign antigen is recognized by the B or T cell.\u00a0 From there, cytokines produced by helper T-cells promote clonal expansion of lymphocytes.\u00a0 From this clonal expansion, two types of B or T cells are produced- cells that directly fight the pathogen invasion and cells that remain behind to provide long-term [pb_glossary id=\"4884\"]immunity[\/pb_glossary].\u00a0 Finally, once the pathogen invasion has been eradicated, the plasma cells and killer T cells go through apoptosis (programmed cell death).\r\nT Cells<\/h1>\r\n<\/div>\r\nThere are multiple types of\u00a0[pb_glossary id=\"4892\"]T cells[\/pb_glossary],<\/strong>\u00a0or T lymphocytes. Major types are [pb_glossary id=\"4893\"]killer (or cytotoxic) T cells[\/pb_glossary] and [pb_glossary id=\"5537\"]helper T cells[\/pb_glossary]. Both types develop from immature T cells that become activated by exposure to an [pb_glossary id=\"2704\"]antigen[\/pb_glossary].\r\nT Cell Activation (or Cell-Mediated Immunity)<\/h2>\r\nT cells must be activated to become either killer T cells or helper T cells. This requires presentation of a foreign antigen by [pb_glossary id=\"4895\"]antigen-presenting cells[\/pb_glossary], as shown in Figure 17.5.2. Antigen-presenting cells may be [pb_glossary id=\"4875\"]dendritic cells<\/span>[\/pb_glossary]<\/span>, [pb_glossary id=\"4340\"]macrophage<\/span>s<\/span>[\/pb_glossary]<\/span>, or [pb_glossary id=\"5539\"]B cell<\/span>s<\/span>[\/pb_glossary]<\/span>. Activation occurs when T cells are presented with a foreign antigen coupled with an MHC self antigen. Helper T cells are more easily activated than killer T cells. Activation of killer T cells is strongly regulated and may require additional stimulation from helper T cells.<\/span>\r\n\r\n[caption id=\"attachment_4898\" align=\"aligncenter\" width=\"580\"] Figure 17.5.2 Exposure to a foreign antigen on an antigen-presenting cell is necessary to activate T cells to become killer T cells or helper T cells.<\/em>[\/caption]\r\nKiller T Cells<\/h2>\r\nActivated killer T cells induce the death of cells that bear a specific [pb_glossary id=\"5463\"]non-self[\/pb_glossary] antigen because they are infected with pathogens or are cancerous. The antigen targets the cell for destruction by killer T cells, which travel through the bloodstream searching for target cells to kill. Killer T cells may use various mechanisms to kill target cells. One way is by releasing toxins in granules that enter and kill infected or cancerous cells (see Figure 17.5.3).\r\n\r\n[caption id=\"attachment_4900\" align=\"aligncenter\" width=\"1024\"] Figure 17.5.3 A killer T cell releases toxins that destroy an infected body cell and the viruses it contains.<\/em>[\/caption]\r\nHelper T Cells<\/h2>\r\nActivated [pb_glossary id=\"5537\"]helper T cells<\/span>[\/pb_glossary]<\/span>\u00a0do not kill infected or cancerous cells. Instead, their role is to \u201cmanage\u201d both [pb_glossary id=\"4820\"]innate[\/pb_glossary] and [pb_glossary id=\"4822\"]adaptive immune[\/pb_glossary] responses by directing other cells to perform these tasks. They control other cells by releasing [pb_glossary id=\"4865\"]cytokine<\/span>s<\/span>[\/pb_glossary]<\/span>, which are proteins that can influence the activity of many cell types, including [pb_glossary id=\"4893\"]killer T cell<\/span>s<\/span>[\/pb_glossary]<\/span>, [pb_glossary id=\"5539\"]B cells[\/pb_glossary], and [pb_glossary id=\"4340\"]macrophages[\/pb_glossary].\u00a0Some cytokines released by helper T cells assist with the activation of killer T cells.<\/span>\r\n\r\nB Cells<\/h1>\r\n<\/div>\r\n[pb_glossary id=\"5539\"]B cell<\/strong>s<\/strong>[\/pb_glossary]<\/strong>, or B lymphocytes, are the major cells involved in the creation of [pb_glossary id=\"5659\"]antibodies[\/pb_glossary] that circulate in blood [pb_glossary id=\"4395\"]plasma[\/pb_glossary] and [pb_glossary id=\"4552\"]lymph[\/pb_glossary]. Antibodies are large, Y-shaped proteins used by the [pb_glossary id=\"4242\"]immune system[\/pb_glossary] to identify and neutralize foreign invaders. Besides producing antibodies, B cells may also function as [pb_glossary id=\"4895\"]antigen-presenting cell<\/span>s<\/span>[\/pb_glossary]<\/span>, or secrete cytokines that help control other immune cells and responses.<\/span>\r\nB Cell Activation (or Antibody-Mediated Immunity)<\/h2>\r\nBefore B cells can actively function to defend the host, they must be activated. As shown in Figure 17.5.4, B cell activation begins when a B cell engulfs and digests an antigen. The antigen may be either free floating in the lymph, or it may be presented by an antigen-presenting cell, such as a dendritic cell or macrophage. In either case, the B cell then displays antigen fragments bound to its own MHC antigens. The MHC-antigen complex on the B cell attracts helper T cells. The helper T cells, in turn, secrete cytokines that help the B cell to multiply, and the daughter cells to mature into plasma cells.\r\n\r\n[caption id=\"attachment_4901\" align=\"aligncenter\" width=\"583\"] Figure 17.5.4 B cells are activated and become antibody-producing plasma cells with the help of helper T cells.<\/em>[\/caption]\r\nPlasma Cells<\/h2>\r\n[caption id=\"attachment_4903\" align=\"alignright\" width=\"325\"] Figure 17.5.5 Each antibody fits its antigen like a lock fits a key.<\/em>[\/caption]\r\n\r\n[pb_glossary id=\"4902\"]Plasma cell<\/strong>s<\/strong>[\/pb_glossary]<\/strong> are antibody-secreting cells that form from activated B cells. Each plasma cell is like a tiny antibody factory. It may secrete millions of copies of an antibody, each of which can bind to the specific antigen that activated the original B cell. The specificity of an antibody to a specific antigen is illustrated in Figure 17.5.5. When antibodies bind with antigens, it makes the cells bearing them easier targets for phagocytes to find and destroy. Antibody-antigen complexes may also trigger the complement system of the innate immune system, which destroys the cells in a cascade of protein enzymes. In addition, the complexes are likely to clump together (agglutinate). If this occurs, they are filtered out of the blood in the spleen or liver.<\/span>\r\n\r\nImmunity<\/h1>\r\n<\/div>\r\nOnce a pathogen has been cleared from the body, most activated T cells and B cells die within a few days.\u00a0A\u00a0few of the cells, however, survive and remain in the body as memory T cells or memory B cells. These [pb_glossary id=\"4904\"]memory cell<\/strong>s[\/pb_glossary] are ready to activate an immediate response if they are exposed to the same antigen again in the future. This is the basis of\u00a0[pb_glossary id=\"4884\"]immunity[\/pb_glossary].<\/strong>\r\n\r\nThe earliest known reference to the concept of immunity relates to the bubonic plaque (see Figure 17.5.6). In 430 B.C., a Greek historian and general named Thucydides noted that people who had recovered from a previous bout of the plague could nurse people who were sick with the plague without contracting the illness a second time. We now know that this is true of many diseases, and that it occurs because of active immunity.\r\n\r\n[caption id=\"attachment_4905\" align=\"aligncenter\" width=\"332\"] Figure 17.5.6 Dead, blackened tissues at the finger tips and other extremities are a sign of the bubonic plague, giving rise to its other name, the black death.<\/em>[\/caption]\r\nActive Immunity<\/h2>\r\n[pb_glossary id=\"4906\"]Active immunity[\/pb_glossary]<\/strong>\u00a0is the ability of the adaptive immune system to resist a specific pathogen because it has formed an immunological memory of the pathogen. Active immunity is adaptive, because it occurs during the lifetime of an individual as an adaptation to infection with a specific pathogen, and prepares the immune system for future challenges from that pathogen. Active immunity can come about naturally or artificially.\r\nNaturally Acquired Active Immunity<\/h3>\r\nActive immunity is acquired naturally when a pathogen invades the body and activates the adaptive immune system. When the initial infection is over, memory B cells and memory T cells remain, providing immunological memory of the pathogen. As long as the memory cells are alive, the immune system is ready to mount an immediate response if the same pathogen tries to infect the body again.\r\nArtificially Acquired Active Immunity<\/h3>\r\nActive immunity can also be acquired artificially through immunization.\u00a0[pb_glossary id=\"4908\"]Immunization[\/pb_glossary]<\/strong> is the deliberate exposure of a person to a pathogen in order to provoke an adaptive immune response and the formation of memory cells specific to that pathogen. The pathogen is introduced in a vaccine \u2014 usually by injection, sometimes by nose or mouth (see Figure 17.5.7) \u2014 so immunization is also called vaccination.\r\n\r\n[caption id=\"attachment_4909\" align=\"alignleft\" width=\"511\"] Figure 17.5.7 This young child is receiving a vaccine. Vaccines are a safe way to create immunity against life-threatening diseases.<\/em>[\/caption]\r\n\r\nTypically, only part of a pathogen, a weakened form of the pathogen, or a dead pathogen is used in a vaccine, which causes an adaptive immune response without making the immunized person sick. This is how you most likely became immune to diseases such as measles<\/a>, mumps<\/a>, and chicken pox. Immunizations may last for a lifetime, or they may require periodic booster shots to maintain immunity. While immunization generally has long-lasting effects, it usually takes several weeks to develop full immunity.\r\n\r\nImmunization is the most effective method ever discovered of preventing infectious diseases. As many as 3 million deaths are prevented each year because of vaccinations. Widespread immunity from vaccinations is largely responsible for the worldwide eradication of smallpox<\/a>, and the near elimination of several other infectious diseases from many populations, including polio<\/a> and measles. Immunization is so successful because it exploits the natural specificity and inducibility of the adaptive immune system.\r\nPassive Immunity<\/h2>\r\n[pb_glossary id=\"4910\"]Passive immunity[\/pb_glossary]<\/strong>\u00a0results when pathogen-specific antibodies or activated T cells are transferred to a person who has never been exposed to the pathogen. Passive immunity provides immediate protection from a pathogen, but the adaptive immune system does not develop immunological memory to protect the host from the same pathogen in the future. Unlike active immunity, passive immunity lasts only as long as the transferred antibodies or T cells survive in the blood \u2014 usually between a few days and a few months. However, like active immunity, passive immunity can be acquired both naturally and artificially.\r\nNaturally Acquired Passive Immunity<\/h3>\r\nPassive immunity is acquired naturally by a fetus through its mother\u2019s blood. [pb_glossary id=\"5659\"]Antibodies[\/pb_glossary] are transported from mother to fetus across the placenta, so babies have high levels of antibodies at birth. Their antibodies have the same range of antigen specificities as their mother\u2019s. Passive immunity may also be acquired by an infant through the mother\u2019s breast milk. This gives young infants protection from common pathogens in their environment while their own immune system matures.\r\nArtificially Acquired Passive Immunity<\/h3>\r\nOlder children and adults can acquire passive immunity artificially through the injection of antibodies or activated T cells, which may be done when there is a high risk of infection and insufficient time for the body to develop active immunity through vaccination. It may also be done to reduce symptoms of ongoing disease, or to compensate for immunodeficiency diseases.\r\n\r\nAdaptive Immune Evasion<\/h1>\r\n<\/div>\r\nMany pathogens have been around for a long time, living with human populations for generations. To persist, some have evolved mechanisms to evade the adaptive immune system of human hosts. One way they have done this is by rapidly changing their non-essential antigens. This is called antigenic variation. An example of a pathogen that takes this approach is human immunodeficiency virus (HIV). It mutates rapidly so the proteins on its viral envelope are constantly changing. By the time the adaptive immune system responds, the virus\u2019s antigens have changed. Antigenic variation is the main reason that efforts to develop a vaccine against HIV have not yet been successful.\r\n\r\nAnother evasion approach that some pathogens may take is to mask pathogen antigens with host molecules so the host\u2019s immune system cannot detect the antigens. HIV takes this approach, as well. The envelope that covers the virus is formed from the outermost membrane of the host cell.\r\n\r\nFeature: My Human Body<\/h1>\r\n<\/div>\r\nIf you think that immunizations are just for kids, think again. There are several vaccines recommended by HealthLinkBC<\/a> for people over the age of 18. The tables below from HealthLinkBC show the vaccine schedules recommended for infants and children, school-aged children, and adults and senior. Additional vaccines may be recommended for certain adults based on specific travel plans, medical conditions or other indications. Are you up to date with your vaccines? You can check with your doctor to be sure.\r\n\r\n\r\n\r\n\r\n<\/div>\r\n\r\n\r\n \r\n\r\n\r\n\r\n17.5 Summary<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- The [pb_glossary id=\"4822\"]adaptive immune system[\/pb_glossary] is a subsystem of the overall immune system that recognizes and makes a tailored attack against specific [pb_glossary id=\"5799\"]pathogens[\/pb_glossary]\u00a0or tumor cells. It is a slower, but more effective response than the [pb_glossary id=\"4820\"]innate immune response[\/pb_glossary], and also leads to immunity to particular pathogens.<\/li>\r\n \t
- [pb_glossary id=\"4832\"]Lymphocytes[\/pb_glossary]\u00a0produced by the [pb_glossary id=\"2911\"]lymphatic system[\/pb_glossary] are the main cells of the adaptive immune system. There are two major types of lymphocytes: [pb_glossary id=\"4892\"]T cells[\/pb_glossary]\u00a0and [pb_glossary id=\"5539\"]B cells[\/pb_glossary]. Both types must be activated by foreign [pb_glossary id=\"2704\"]antigens[\/pb_glossary]\u00a0to become functioning immune cells.<\/li>\r\n \t
- Most activated T cells become either [pb_glossary id=\"4893\"]killer T cells[\/pb_glossary]\u00a0or [pb_glossary id=\"5537\"]helper T cells[\/pb_glossary]. Killer T cells destroy cells that are infected with pathogens or are cancerous. Helper T cells manage immune responses by releasing [pb_glossary id=\"4865\"]cytokines[\/pb_glossary]\u00a0that control other types of leukocytes.<\/li>\r\n \t
- Activated B cells form [pb_glossary id=\"4902\"]plasma cells[\/pb_glossary]\u00a0that secrete [pb_glossary id=\"5659\"]antibodies[\/pb_glossary], which bind to specific antigens on pathogens or infected cells. The antibody-antigen complexes generally lead to the destruction of the cells, for example, by attracting phagocytes or triggering the [pb_glossary id=\"4868\"]complement system[\/pb_glossary].<\/li>\r\n \t
- After an adaptive immune response occurs, long-lasting [pb_glossary id=\"4904\"]memory B cells and memory T cells[\/pb_glossary] may remain to confer immunity to the specific pathogen that caused the adaptive immune response. These memory cells are ready to activate an immediate response if they are exposed to the same antigen again in the future.<\/li>\r\n \t
- Immunity may be active or passive. Active immunity occurs when the immune system has been presented with antigens that elicit an adaptive immune response. This may occur naturally as the result of an infection, or artificially as the result of immunization. Active immunity may last for years or even for life.<\/li>\r\n \t
- Passive immunity occurs without an adaptive immune response by the transfer of antibodies or activated T cells. This may occur naturally between a mother and her fetus or her nursing infant, or it may occur artificially by injection. Passive immunity lasts only as long as the antibodies or activated T cells remain alive in the body, generally just weeks or months.<\/li>\r\n \t
- Many pathogens have evolved mechanisms to evade the adaptive immune system. For example, human immunodeficiency virus ([pb_glossary id=\"4501\"]HIV[\/pb_glossary]) evades the adaptive immune system by frequently changing its antigens and by forming its outer envelope from the host\u2019s cell membrane.<\/li>\r\n<\/ul>\r\n<\/div>\r\n<\/div>\r\n
\r\n17.5 Review Questions<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- What is the adaptive immune system?<\/li>\r\n \t
- Define immunity.<\/li>\r\n \t
- [h5p id=\"639\"]<\/li>\r\n \t
- How are lymphocytes activated?<\/li>\r\n \t
- Identify two common types of T cells and their functions.<\/li>\r\n \t
- How do activated B cells help defend against pathogens?<\/li>\r\n \t
- How does passive immunity differ from active immunity?\u00a0How may passive immunity occur?<\/li>\r\n \t
- What are two ways that active immunity may come about?<\/li>\r\n \t
- What ways of evading the human adaptive immune system evolved in human immunodeficiency virus (HIV)?<\/li>\r\n \t
- Why do vaccinations expose\u00a0a person to a version of a pathogen?<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n
\r\n17.5 Explore More<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\nhttps:\/\/youtu.be\/rb7TVW77ZCs\r\nHow do vaccines work? - Kelwalin Dhanasarnsombut, TED-Ed, 2015.<\/p>\r\nhttps:\/\/youtu.be\/yqUFy-t4MlQ\r\n
How we conquered the deadly smallpox virus - Simona Zompi, TED-Ed, 2013.<\/p>\r\nhttps:\/\/youtu.be\/5THf6gTNqO8\r\n
Why Do We Need A New Flu Shot Every Year? Seeker, 2015.<\/p>\r\nhttps:\/\/youtu.be\/X-rC78MKZvw\r\n
An HIV Vaccine: Mapping Uncharted Territory, NIAID, 2016.<\/p>\r\n\r\n<\/div>\r\n<\/div>\r\n \r\n
Attributions<\/h2>\r\nFigure 17.5.1<\/strong>\r\n\r\nKiller_T_cells_surround_a_cancer_cell<\/a> by Alex Ritter, Jennifer Lippincott Schwartz and Gillian Griffiths at the National Institutes of Health\/ Visuals Online<\/a> on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).\r\n\r\nFigure 17.5.2<\/strong>\r\n\r\nT_cell_activation.svg<\/a> by Rehua<\/a> (derivative work) on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).\u00a0(Original image: T_cell_activation.png<\/a>:\u00a0 from The Immune System - NIH Publication No. 03\u20135423<\/a>)\r\n\r\nFigure 17.5.3<\/strong>\r\n\r\nCytotoxic T Cell function<\/a>\u00a0by 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 17.5.4<\/strong>\r\n\r\nB_cell_activation.svg<\/a> by Fred the Oyster<\/a> on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain). (Original from The Immune System - NIH Publication No. 03\u20135423<\/a>)\r\n\r\nFigure 17.5.5<\/strong>\r\n\r\nAntibody.svg<\/a> by Fvasconcellos<\/a>\u00a0 on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain). (Original black and white image from the National Human Genome Research Institute's Talking Genetics Glossary<\/a>)\r\n\r\nFigure 17.5.7<\/strong>\r\n\r\nimmunizations<\/a> by U.S. Air Force photo by Airman 1st Class Destinee Dougherty from Military Health System website, Health.mil, is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).\r\n\r\n \r\nReferences<\/h2>\r\n
HealthLinkBC. (2018). B.C. immunization schedules. \u00a0Gov.BC.CA. \u00a0https:\/\/www.healthlinkbc.ca\/tools-videos\/bc-immunization-schedules<\/p>\r\n
Mayo Clinic Staff. (n.d.). Measles [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/measles\/symptoms-causes\/syc-20374857<\/p>\r\n
Mayo Clinic Staff. (n.d.). Mumps [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/mumps\/symptoms-causes\/syc-20375361<\/p>\r\n
Mayo Clinic Staff. (n.d.). Polio [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/polio\/symptoms-causes\/syc-20376512<\/p>\r\n
Mayo Clinic Staff. (n.d.). Smallpox [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/smallpox\/symptoms-causes\/syc-20353027<\/p>\r\n
NIAID. (2016, August 11). An HIV vaccine: Mapping uncharted territory. YouTube. https:\/\/www.youtube.com\/watch?v=X-rC78MKZvw&feature=youtu.be<\/p>\r\n
OpenStax. (2016, March 23). Figure 4 Na\u00efve\u00a0CD4<\/mtext>+<\/sup>\u00a0T cells engage MHC II molecules on antigen-presenting cells (APCs) and become activated. Clones of the activated helper T cell, in turn, activate B cells and CD8+<\/sup> T cells, which become cytotoxic T cells. Cytotoxic T cells kill infected cells [digital image].\u00a0In OpenStax, Biology<\/em> (Section 42.2). OpenStax CNX. https:\/\/cnx.org\/contents\/GFy_h8cu@10.53:etZobsU-@6\/Adaptive-Immune-Response<\/p>\r\nSeeker. (2015, September 2). Why do we need a new flu shot every year? YouTube. https:\/\/www.youtube.com\/watch?v=5THf6gTNqO8<\/p>\r\n
TED-Ed. (2015, January 12). How do vaccines work? - Kelwalin Dhanasarnsombut. YouTube. https:\/\/www.youtube.com\/watch?v=rb7TVW77ZCs&feature=youtu.be<\/p>\r\n
TED-Ed. (2013, October 28). How we conquered the deadly smallpox virus - Simona Zompi. YouTube. https:\/\/www.youtube.com\/watch?v=yqUFy-t4MlQ&feature=youtu.be<\/p>","rendered":"
<\/p>\nFigure 17.5.1 Kill the cancer.<\/em><\/figcaption><\/figure>\n\nThe Kiss of Death<\/h1>\n<\/div>\n
The photomicrograph in Figure 17.5.1 shows a group of killer T cells (green and red) surrounding a cancer<\/a> cell (blue, centre). When a killer T cell makes contact with the cancer cell, it attaches to and spreads over the dangerous target. The killer T cell then uses special chemicals stored in vesicles<\/span><\/a><\/span>\u00a0(red) to deliver the killing blow. This event has thus been nicknamed \u201cthe kiss of death.\u201d After the target cell is killed, the killer T cells move on to find the next victim. Killer T cells like these are important players in the adaptive immune system.<\/span><\/p>\n\nWhat Is the Adaptive Immune System?<\/h1>\n<\/div>\n
The\u00a0adaptive immune system<\/a><\/strong>\u00a0is a subsystem of the overall immune system. It is composed of highly specialized cells and processes that eliminate specific pathogens<\/span><\/a><\/span>\u00a0and tumor cells. An adaptive immune response is set in motion by antigen<\/span>s<\/span><\/a>\u00a0that the immune system recognizes as foreign. Unlike an innate immune response, an adaptive immune response is highly specific to a particular pathogen (or its antigen). An important function of the adaptive immune system that is\u00a0<\/span>not<\/em>\u00a0shared by the innate immune system is the creation of immunological memory \u2014 or immunity<\/a> \u2014 which occurs after the initial response to a specific pathogen. It allows for a faster, stronger response on subsequent encounters with the same pathogen, usually before the pathogen can even cause symptoms of illness.<\/span><\/p>\nLymphocytes<\/span><\/a><\/span>\u00a0are the main cells of the adaptive immune system. They are leukocyte<\/span>s<\/span><\/a><\/span>\u00a0that arise and mature in organs of the lymphatic system, including the bone marrow<\/a> and thymus<\/a>. The human body normally has about 2 trillion lymphocytes, which constitute about 1\/3 of all leukocytes. Most of the lymphocytes are normally sequestered within tissue fluid or organs of the lymphatic system, including the tonsil<\/span>s<\/span><\/a><\/span>, spleen<\/a>, and lymph node<\/span>s<\/span><\/a><\/span>. Only about 2% of the lymphocytes are normally circulating in the blood. There are two main types of lymphocytes involved in adaptive immune responses, called T cells and B cells. T cells destroy infected cells or release chemicals that regulate immune responses. B cells secrete antibodies<\/a> that bind with antigen<\/span>s<\/span>[<\/span>\/pb_glossary]<\/span>\u00a0of [pb_glossary id=\"271\"]pathogen<\/span>s<\/span><\/a><\/span>\u00a0so they can be removed by other immune cells or processes.<\/span><\/p>\n\nPathways of the Adaptive Immune Response<\/h1>\n
There are some general similarities in the way in which the separate adaptive immune responses occur in T cell and B cell responses.\u00a0 In both pathways, a foreign antigen is recognized by the B or T cell.\u00a0 From there, cytokines produced by helper T-cells promote clonal expansion of lymphocytes.\u00a0 From this clonal expansion, two types of B or T cells are produced- cells that directly fight the pathogen invasion and cells that remain behind to provide long-term immunity<\/a>.\u00a0 Finally, once the pathogen invasion has been eradicated, the plasma cells and killer T cells go through apoptosis (programmed cell death).<\/p>\nT Cells<\/h1>\n<\/div>\n
There are multiple types of\u00a0T cells<\/a>,<\/strong>\u00a0or T lymphocytes. Major types are killer (or cytotoxic) T cells<\/a> and helper T cells<\/a>. Both types develop from immature T cells that become activated by exposure to an antigen<\/a>.<\/p>\nT Cell Activation (or Cell-Mediated Immunity)<\/h2>\n
T cells must be activated to become either killer T cells or helper T cells. This requires presentation of a foreign antigen by antigen-presenting cells<\/a>, as shown in Figure 17.5.2. Antigen-presenting cells may be dendritic cells<\/span><\/a><\/span>, macrophage<\/span>s<\/span><\/a><\/span>, or B cell<\/span>s<\/span><\/a><\/span>. Activation occurs when T cells are presented with a foreign antigen coupled with an MHC self antigen. Helper T cells are more easily activated than killer T cells. Activation of killer T cells is strongly regulated and may require additional stimulation from helper T cells.<\/span><\/p>\nFigure 17.5.2 Exposure to a foreign antigen on an antigen-presenting cell is necessary to activate T cells to become killer T cells or helper T cells.<\/em><\/figcaption><\/figure>\nKiller T Cells<\/h2>\n
Activated killer T cells induce the death of cells that bear a specific non-self<\/a> antigen because they are infected with pathogens or are cancerous. The antigen targets the cell for destruction by killer T cells, which travel through the bloodstream searching for target cells to kill. Killer T cells may use various mechanisms to kill target cells. One way is by releasing toxins in granules that enter and kill infected or cancerous cells (see Figure 17.5.3).<\/p>\nFigure 17.5.3 A killer T cell releases toxins that destroy an infected body cell and the viruses it contains.<\/em><\/figcaption><\/figure>\nHelper T Cells<\/h2>\n
Activated helper T cells<\/span><\/a><\/span>\u00a0do not kill infected or cancerous cells. Instead, their role is to \u201cmanage\u201d both innate<\/a> and adaptive immune<\/a> responses by directing other cells to perform these tasks. They control other cells by releasing cytokine<\/span>s<\/span><\/a><\/span>, which are proteins that can influence the activity of many cell types, including killer T cell<\/span>s<\/span><\/a><\/span>, B cells<\/a>, and macrophages<\/a>.\u00a0Some cytokines released by helper T cells assist with the activation of killer T cells.<\/span><\/p>\n\nB Cells<\/h1>\n<\/div>\n
B cell<\/strong>s<\/strong><\/a><\/strong>, or B lymphocytes, are the major cells involved in the creation of antibodies<\/a> that circulate in blood plasma<\/a> and lymph<\/a>. Antibodies are large, Y-shaped proteins used by the immune system to identify and neutralize foreign invaders. Besides producing antibodies, B cells may also function as antigen-presenting cell<\/span>s<\/span><\/a><\/span>, or secrete cytokines that help control other immune cells and responses.<\/span><\/p>\nB Cell Activation (or Antibody-Mediated Immunity)<\/h2>\n
Before B cells can actively function to defend the host, they must be activated. As shown in Figure 17.5.4, B cell activation begins when a B cell engulfs and digests an antigen. The antigen may be either free floating in the lymph, or it may be presented by an antigen-presenting cell, such as a dendritic cell or macrophage. In either case, the B cell then displays antigen fragments bound to its own MHC antigens. The MHC-antigen complex on the B cell attracts helper T cells. The helper T cells, in turn, secrete cytokines that help the B cell to multiply, and the daughter cells to mature into plasma cells.<\/p>\nFigure 17.5.4 B cells are activated and become antibody-producing plasma cells with the help of helper T cells.<\/em><\/figcaption><\/figure>\nPlasma Cells<\/h2>\nFigure 17.5.5 Each antibody fits its antigen like a lock fits a key.<\/em><\/figcaption><\/figure>\n
T Cell Activation (or Cell-Mediated Immunity)<\/h2>\r\nT cells must be activated to become either killer T cells or helper T cells. This requires presentation of a foreign antigen by [pb_glossary id=\"4895\"]antigen-presenting cells[\/pb_glossary], as shown in Figure 17.5.2. Antigen-presenting cells may be [pb_glossary id=\"4875\"]dendritic cells<\/span>[\/pb_glossary]<\/span>, [pb_glossary id=\"4340\"]macrophage<\/span>s<\/span>[\/pb_glossary]<\/span>, or [pb_glossary id=\"5539\"]B cell<\/span>s<\/span>[\/pb_glossary]<\/span>. Activation occurs when T cells are presented with a foreign antigen coupled with an MHC self antigen. Helper T cells are more easily activated than killer T cells. Activation of killer T cells is strongly regulated and may require additional stimulation from helper T cells.<\/span>\r\n\r\n[caption id=\"attachment_4898\" align=\"aligncenter\" width=\"580\"] Figure 17.5.2 Exposure to a foreign antigen on an antigen-presenting cell is necessary to activate T cells to become killer T cells or helper T cells.<\/em>[\/caption]\r\nKiller T Cells<\/h2>\r\nActivated killer T cells induce the death of cells that bear a specific [pb_glossary id=\"5463\"]non-self[\/pb_glossary] antigen because they are infected with pathogens or are cancerous. The antigen targets the cell for destruction by killer T cells, which travel through the bloodstream searching for target cells to kill. Killer T cells may use various mechanisms to kill target cells. One way is by releasing toxins in granules that enter and kill infected or cancerous cells (see Figure 17.5.3).\r\n\r\n[caption id=\"attachment_4900\" align=\"aligncenter\" width=\"1024\"] Figure 17.5.3 A killer T cell releases toxins that destroy an infected body cell and the viruses it contains.<\/em>[\/caption]\r\nHelper T Cells<\/h2>\r\nActivated [pb_glossary id=\"5537\"]helper T cells<\/span>[\/pb_glossary]<\/span>\u00a0do not kill infected or cancerous cells. Instead, their role is to \u201cmanage\u201d both [pb_glossary id=\"4820\"]innate[\/pb_glossary] and [pb_glossary id=\"4822\"]adaptive immune[\/pb_glossary] responses by directing other cells to perform these tasks. They control other cells by releasing [pb_glossary id=\"4865\"]cytokine<\/span>s<\/span>[\/pb_glossary]<\/span>, which are proteins that can influence the activity of many cell types, including [pb_glossary id=\"4893\"]killer T cell<\/span>s<\/span>[\/pb_glossary]<\/span>, [pb_glossary id=\"5539\"]B cells[\/pb_glossary], and [pb_glossary id=\"4340\"]macrophages[\/pb_glossary].\u00a0Some cytokines released by helper T cells assist with the activation of killer T cells.<\/span>\r\n\r\nB Cells<\/h1>\r\n<\/div>\r\n[pb_glossary id=\"5539\"]B cell<\/strong>s<\/strong>[\/pb_glossary]<\/strong>, or B lymphocytes, are the major cells involved in the creation of [pb_glossary id=\"5659\"]antibodies[\/pb_glossary] that circulate in blood [pb_glossary id=\"4395\"]plasma[\/pb_glossary] and [pb_glossary id=\"4552\"]lymph[\/pb_glossary]. Antibodies are large, Y-shaped proteins used by the [pb_glossary id=\"4242\"]immune system[\/pb_glossary] to identify and neutralize foreign invaders. Besides producing antibodies, B cells may also function as [pb_glossary id=\"4895\"]antigen-presenting cell<\/span>s<\/span>[\/pb_glossary]<\/span>, or secrete cytokines that help control other immune cells and responses.<\/span>\r\nB Cell Activation (or Antibody-Mediated Immunity)<\/h2>\r\nBefore B cells can actively function to defend the host, they must be activated. As shown in Figure 17.5.4, B cell activation begins when a B cell engulfs and digests an antigen. The antigen may be either free floating in the lymph, or it may be presented by an antigen-presenting cell, such as a dendritic cell or macrophage. In either case, the B cell then displays antigen fragments bound to its own MHC antigens. The MHC-antigen complex on the B cell attracts helper T cells. The helper T cells, in turn, secrete cytokines that help the B cell to multiply, and the daughter cells to mature into plasma cells.\r\n\r\n[caption id=\"attachment_4901\" align=\"aligncenter\" width=\"583\"] Figure 17.5.4 B cells are activated and become antibody-producing plasma cells with the help of helper T cells.<\/em>[\/caption]\r\nPlasma Cells<\/h2>\r\n[caption id=\"attachment_4903\" align=\"alignright\" width=\"325\"] Figure 17.5.5 Each antibody fits its antigen like a lock fits a key.<\/em>[\/caption]\r\n\r\n[pb_glossary id=\"4902\"]Plasma cell<\/strong>s<\/strong>[\/pb_glossary]<\/strong> are antibody-secreting cells that form from activated B cells. Each plasma cell is like a tiny antibody factory. It may secrete millions of copies of an antibody, each of which can bind to the specific antigen that activated the original B cell. The specificity of an antibody to a specific antigen is illustrated in Figure 17.5.5. When antibodies bind with antigens, it makes the cells bearing them easier targets for phagocytes to find and destroy. Antibody-antigen complexes may also trigger the complement system of the innate immune system, which destroys the cells in a cascade of protein enzymes. In addition, the complexes are likely to clump together (agglutinate). If this occurs, they are filtered out of the blood in the spleen or liver.<\/span>\r\n\r\nImmunity<\/h1>\r\n<\/div>\r\nOnce a pathogen has been cleared from the body, most activated T cells and B cells die within a few days.\u00a0A\u00a0few of the cells, however, survive and remain in the body as memory T cells or memory B cells. These [pb_glossary id=\"4904\"]memory cell<\/strong>s[\/pb_glossary] are ready to activate an immediate response if they are exposed to the same antigen again in the future. This is the basis of\u00a0[pb_glossary id=\"4884\"]immunity[\/pb_glossary].<\/strong>\r\n\r\nThe earliest known reference to the concept of immunity relates to the bubonic plaque (see Figure 17.5.6). In 430 B.C., a Greek historian and general named Thucydides noted that people who had recovered from a previous bout of the plague could nurse people who were sick with the plague without contracting the illness a second time. We now know that this is true of many diseases, and that it occurs because of active immunity.\r\n\r\n[caption id=\"attachment_4905\" align=\"aligncenter\" width=\"332\"] Figure 17.5.6 Dead, blackened tissues at the finger tips and other extremities are a sign of the bubonic plague, giving rise to its other name, the black death.<\/em>[\/caption]\r\nActive Immunity<\/h2>\r\n[pb_glossary id=\"4906\"]Active immunity[\/pb_glossary]<\/strong>\u00a0is the ability of the adaptive immune system to resist a specific pathogen because it has formed an immunological memory of the pathogen. Active immunity is adaptive, because it occurs during the lifetime of an individual as an adaptation to infection with a specific pathogen, and prepares the immune system for future challenges from that pathogen. Active immunity can come about naturally or artificially.\r\nNaturally Acquired Active Immunity<\/h3>\r\nActive immunity is acquired naturally when a pathogen invades the body and activates the adaptive immune system. When the initial infection is over, memory B cells and memory T cells remain, providing immunological memory of the pathogen. As long as the memory cells are alive, the immune system is ready to mount an immediate response if the same pathogen tries to infect the body again.\r\nArtificially Acquired Active Immunity<\/h3>\r\nActive immunity can also be acquired artificially through immunization.\u00a0[pb_glossary id=\"4908\"]Immunization[\/pb_glossary]<\/strong> is the deliberate exposure of a person to a pathogen in order to provoke an adaptive immune response and the formation of memory cells specific to that pathogen. The pathogen is introduced in a vaccine \u2014 usually by injection, sometimes by nose or mouth (see Figure 17.5.7) \u2014 so immunization is also called vaccination.\r\n\r\n[caption id=\"attachment_4909\" align=\"alignleft\" width=\"511\"] Figure 17.5.7 This young child is receiving a vaccine. Vaccines are a safe way to create immunity against life-threatening diseases.<\/em>[\/caption]\r\n\r\nTypically, only part of a pathogen, a weakened form of the pathogen, or a dead pathogen is used in a vaccine, which causes an adaptive immune response without making the immunized person sick. This is how you most likely became immune to diseases such as measles<\/a>, mumps<\/a>, and chicken pox. Immunizations may last for a lifetime, or they may require periodic booster shots to maintain immunity. While immunization generally has long-lasting effects, it usually takes several weeks to develop full immunity.\r\n\r\nImmunization is the most effective method ever discovered of preventing infectious diseases. As many as 3 million deaths are prevented each year because of vaccinations. Widespread immunity from vaccinations is largely responsible for the worldwide eradication of smallpox<\/a>, and the near elimination of several other infectious diseases from many populations, including polio<\/a> and measles. Immunization is so successful because it exploits the natural specificity and inducibility of the adaptive immune system.\r\nPassive Immunity<\/h2>\r\n[pb_glossary id=\"4910\"]Passive immunity[\/pb_glossary]<\/strong>\u00a0results when pathogen-specific antibodies or activated T cells are transferred to a person who has never been exposed to the pathogen. Passive immunity provides immediate protection from a pathogen, but the adaptive immune system does not develop immunological memory to protect the host from the same pathogen in the future. Unlike active immunity, passive immunity lasts only as long as the transferred antibodies or T cells survive in the blood \u2014 usually between a few days and a few months. However, like active immunity, passive immunity can be acquired both naturally and artificially.\r\nNaturally Acquired Passive Immunity<\/h3>\r\nPassive immunity is acquired naturally by a fetus through its mother\u2019s blood. [pb_glossary id=\"5659\"]Antibodies[\/pb_glossary] are transported from mother to fetus across the placenta, so babies have high levels of antibodies at birth. Their antibodies have the same range of antigen specificities as their mother\u2019s. Passive immunity may also be acquired by an infant through the mother\u2019s breast milk. This gives young infants protection from common pathogens in their environment while their own immune system matures.\r\nArtificially Acquired Passive Immunity<\/h3>\r\nOlder children and adults can acquire passive immunity artificially through the injection of antibodies or activated T cells, which may be done when there is a high risk of infection and insufficient time for the body to develop active immunity through vaccination. It may also be done to reduce symptoms of ongoing disease, or to compensate for immunodeficiency diseases.\r\n\r\nAdaptive Immune Evasion<\/h1>\r\n<\/div>\r\nMany pathogens have been around for a long time, living with human populations for generations. To persist, some have evolved mechanisms to evade the adaptive immune system of human hosts. One way they have done this is by rapidly changing their non-essential antigens. This is called antigenic variation. An example of a pathogen that takes this approach is human immunodeficiency virus (HIV). It mutates rapidly so the proteins on its viral envelope are constantly changing. By the time the adaptive immune system responds, the virus\u2019s antigens have changed. Antigenic variation is the main reason that efforts to develop a vaccine against HIV have not yet been successful.\r\n\r\nAnother evasion approach that some pathogens may take is to mask pathogen antigens with host molecules so the host\u2019s immune system cannot detect the antigens. HIV takes this approach, as well. The envelope that covers the virus is formed from the outermost membrane of the host cell.\r\n\r\nFeature: My Human Body<\/h1>\r\n<\/div>\r\nIf you think that immunizations are just for kids, think again. There are several vaccines recommended by HealthLinkBC<\/a> for people over the age of 18. The tables below from HealthLinkBC show the vaccine schedules recommended for infants and children, school-aged children, and adults and senior. Additional vaccines may be recommended for certain adults based on specific travel plans, medical conditions or other indications. Are you up to date with your vaccines? You can check with your doctor to be sure.\r\n\r\n\r\n\r\n\r\n<\/div>\r\n\r\n\r\n \r\n\r\n\r\n\r\n17.5 Summary<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- The [pb_glossary id=\"4822\"]adaptive immune system[\/pb_glossary] is a subsystem of the overall immune system that recognizes and makes a tailored attack against specific [pb_glossary id=\"5799\"]pathogens[\/pb_glossary]\u00a0or tumor cells. It is a slower, but more effective response than the [pb_glossary id=\"4820\"]innate immune response[\/pb_glossary], and also leads to immunity to particular pathogens.<\/li>\r\n \t
- [pb_glossary id=\"4832\"]Lymphocytes[\/pb_glossary]\u00a0produced by the [pb_glossary id=\"2911\"]lymphatic system[\/pb_glossary] are the main cells of the adaptive immune system. There are two major types of lymphocytes: [pb_glossary id=\"4892\"]T cells[\/pb_glossary]\u00a0and [pb_glossary id=\"5539\"]B cells[\/pb_glossary]. Both types must be activated by foreign [pb_glossary id=\"2704\"]antigens[\/pb_glossary]\u00a0to become functioning immune cells.<\/li>\r\n \t
- Most activated T cells become either [pb_glossary id=\"4893\"]killer T cells[\/pb_glossary]\u00a0or [pb_glossary id=\"5537\"]helper T cells[\/pb_glossary]. Killer T cells destroy cells that are infected with pathogens or are cancerous. Helper T cells manage immune responses by releasing [pb_glossary id=\"4865\"]cytokines[\/pb_glossary]\u00a0that control other types of leukocytes.<\/li>\r\n \t
- Activated B cells form [pb_glossary id=\"4902\"]plasma cells[\/pb_glossary]\u00a0that secrete [pb_glossary id=\"5659\"]antibodies[\/pb_glossary], which bind to specific antigens on pathogens or infected cells. The antibody-antigen complexes generally lead to the destruction of the cells, for example, by attracting phagocytes or triggering the [pb_glossary id=\"4868\"]complement system[\/pb_glossary].<\/li>\r\n \t
- After an adaptive immune response occurs, long-lasting [pb_glossary id=\"4904\"]memory B cells and memory T cells[\/pb_glossary] may remain to confer immunity to the specific pathogen that caused the adaptive immune response. These memory cells are ready to activate an immediate response if they are exposed to the same antigen again in the future.<\/li>\r\n \t
- Immunity may be active or passive. Active immunity occurs when the immune system has been presented with antigens that elicit an adaptive immune response. This may occur naturally as the result of an infection, or artificially as the result of immunization. Active immunity may last for years or even for life.<\/li>\r\n \t
- Passive immunity occurs without an adaptive immune response by the transfer of antibodies or activated T cells. This may occur naturally between a mother and her fetus or her nursing infant, or it may occur artificially by injection. Passive immunity lasts only as long as the antibodies or activated T cells remain alive in the body, generally just weeks or months.<\/li>\r\n \t
- Many pathogens have evolved mechanisms to evade the adaptive immune system. For example, human immunodeficiency virus ([pb_glossary id=\"4501\"]HIV[\/pb_glossary]) evades the adaptive immune system by frequently changing its antigens and by forming its outer envelope from the host\u2019s cell membrane.<\/li>\r\n<\/ul>\r\n<\/div>\r\n<\/div>\r\n
\r\n17.5 Review Questions<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- What is the adaptive immune system?<\/li>\r\n \t
- Define immunity.<\/li>\r\n \t
- [h5p id=\"639\"]<\/li>\r\n \t
- How are lymphocytes activated?<\/li>\r\n \t
- Identify two common types of T cells and their functions.<\/li>\r\n \t
- How do activated B cells help defend against pathogens?<\/li>\r\n \t
- How does passive immunity differ from active immunity?\u00a0How may passive immunity occur?<\/li>\r\n \t
- What are two ways that active immunity may come about?<\/li>\r\n \t
- What ways of evading the human adaptive immune system evolved in human immunodeficiency virus (HIV)?<\/li>\r\n \t
- Why do vaccinations expose\u00a0a person to a version of a pathogen?<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n
\r\n17.5 Explore More<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\nhttps:\/\/youtu.be\/rb7TVW77ZCs\r\nHow do vaccines work? - Kelwalin Dhanasarnsombut, TED-Ed, 2015.<\/p>\r\nhttps:\/\/youtu.be\/yqUFy-t4MlQ\r\n
How we conquered the deadly smallpox virus - Simona Zompi, TED-Ed, 2013.<\/p>\r\nhttps:\/\/youtu.be\/5THf6gTNqO8\r\n
Why Do We Need A New Flu Shot Every Year? Seeker, 2015.<\/p>\r\nhttps:\/\/youtu.be\/X-rC78MKZvw\r\n
An HIV Vaccine: Mapping Uncharted Territory, NIAID, 2016.<\/p>\r\n\r\n<\/div>\r\n<\/div>\r\n \r\n
Attributions<\/h2>\r\nFigure 17.5.1<\/strong>\r\n\r\nKiller_T_cells_surround_a_cancer_cell<\/a> by Alex Ritter, Jennifer Lippincott Schwartz and Gillian Griffiths at the National Institutes of Health\/ Visuals Online<\/a> on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).\r\n\r\nFigure 17.5.2<\/strong>\r\n\r\nT_cell_activation.svg<\/a> by Rehua<\/a> (derivative work) on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).\u00a0(Original image: T_cell_activation.png<\/a>:\u00a0 from The Immune System - NIH Publication No. 03\u20135423<\/a>)\r\n\r\nFigure 17.5.3<\/strong>\r\n\r\nCytotoxic T Cell function<\/a>\u00a0by 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 17.5.4<\/strong>\r\n\r\nB_cell_activation.svg<\/a> by Fred the Oyster<\/a> on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain). (Original from The Immune System - NIH Publication No. 03\u20135423<\/a>)\r\n\r\nFigure 17.5.5<\/strong>\r\n\r\nAntibody.svg<\/a> by Fvasconcellos<\/a>\u00a0 on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain). (Original black and white image from the National Human Genome Research Institute's Talking Genetics Glossary<\/a>)\r\n\r\nFigure 17.5.7<\/strong>\r\n\r\nimmunizations<\/a> by U.S. Air Force photo by Airman 1st Class Destinee Dougherty from Military Health System website, Health.mil, is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).\r\n\r\n \r\nReferences<\/h2>\r\n
HealthLinkBC. (2018). B.C. immunization schedules. \u00a0Gov.BC.CA. \u00a0https:\/\/www.healthlinkbc.ca\/tools-videos\/bc-immunization-schedules<\/p>\r\n
Mayo Clinic Staff. (n.d.). Measles [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/measles\/symptoms-causes\/syc-20374857<\/p>\r\n
Mayo Clinic Staff. (n.d.). Mumps [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/mumps\/symptoms-causes\/syc-20375361<\/p>\r\n
Mayo Clinic Staff. (n.d.). Polio [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/polio\/symptoms-causes\/syc-20376512<\/p>\r\n
Mayo Clinic Staff. (n.d.). Smallpox [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/smallpox\/symptoms-causes\/syc-20353027<\/p>\r\n
NIAID. (2016, August 11). An HIV vaccine: Mapping uncharted territory. YouTube. https:\/\/www.youtube.com\/watch?v=X-rC78MKZvw&feature=youtu.be<\/p>\r\n
OpenStax. (2016, March 23). Figure 4 Na\u00efve\u00a0CD4<\/mtext>+<\/sup>\u00a0T cells engage MHC II molecules on antigen-presenting cells (APCs) and become activated. Clones of the activated helper T cell, in turn, activate B cells and CD8+<\/sup> T cells, which become cytotoxic T cells. Cytotoxic T cells kill infected cells [digital image].\u00a0In OpenStax, Biology<\/em> (Section 42.2). OpenStax CNX. https:\/\/cnx.org\/contents\/GFy_h8cu@10.53:etZobsU-@6\/Adaptive-Immune-Response<\/p>\r\nSeeker. (2015, September 2). Why do we need a new flu shot every year? YouTube. https:\/\/www.youtube.com\/watch?v=5THf6gTNqO8<\/p>\r\n
TED-Ed. (2015, January 12). How do vaccines work? - Kelwalin Dhanasarnsombut. YouTube. https:\/\/www.youtube.com\/watch?v=rb7TVW77ZCs&feature=youtu.be<\/p>\r\n
TED-Ed. (2013, October 28). How we conquered the deadly smallpox virus - Simona Zompi. YouTube. https:\/\/www.youtube.com\/watch?v=yqUFy-t4MlQ&feature=youtu.be<\/p>","rendered":"
<\/p>\nFigure 17.5.1 Kill the cancer.<\/em><\/figcaption><\/figure>\n\nThe Kiss of Death<\/h1>\n<\/div>\n
The photomicrograph in Figure 17.5.1 shows a group of killer T cells (green and red) surrounding a cancer<\/a> cell (blue, centre). When a killer T cell makes contact with the cancer cell, it attaches to and spreads over the dangerous target. The killer T cell then uses special chemicals stored in vesicles<\/span><\/a><\/span>\u00a0(red) to deliver the killing blow. This event has thus been nicknamed \u201cthe kiss of death.\u201d After the target cell is killed, the killer T cells move on to find the next victim. Killer T cells like these are important players in the adaptive immune system.<\/span><\/p>\n\nWhat Is the Adaptive Immune System?<\/h1>\n<\/div>\n
The\u00a0adaptive immune system<\/a><\/strong>\u00a0is a subsystem of the overall immune system. It is composed of highly specialized cells and processes that eliminate specific pathogens<\/span><\/a><\/span>\u00a0and tumor cells. An adaptive immune response is set in motion by antigen<\/span>s<\/span><\/a>\u00a0that the immune system recognizes as foreign. Unlike an innate immune response, an adaptive immune response is highly specific to a particular pathogen (or its antigen). An important function of the adaptive immune system that is\u00a0<\/span>not<\/em>\u00a0shared by the innate immune system is the creation of immunological memory \u2014 or immunity<\/a> \u2014 which occurs after the initial response to a specific pathogen. It allows for a faster, stronger response on subsequent encounters with the same pathogen, usually before the pathogen can even cause symptoms of illness.<\/span><\/p>\nLymphocytes<\/span><\/a><\/span>\u00a0are the main cells of the adaptive immune system. They are leukocyte<\/span>s<\/span><\/a><\/span>\u00a0that arise and mature in organs of the lymphatic system, including the bone marrow<\/a> and thymus<\/a>. The human body normally has about 2 trillion lymphocytes, which constitute about 1\/3 of all leukocytes. Most of the lymphocytes are normally sequestered within tissue fluid or organs of the lymphatic system, including the tonsil<\/span>s<\/span><\/a><\/span>, spleen<\/a>, and lymph node<\/span>s<\/span><\/a><\/span>. Only about 2% of the lymphocytes are normally circulating in the blood. There are two main types of lymphocytes involved in adaptive immune responses, called T cells and B cells. T cells destroy infected cells or release chemicals that regulate immune responses. B cells secrete antibodies<\/a> that bind with antigen<\/span>s<\/span>[<\/span>\/pb_glossary]<\/span>\u00a0of [pb_glossary id=\"271\"]pathogen<\/span>s<\/span><\/a><\/span>\u00a0so they can be removed by other immune cells or processes.<\/span><\/p>\n\nPathways of the Adaptive Immune Response<\/h1>\n
There are some general similarities in the way in which the separate adaptive immune responses occur in T cell and B cell responses.\u00a0 In both pathways, a foreign antigen is recognized by the B or T cell.\u00a0 From there, cytokines produced by helper T-cells promote clonal expansion of lymphocytes.\u00a0 From this clonal expansion, two types of B or T cells are produced- cells that directly fight the pathogen invasion and cells that remain behind to provide long-term immunity<\/a>.\u00a0 Finally, once the pathogen invasion has been eradicated, the plasma cells and killer T cells go through apoptosis (programmed cell death).<\/p>\nT Cells<\/h1>\n<\/div>\n
There are multiple types of\u00a0T cells<\/a>,<\/strong>\u00a0or T lymphocytes. Major types are killer (or cytotoxic) T cells<\/a> and helper T cells<\/a>. Both types develop from immature T cells that become activated by exposure to an antigen<\/a>.<\/p>\nT Cell Activation (or Cell-Mediated Immunity)<\/h2>\n
T cells must be activated to become either killer T cells or helper T cells. This requires presentation of a foreign antigen by antigen-presenting cells<\/a>, as shown in Figure 17.5.2. Antigen-presenting cells may be dendritic cells<\/span><\/a><\/span>, macrophage<\/span>s<\/span><\/a><\/span>, or B cell<\/span>s<\/span><\/a><\/span>. Activation occurs when T cells are presented with a foreign antigen coupled with an MHC self antigen. Helper T cells are more easily activated than killer T cells. Activation of killer T cells is strongly regulated and may require additional stimulation from helper T cells.<\/span><\/p>\nFigure 17.5.2 Exposure to a foreign antigen on an antigen-presenting cell is necessary to activate T cells to become killer T cells or helper T cells.<\/em><\/figcaption><\/figure>\nKiller T Cells<\/h2>\n
Activated killer T cells induce the death of cells that bear a specific non-self<\/a> antigen because they are infected with pathogens or are cancerous. The antigen targets the cell for destruction by killer T cells, which travel through the bloodstream searching for target cells to kill. Killer T cells may use various mechanisms to kill target cells. One way is by releasing toxins in granules that enter and kill infected or cancerous cells (see Figure 17.5.3).<\/p>\nFigure 17.5.3 A killer T cell releases toxins that destroy an infected body cell and the viruses it contains.<\/em><\/figcaption><\/figure>\nHelper T Cells<\/h2>\n
Activated helper T cells<\/span><\/a><\/span>\u00a0do not kill infected or cancerous cells. Instead, their role is to \u201cmanage\u201d both innate<\/a> and adaptive immune<\/a> responses by directing other cells to perform these tasks. They control other cells by releasing cytokine<\/span>s<\/span><\/a><\/span>, which are proteins that can influence the activity of many cell types, including killer T cell<\/span>s<\/span><\/a><\/span>, B cells<\/a>, and macrophages<\/a>.\u00a0Some cytokines released by helper T cells assist with the activation of killer T cells.<\/span><\/p>\n\nB Cells<\/h1>\n<\/div>\n
B cell<\/strong>s<\/strong><\/a><\/strong>, or B lymphocytes, are the major cells involved in the creation of antibodies<\/a> that circulate in blood plasma<\/a> and lymph<\/a>. Antibodies are large, Y-shaped proteins used by the immune system to identify and neutralize foreign invaders. Besides producing antibodies, B cells may also function as antigen-presenting cell<\/span>s<\/span><\/a><\/span>, or secrete cytokines that help control other immune cells and responses.<\/span><\/p>\nB Cell Activation (or Antibody-Mediated Immunity)<\/h2>\n
Before B cells can actively function to defend the host, they must be activated. As shown in Figure 17.5.4, B cell activation begins when a B cell engulfs and digests an antigen. The antigen may be either free floating in the lymph, or it may be presented by an antigen-presenting cell, such as a dendritic cell or macrophage. In either case, the B cell then displays antigen fragments bound to its own MHC antigens. The MHC-antigen complex on the B cell attracts helper T cells. The helper T cells, in turn, secrete cytokines that help the B cell to multiply, and the daughter cells to mature into plasma cells.<\/p>\nFigure 17.5.4 B cells are activated and become antibody-producing plasma cells with the help of helper T cells.<\/em><\/figcaption><\/figure>\nPlasma Cells<\/h2>\nFigure 17.5.5 Each antibody fits its antigen like a lock fits a key.<\/em><\/figcaption><\/figure>\n
Figure 17.5.3 A killer T cell releases toxins that destroy an infected body cell and the viruses it contains.<\/em>[\/caption]\r\nHelper T Cells<\/h2>\r\nActivated [pb_glossary id=\"5537\"]helper T cells<\/span>[\/pb_glossary]<\/span>\u00a0do not kill infected or cancerous cells. Instead, their role is to \u201cmanage\u201d both [pb_glossary id=\"4820\"]innate[\/pb_glossary] and [pb_glossary id=\"4822\"]adaptive immune[\/pb_glossary] responses by directing other cells to perform these tasks. They control other cells by releasing [pb_glossary id=\"4865\"]cytokine<\/span>s<\/span>[\/pb_glossary]<\/span>, which are proteins that can influence the activity of many cell types, including [pb_glossary id=\"4893\"]killer T cell<\/span>s<\/span>[\/pb_glossary]<\/span>, [pb_glossary id=\"5539\"]B cells[\/pb_glossary], and [pb_glossary id=\"4340\"]macrophages[\/pb_glossary].\u00a0Some cytokines released by helper T cells assist with the activation of killer T cells.<\/span>\r\n\r\nB Cells<\/h1>\r\n<\/div>\r\n[pb_glossary id=\"5539\"]B cell<\/strong>s<\/strong>[\/pb_glossary]<\/strong>, or B lymphocytes, are the major cells involved in the creation of [pb_glossary id=\"5659\"]antibodies[\/pb_glossary] that circulate in blood [pb_glossary id=\"4395\"]plasma[\/pb_glossary] and [pb_glossary id=\"4552\"]lymph[\/pb_glossary]. Antibodies are large, Y-shaped proteins used by the [pb_glossary id=\"4242\"]immune system[\/pb_glossary] to identify and neutralize foreign invaders. Besides producing antibodies, B cells may also function as [pb_glossary id=\"4895\"]antigen-presenting cell<\/span>s<\/span>[\/pb_glossary]<\/span>, or secrete cytokines that help control other immune cells and responses.<\/span>\r\nB Cell Activation (or Antibody-Mediated Immunity)<\/h2>\r\nBefore B cells can actively function to defend the host, they must be activated. As shown in Figure 17.5.4, B cell activation begins when a B cell engulfs and digests an antigen. The antigen may be either free floating in the lymph, or it may be presented by an antigen-presenting cell, such as a dendritic cell or macrophage. In either case, the B cell then displays antigen fragments bound to its own MHC antigens. The MHC-antigen complex on the B cell attracts helper T cells. The helper T cells, in turn, secrete cytokines that help the B cell to multiply, and the daughter cells to mature into plasma cells.\r\n\r\n[caption id=\"attachment_4901\" align=\"aligncenter\" width=\"583\"] Figure 17.5.4 B cells are activated and become antibody-producing plasma cells with the help of helper T cells.<\/em>[\/caption]\r\nPlasma Cells<\/h2>\r\n[caption id=\"attachment_4903\" align=\"alignright\" width=\"325\"] Figure 17.5.5 Each antibody fits its antigen like a lock fits a key.<\/em>[\/caption]\r\n\r\n[pb_glossary id=\"4902\"]Plasma cell<\/strong>s<\/strong>[\/pb_glossary]<\/strong> are antibody-secreting cells that form from activated B cells. Each plasma cell is like a tiny antibody factory. It may secrete millions of copies of an antibody, each of which can bind to the specific antigen that activated the original B cell. The specificity of an antibody to a specific antigen is illustrated in Figure 17.5.5. When antibodies bind with antigens, it makes the cells bearing them easier targets for phagocytes to find and destroy. Antibody-antigen complexes may also trigger the complement system of the innate immune system, which destroys the cells in a cascade of protein enzymes. In addition, the complexes are likely to clump together (agglutinate). If this occurs, they are filtered out of the blood in the spleen or liver.<\/span>\r\n\r\nImmunity<\/h1>\r\n<\/div>\r\nOnce a pathogen has been cleared from the body, most activated T cells and B cells die within a few days.\u00a0A\u00a0few of the cells, however, survive and remain in the body as memory T cells or memory B cells. These [pb_glossary id=\"4904\"]memory cell<\/strong>s[\/pb_glossary] are ready to activate an immediate response if they are exposed to the same antigen again in the future. This is the basis of\u00a0[pb_glossary id=\"4884\"]immunity[\/pb_glossary].<\/strong>\r\n\r\nThe earliest known reference to the concept of immunity relates to the bubonic plaque (see Figure 17.5.6). In 430 B.C., a Greek historian and general named Thucydides noted that people who had recovered from a previous bout of the plague could nurse people who were sick with the plague without contracting the illness a second time. We now know that this is true of many diseases, and that it occurs because of active immunity.\r\n\r\n[caption id=\"attachment_4905\" align=\"aligncenter\" width=\"332\"] Figure 17.5.6 Dead, blackened tissues at the finger tips and other extremities are a sign of the bubonic plague, giving rise to its other name, the black death.<\/em>[\/caption]\r\nActive Immunity<\/h2>\r\n[pb_glossary id=\"4906\"]Active immunity[\/pb_glossary]<\/strong>\u00a0is the ability of the adaptive immune system to resist a specific pathogen because it has formed an immunological memory of the pathogen. Active immunity is adaptive, because it occurs during the lifetime of an individual as an adaptation to infection with a specific pathogen, and prepares the immune system for future challenges from that pathogen. Active immunity can come about naturally or artificially.\r\nNaturally Acquired Active Immunity<\/h3>\r\nActive immunity is acquired naturally when a pathogen invades the body and activates the adaptive immune system. When the initial infection is over, memory B cells and memory T cells remain, providing immunological memory of the pathogen. As long as the memory cells are alive, the immune system is ready to mount an immediate response if the same pathogen tries to infect the body again.\r\nArtificially Acquired Active Immunity<\/h3>\r\nActive immunity can also be acquired artificially through immunization.\u00a0[pb_glossary id=\"4908\"]Immunization[\/pb_glossary]<\/strong> is the deliberate exposure of a person to a pathogen in order to provoke an adaptive immune response and the formation of memory cells specific to that pathogen. The pathogen is introduced in a vaccine \u2014 usually by injection, sometimes by nose or mouth (see Figure 17.5.7) \u2014 so immunization is also called vaccination.\r\n\r\n[caption id=\"attachment_4909\" align=\"alignleft\" width=\"511\"] Figure 17.5.7 This young child is receiving a vaccine. Vaccines are a safe way to create immunity against life-threatening diseases.<\/em>[\/caption]\r\n\r\nTypically, only part of a pathogen, a weakened form of the pathogen, or a dead pathogen is used in a vaccine, which causes an adaptive immune response without making the immunized person sick. This is how you most likely became immune to diseases such as measles<\/a>, mumps<\/a>, and chicken pox. Immunizations may last for a lifetime, or they may require periodic booster shots to maintain immunity. While immunization generally has long-lasting effects, it usually takes several weeks to develop full immunity.\r\n\r\nImmunization is the most effective method ever discovered of preventing infectious diseases. As many as 3 million deaths are prevented each year because of vaccinations. Widespread immunity from vaccinations is largely responsible for the worldwide eradication of smallpox<\/a>, and the near elimination of several other infectious diseases from many populations, including polio<\/a> and measles. Immunization is so successful because it exploits the natural specificity and inducibility of the adaptive immune system.\r\nPassive Immunity<\/h2>\r\n[pb_glossary id=\"4910\"]Passive immunity[\/pb_glossary]<\/strong>\u00a0results when pathogen-specific antibodies or activated T cells are transferred to a person who has never been exposed to the pathogen. Passive immunity provides immediate protection from a pathogen, but the adaptive immune system does not develop immunological memory to protect the host from the same pathogen in the future. Unlike active immunity, passive immunity lasts only as long as the transferred antibodies or T cells survive in the blood \u2014 usually between a few days and a few months. However, like active immunity, passive immunity can be acquired both naturally and artificially.\r\nNaturally Acquired Passive Immunity<\/h3>\r\nPassive immunity is acquired naturally by a fetus through its mother\u2019s blood. [pb_glossary id=\"5659\"]Antibodies[\/pb_glossary] are transported from mother to fetus across the placenta, so babies have high levels of antibodies at birth. Their antibodies have the same range of antigen specificities as their mother\u2019s. Passive immunity may also be acquired by an infant through the mother\u2019s breast milk. This gives young infants protection from common pathogens in their environment while their own immune system matures.\r\nArtificially Acquired Passive Immunity<\/h3>\r\nOlder children and adults can acquire passive immunity artificially through the injection of antibodies or activated T cells, which may be done when there is a high risk of infection and insufficient time for the body to develop active immunity through vaccination. It may also be done to reduce symptoms of ongoing disease, or to compensate for immunodeficiency diseases.\r\n\r\nAdaptive Immune Evasion<\/h1>\r\n<\/div>\r\nMany pathogens have been around for a long time, living with human populations for generations. To persist, some have evolved mechanisms to evade the adaptive immune system of human hosts. One way they have done this is by rapidly changing their non-essential antigens. This is called antigenic variation. An example of a pathogen that takes this approach is human immunodeficiency virus (HIV). It mutates rapidly so the proteins on its viral envelope are constantly changing. By the time the adaptive immune system responds, the virus\u2019s antigens have changed. Antigenic variation is the main reason that efforts to develop a vaccine against HIV have not yet been successful.\r\n\r\nAnother evasion approach that some pathogens may take is to mask pathogen antigens with host molecules so the host\u2019s immune system cannot detect the antigens. HIV takes this approach, as well. The envelope that covers the virus is formed from the outermost membrane of the host cell.\r\n\r\nFeature: My Human Body<\/h1>\r\n<\/div>\r\nIf you think that immunizations are just for kids, think again. There are several vaccines recommended by HealthLinkBC<\/a> for people over the age of 18. The tables below from HealthLinkBC show the vaccine schedules recommended for infants and children, school-aged children, and adults and senior. Additional vaccines may be recommended for certain adults based on specific travel plans, medical conditions or other indications. Are you up to date with your vaccines? You can check with your doctor to be sure.\r\n\r\n\r\n\r\n\r\n<\/div>\r\n\r\n\r\n \r\n\r\n\r\n\r\n17.5 Summary<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- The [pb_glossary id=\"4822\"]adaptive immune system[\/pb_glossary] is a subsystem of the overall immune system that recognizes and makes a tailored attack against specific [pb_glossary id=\"5799\"]pathogens[\/pb_glossary]\u00a0or tumor cells. It is a slower, but more effective response than the [pb_glossary id=\"4820\"]innate immune response[\/pb_glossary], and also leads to immunity to particular pathogens.<\/li>\r\n \t
- [pb_glossary id=\"4832\"]Lymphocytes[\/pb_glossary]\u00a0produced by the [pb_glossary id=\"2911\"]lymphatic system[\/pb_glossary] are the main cells of the adaptive immune system. There are two major types of lymphocytes: [pb_glossary id=\"4892\"]T cells[\/pb_glossary]\u00a0and [pb_glossary id=\"5539\"]B cells[\/pb_glossary]. Both types must be activated by foreign [pb_glossary id=\"2704\"]antigens[\/pb_glossary]\u00a0to become functioning immune cells.<\/li>\r\n \t
- Most activated T cells become either [pb_glossary id=\"4893\"]killer T cells[\/pb_glossary]\u00a0or [pb_glossary id=\"5537\"]helper T cells[\/pb_glossary]. Killer T cells destroy cells that are infected with pathogens or are cancerous. Helper T cells manage immune responses by releasing [pb_glossary id=\"4865\"]cytokines[\/pb_glossary]\u00a0that control other types of leukocytes.<\/li>\r\n \t
- Activated B cells form [pb_glossary id=\"4902\"]plasma cells[\/pb_glossary]\u00a0that secrete [pb_glossary id=\"5659\"]antibodies[\/pb_glossary], which bind to specific antigens on pathogens or infected cells. The antibody-antigen complexes generally lead to the destruction of the cells, for example, by attracting phagocytes or triggering the [pb_glossary id=\"4868\"]complement system[\/pb_glossary].<\/li>\r\n \t
- After an adaptive immune response occurs, long-lasting [pb_glossary id=\"4904\"]memory B cells and memory T cells[\/pb_glossary] may remain to confer immunity to the specific pathogen that caused the adaptive immune response. These memory cells are ready to activate an immediate response if they are exposed to the same antigen again in the future.<\/li>\r\n \t
- Immunity may be active or passive. Active immunity occurs when the immune system has been presented with antigens that elicit an adaptive immune response. This may occur naturally as the result of an infection, or artificially as the result of immunization. Active immunity may last for years or even for life.<\/li>\r\n \t
- Passive immunity occurs without an adaptive immune response by the transfer of antibodies or activated T cells. This may occur naturally between a mother and her fetus or her nursing infant, or it may occur artificially by injection. Passive immunity lasts only as long as the antibodies or activated T cells remain alive in the body, generally just weeks or months.<\/li>\r\n \t
- Many pathogens have evolved mechanisms to evade the adaptive immune system. For example, human immunodeficiency virus ([pb_glossary id=\"4501\"]HIV[\/pb_glossary]) evades the adaptive immune system by frequently changing its antigens and by forming its outer envelope from the host\u2019s cell membrane.<\/li>\r\n<\/ul>\r\n<\/div>\r\n<\/div>\r\n
\r\n17.5 Review Questions<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- What is the adaptive immune system?<\/li>\r\n \t
- Define immunity.<\/li>\r\n \t
- [h5p id=\"639\"]<\/li>\r\n \t
- How are lymphocytes activated?<\/li>\r\n \t
- Identify two common types of T cells and their functions.<\/li>\r\n \t
- How do activated B cells help defend against pathogens?<\/li>\r\n \t
- How does passive immunity differ from active immunity?\u00a0How may passive immunity occur?<\/li>\r\n \t
- What are two ways that active immunity may come about?<\/li>\r\n \t
- What ways of evading the human adaptive immune system evolved in human immunodeficiency virus (HIV)?<\/li>\r\n \t
- Why do vaccinations expose\u00a0a person to a version of a pathogen?<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n
\r\n17.5 Explore More<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\nhttps:\/\/youtu.be\/rb7TVW77ZCs\r\nHow do vaccines work? - Kelwalin Dhanasarnsombut, TED-Ed, 2015.<\/p>\r\nhttps:\/\/youtu.be\/yqUFy-t4MlQ\r\n
How we conquered the deadly smallpox virus - Simona Zompi, TED-Ed, 2013.<\/p>\r\nhttps:\/\/youtu.be\/5THf6gTNqO8\r\n
Why Do We Need A New Flu Shot Every Year? Seeker, 2015.<\/p>\r\nhttps:\/\/youtu.be\/X-rC78MKZvw\r\n
An HIV Vaccine: Mapping Uncharted Territory, NIAID, 2016.<\/p>\r\n\r\n<\/div>\r\n<\/div>\r\n \r\n
Attributions<\/h2>\r\nFigure 17.5.1<\/strong>\r\n\r\nKiller_T_cells_surround_a_cancer_cell<\/a> by Alex Ritter, Jennifer Lippincott Schwartz and Gillian Griffiths at the National Institutes of Health\/ Visuals Online<\/a> on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).\r\n\r\nFigure 17.5.2<\/strong>\r\n\r\nT_cell_activation.svg<\/a> by Rehua<\/a> (derivative work) on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).\u00a0(Original image: T_cell_activation.png<\/a>:\u00a0 from The Immune System - NIH Publication No. 03\u20135423<\/a>)\r\n\r\nFigure 17.5.3<\/strong>\r\n\r\nCytotoxic T Cell function<\/a>\u00a0by 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 17.5.4<\/strong>\r\n\r\nB_cell_activation.svg<\/a> by Fred the Oyster<\/a> on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain). (Original from The Immune System - NIH Publication No. 03\u20135423<\/a>)\r\n\r\nFigure 17.5.5<\/strong>\r\n\r\nAntibody.svg<\/a> by Fvasconcellos<\/a>\u00a0 on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain). (Original black and white image from the National Human Genome Research Institute's Talking Genetics Glossary<\/a>)\r\n\r\nFigure 17.5.7<\/strong>\r\n\r\nimmunizations<\/a> by U.S. Air Force photo by Airman 1st Class Destinee Dougherty from Military Health System website, Health.mil, is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).\r\n\r\n \r\nReferences<\/h2>\r\n
HealthLinkBC. (2018). B.C. immunization schedules. \u00a0Gov.BC.CA. \u00a0https:\/\/www.healthlinkbc.ca\/tools-videos\/bc-immunization-schedules<\/p>\r\n
Mayo Clinic Staff. (n.d.). Measles [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/measles\/symptoms-causes\/syc-20374857<\/p>\r\n
Mayo Clinic Staff. (n.d.). Mumps [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/mumps\/symptoms-causes\/syc-20375361<\/p>\r\n
Mayo Clinic Staff. (n.d.). Polio [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/polio\/symptoms-causes\/syc-20376512<\/p>\r\n
Mayo Clinic Staff. (n.d.). Smallpox [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/smallpox\/symptoms-causes\/syc-20353027<\/p>\r\n
NIAID. (2016, August 11). An HIV vaccine: Mapping uncharted territory. YouTube. https:\/\/www.youtube.com\/watch?v=X-rC78MKZvw&feature=youtu.be<\/p>\r\n
OpenStax. (2016, March 23). Figure 4 Na\u00efve\u00a0CD4<\/mtext>+<\/sup>\u00a0T cells engage MHC II molecules on antigen-presenting cells (APCs) and become activated. Clones of the activated helper T cell, in turn, activate B cells and CD8+<\/sup> T cells, which become cytotoxic T cells. Cytotoxic T cells kill infected cells [digital image].\u00a0In OpenStax, Biology<\/em> (Section 42.2). OpenStax CNX. https:\/\/cnx.org\/contents\/GFy_h8cu@10.53:etZobsU-@6\/Adaptive-Immune-Response<\/p>\r\nSeeker. (2015, September 2). Why do we need a new flu shot every year? YouTube. https:\/\/www.youtube.com\/watch?v=5THf6gTNqO8<\/p>\r\n
TED-Ed. (2015, January 12). How do vaccines work? - Kelwalin Dhanasarnsombut. YouTube. https:\/\/www.youtube.com\/watch?v=rb7TVW77ZCs&feature=youtu.be<\/p>\r\n
TED-Ed. (2013, October 28). How we conquered the deadly smallpox virus - Simona Zompi. YouTube. https:\/\/www.youtube.com\/watch?v=yqUFy-t4MlQ&feature=youtu.be<\/p>","rendered":"
<\/p>\nFigure 17.5.1 Kill the cancer.<\/em><\/figcaption><\/figure>\n\nThe Kiss of Death<\/h1>\n<\/div>\n
The photomicrograph in Figure 17.5.1 shows a group of killer T cells (green and red) surrounding a cancer<\/a> cell (blue, centre). When a killer T cell makes contact with the cancer cell, it attaches to and spreads over the dangerous target. The killer T cell then uses special chemicals stored in vesicles<\/span><\/a><\/span>\u00a0(red) to deliver the killing blow. This event has thus been nicknamed \u201cthe kiss of death.\u201d After the target cell is killed, the killer T cells move on to find the next victim. Killer T cells like these are important players in the adaptive immune system.<\/span><\/p>\n\nWhat Is the Adaptive Immune System?<\/h1>\n<\/div>\n
The\u00a0adaptive immune system<\/a><\/strong>\u00a0is a subsystem of the overall immune system. It is composed of highly specialized cells and processes that eliminate specific pathogens<\/span><\/a><\/span>\u00a0and tumor cells. An adaptive immune response is set in motion by antigen<\/span>s<\/span><\/a>\u00a0that the immune system recognizes as foreign. Unlike an innate immune response, an adaptive immune response is highly specific to a particular pathogen (or its antigen). An important function of the adaptive immune system that is\u00a0<\/span>not<\/em>\u00a0shared by the innate immune system is the creation of immunological memory \u2014 or immunity<\/a> \u2014 which occurs after the initial response to a specific pathogen. It allows for a faster, stronger response on subsequent encounters with the same pathogen, usually before the pathogen can even cause symptoms of illness.<\/span><\/p>\nLymphocytes<\/span><\/a><\/span>\u00a0are the main cells of the adaptive immune system. They are leukocyte<\/span>s<\/span><\/a><\/span>\u00a0that arise and mature in organs of the lymphatic system, including the bone marrow<\/a> and thymus<\/a>. The human body normally has about 2 trillion lymphocytes, which constitute about 1\/3 of all leukocytes. Most of the lymphocytes are normally sequestered within tissue fluid or organs of the lymphatic system, including the tonsil<\/span>s<\/span><\/a><\/span>, spleen<\/a>, and lymph node<\/span>s<\/span><\/a><\/span>. Only about 2% of the lymphocytes are normally circulating in the blood. There are two main types of lymphocytes involved in adaptive immune responses, called T cells and B cells. T cells destroy infected cells or release chemicals that regulate immune responses. B cells secrete antibodies<\/a> that bind with antigen<\/span>s<\/span>[<\/span>\/pb_glossary]<\/span>\u00a0of [pb_glossary id=\"271\"]pathogen<\/span>s<\/span><\/a><\/span>\u00a0so they can be removed by other immune cells or processes.<\/span><\/p>\n\nPathways of the Adaptive Immune Response<\/h1>\n
There are some general similarities in the way in which the separate adaptive immune responses occur in T cell and B cell responses.\u00a0 In both pathways, a foreign antigen is recognized by the B or T cell.\u00a0 From there, cytokines produced by helper T-cells promote clonal expansion of lymphocytes.\u00a0 From this clonal expansion, two types of B or T cells are produced- cells that directly fight the pathogen invasion and cells that remain behind to provide long-term immunity<\/a>.\u00a0 Finally, once the pathogen invasion has been eradicated, the plasma cells and killer T cells go through apoptosis (programmed cell death).<\/p>\nT Cells<\/h1>\n<\/div>\n
There are multiple types of\u00a0T cells<\/a>,<\/strong>\u00a0or T lymphocytes. Major types are killer (or cytotoxic) T cells<\/a> and helper T cells<\/a>. Both types develop from immature T cells that become activated by exposure to an antigen<\/a>.<\/p>\nT Cell Activation (or Cell-Mediated Immunity)<\/h2>\n
T cells must be activated to become either killer T cells or helper T cells. This requires presentation of a foreign antigen by antigen-presenting cells<\/a>, as shown in Figure 17.5.2. Antigen-presenting cells may be dendritic cells<\/span><\/a><\/span>, macrophage<\/span>s<\/span><\/a><\/span>, or B cell<\/span>s<\/span><\/a><\/span>. Activation occurs when T cells are presented with a foreign antigen coupled with an MHC self antigen. Helper T cells are more easily activated than killer T cells. Activation of killer T cells is strongly regulated and may require additional stimulation from helper T cells.<\/span><\/p>\nFigure 17.5.2 Exposure to a foreign antigen on an antigen-presenting cell is necessary to activate T cells to become killer T cells or helper T cells.<\/em><\/figcaption><\/figure>\nKiller T Cells<\/h2>\n
Activated killer T cells induce the death of cells that bear a specific non-self<\/a> antigen because they are infected with pathogens or are cancerous. The antigen targets the cell for destruction by killer T cells, which travel through the bloodstream searching for target cells to kill. Killer T cells may use various mechanisms to kill target cells. One way is by releasing toxins in granules that enter and kill infected or cancerous cells (see Figure 17.5.3).<\/p>\nFigure 17.5.3 A killer T cell releases toxins that destroy an infected body cell and the viruses it contains.<\/em><\/figcaption><\/figure>\nHelper T Cells<\/h2>\n
Activated helper T cells<\/span><\/a><\/span>\u00a0do not kill infected or cancerous cells. Instead, their role is to \u201cmanage\u201d both innate<\/a> and adaptive immune<\/a> responses by directing other cells to perform these tasks. They control other cells by releasing cytokine<\/span>s<\/span><\/a><\/span>, which are proteins that can influence the activity of many cell types, including killer T cell<\/span>s<\/span><\/a><\/span>, B cells<\/a>, and macrophages<\/a>.\u00a0Some cytokines released by helper T cells assist with the activation of killer T cells.<\/span><\/p>\n\nB Cells<\/h1>\n<\/div>\n
B cell<\/strong>s<\/strong><\/a><\/strong>, or B lymphocytes, are the major cells involved in the creation of antibodies<\/a> that circulate in blood plasma<\/a> and lymph<\/a>. Antibodies are large, Y-shaped proteins used by the immune system to identify and neutralize foreign invaders. Besides producing antibodies, B cells may also function as antigen-presenting cell<\/span>s<\/span><\/a><\/span>, or secrete cytokines that help control other immune cells and responses.<\/span><\/p>\nB Cell Activation (or Antibody-Mediated Immunity)<\/h2>\n
Before B cells can actively function to defend the host, they must be activated. As shown in Figure 17.5.4, B cell activation begins when a B cell engulfs and digests an antigen. The antigen may be either free floating in the lymph, or it may be presented by an antigen-presenting cell, such as a dendritic cell or macrophage. In either case, the B cell then displays antigen fragments bound to its own MHC antigens. The MHC-antigen complex on the B cell attracts helper T cells. The helper T cells, in turn, secrete cytokines that help the B cell to multiply, and the daughter cells to mature into plasma cells.<\/p>\nFigure 17.5.4 B cells are activated and become antibody-producing plasma cells with the help of helper T cells.<\/em><\/figcaption><\/figure>\nPlasma Cells<\/h2>\nFigure 17.5.5 Each antibody fits its antigen like a lock fits a key.<\/em><\/figcaption><\/figure>\n
B Cells<\/h1>\r\n<\/div>\r\n[pb_glossary id=\"5539\"]B cell<\/strong>s<\/strong>[\/pb_glossary]<\/strong>, or B lymphocytes, are the major cells involved in the creation of [pb_glossary id=\"5659\"]antibodies[\/pb_glossary] that circulate in blood [pb_glossary id=\"4395\"]plasma[\/pb_glossary] and [pb_glossary id=\"4552\"]lymph[\/pb_glossary]. Antibodies are large, Y-shaped proteins used by the [pb_glossary id=\"4242\"]immune system[\/pb_glossary] to identify and neutralize foreign invaders. Besides producing antibodies, B cells may also function as [pb_glossary id=\"4895\"]antigen-presenting cell<\/span>s<\/span>[\/pb_glossary]<\/span>, or secrete cytokines that help control other immune cells and responses.<\/span>\r\nB Cell Activation (or Antibody-Mediated Immunity)<\/h2>\r\nBefore B cells can actively function to defend the host, they must be activated. As shown in Figure 17.5.4, B cell activation begins when a B cell engulfs and digests an antigen. The antigen may be either free floating in the lymph, or it may be presented by an antigen-presenting cell, such as a dendritic cell or macrophage. In either case, the B cell then displays antigen fragments bound to its own MHC antigens. The MHC-antigen complex on the B cell attracts helper T cells. The helper T cells, in turn, secrete cytokines that help the B cell to multiply, and the daughter cells to mature into plasma cells.\r\n\r\n[caption id=\"attachment_4901\" align=\"aligncenter\" width=\"583\"] Figure 17.5.4 B cells are activated and become antibody-producing plasma cells with the help of helper T cells.<\/em>[\/caption]\r\nPlasma Cells<\/h2>\r\n[caption id=\"attachment_4903\" align=\"alignright\" width=\"325\"] Figure 17.5.5 Each antibody fits its antigen like a lock fits a key.<\/em>[\/caption]\r\n\r\n[pb_glossary id=\"4902\"]Plasma cell<\/strong>s<\/strong>[\/pb_glossary]<\/strong> are antibody-secreting cells that form from activated B cells. Each plasma cell is like a tiny antibody factory. It may secrete millions of copies of an antibody, each of which can bind to the specific antigen that activated the original B cell. The specificity of an antibody to a specific antigen is illustrated in Figure 17.5.5. When antibodies bind with antigens, it makes the cells bearing them easier targets for phagocytes to find and destroy. Antibody-antigen complexes may also trigger the complement system of the innate immune system, which destroys the cells in a cascade of protein enzymes. In addition, the complexes are likely to clump together (agglutinate). If this occurs, they are filtered out of the blood in the spleen or liver.<\/span>\r\n\r\nImmunity<\/h1>\r\n<\/div>\r\nOnce a pathogen has been cleared from the body, most activated T cells and B cells die within a few days.\u00a0A\u00a0few of the cells, however, survive and remain in the body as memory T cells or memory B cells. These [pb_glossary id=\"4904\"]memory cell<\/strong>s[\/pb_glossary] are ready to activate an immediate response if they are exposed to the same antigen again in the future. This is the basis of\u00a0[pb_glossary id=\"4884\"]immunity[\/pb_glossary].<\/strong>\r\n\r\nThe earliest known reference to the concept of immunity relates to the bubonic plaque (see Figure 17.5.6). In 430 B.C., a Greek historian and general named Thucydides noted that people who had recovered from a previous bout of the plague could nurse people who were sick with the plague without contracting the illness a second time. We now know that this is true of many diseases, and that it occurs because of active immunity.\r\n\r\n[caption id=\"attachment_4905\" align=\"aligncenter\" width=\"332\"] Figure 17.5.6 Dead, blackened tissues at the finger tips and other extremities are a sign of the bubonic plague, giving rise to its other name, the black death.<\/em>[\/caption]\r\nActive Immunity<\/h2>\r\n[pb_glossary id=\"4906\"]Active immunity[\/pb_glossary]<\/strong>\u00a0is the ability of the adaptive immune system to resist a specific pathogen because it has formed an immunological memory of the pathogen. Active immunity is adaptive, because it occurs during the lifetime of an individual as an adaptation to infection with a specific pathogen, and prepares the immune system for future challenges from that pathogen. Active immunity can come about naturally or artificially.\r\nNaturally Acquired Active Immunity<\/h3>\r\nActive immunity is acquired naturally when a pathogen invades the body and activates the adaptive immune system. When the initial infection is over, memory B cells and memory T cells remain, providing immunological memory of the pathogen. As long as the memory cells are alive, the immune system is ready to mount an immediate response if the same pathogen tries to infect the body again.\r\nArtificially Acquired Active Immunity<\/h3>\r\nActive immunity can also be acquired artificially through immunization.\u00a0[pb_glossary id=\"4908\"]Immunization[\/pb_glossary]<\/strong> is the deliberate exposure of a person to a pathogen in order to provoke an adaptive immune response and the formation of memory cells specific to that pathogen. The pathogen is introduced in a vaccine \u2014 usually by injection, sometimes by nose or mouth (see Figure 17.5.7) \u2014 so immunization is also called vaccination.\r\n\r\n[caption id=\"attachment_4909\" align=\"alignleft\" width=\"511\"] Figure 17.5.7 This young child is receiving a vaccine. Vaccines are a safe way to create immunity against life-threatening diseases.<\/em>[\/caption]\r\n\r\nTypically, only part of a pathogen, a weakened form of the pathogen, or a dead pathogen is used in a vaccine, which causes an adaptive immune response without making the immunized person sick. This is how you most likely became immune to diseases such as measles<\/a>, mumps<\/a>, and chicken pox. Immunizations may last for a lifetime, or they may require periodic booster shots to maintain immunity. While immunization generally has long-lasting effects, it usually takes several weeks to develop full immunity.\r\n\r\nImmunization is the most effective method ever discovered of preventing infectious diseases. As many as 3 million deaths are prevented each year because of vaccinations. Widespread immunity from vaccinations is largely responsible for the worldwide eradication of smallpox<\/a>, and the near elimination of several other infectious diseases from many populations, including polio<\/a> and measles. Immunization is so successful because it exploits the natural specificity and inducibility of the adaptive immune system.\r\nPassive Immunity<\/h2>\r\n[pb_glossary id=\"4910\"]Passive immunity[\/pb_glossary]<\/strong>\u00a0results when pathogen-specific antibodies or activated T cells are transferred to a person who has never been exposed to the pathogen. Passive immunity provides immediate protection from a pathogen, but the adaptive immune system does not develop immunological memory to protect the host from the same pathogen in the future. Unlike active immunity, passive immunity lasts only as long as the transferred antibodies or T cells survive in the blood \u2014 usually between a few days and a few months. However, like active immunity, passive immunity can be acquired both naturally and artificially.\r\nNaturally Acquired Passive Immunity<\/h3>\r\nPassive immunity is acquired naturally by a fetus through its mother\u2019s blood. [pb_glossary id=\"5659\"]Antibodies[\/pb_glossary] are transported from mother to fetus across the placenta, so babies have high levels of antibodies at birth. Their antibodies have the same range of antigen specificities as their mother\u2019s. Passive immunity may also be acquired by an infant through the mother\u2019s breast milk. This gives young infants protection from common pathogens in their environment while their own immune system matures.\r\nArtificially Acquired Passive Immunity<\/h3>\r\nOlder children and adults can acquire passive immunity artificially through the injection of antibodies or activated T cells, which may be done when there is a high risk of infection and insufficient time for the body to develop active immunity through vaccination. It may also be done to reduce symptoms of ongoing disease, or to compensate for immunodeficiency diseases.\r\n\r\nAdaptive Immune Evasion<\/h1>\r\n<\/div>\r\nMany pathogens have been around for a long time, living with human populations for generations. To persist, some have evolved mechanisms to evade the adaptive immune system of human hosts. One way they have done this is by rapidly changing their non-essential antigens. This is called antigenic variation. An example of a pathogen that takes this approach is human immunodeficiency virus (HIV). It mutates rapidly so the proteins on its viral envelope are constantly changing. By the time the adaptive immune system responds, the virus\u2019s antigens have changed. Antigenic variation is the main reason that efforts to develop a vaccine against HIV have not yet been successful.\r\n\r\nAnother evasion approach that some pathogens may take is to mask pathogen antigens with host molecules so the host\u2019s immune system cannot detect the antigens. HIV takes this approach, as well. The envelope that covers the virus is formed from the outermost membrane of the host cell.\r\n\r\nFeature: My Human Body<\/h1>\r\n<\/div>\r\nIf you think that immunizations are just for kids, think again. There are several vaccines recommended by HealthLinkBC<\/a> for people over the age of 18. The tables below from HealthLinkBC show the vaccine schedules recommended for infants and children, school-aged children, and adults and senior. Additional vaccines may be recommended for certain adults based on specific travel plans, medical conditions or other indications. Are you up to date with your vaccines? You can check with your doctor to be sure.\r\n\r\n\r\n\r\n\r\n<\/div>\r\n\r\n\r\n \r\n\r\n\r\n\r\n17.5 Summary<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- The [pb_glossary id=\"4822\"]adaptive immune system[\/pb_glossary] is a subsystem of the overall immune system that recognizes and makes a tailored attack against specific [pb_glossary id=\"5799\"]pathogens[\/pb_glossary]\u00a0or tumor cells. It is a slower, but more effective response than the [pb_glossary id=\"4820\"]innate immune response[\/pb_glossary], and also leads to immunity to particular pathogens.<\/li>\r\n \t
- [pb_glossary id=\"4832\"]Lymphocytes[\/pb_glossary]\u00a0produced by the [pb_glossary id=\"2911\"]lymphatic system[\/pb_glossary] are the main cells of the adaptive immune system. There are two major types of lymphocytes: [pb_glossary id=\"4892\"]T cells[\/pb_glossary]\u00a0and [pb_glossary id=\"5539\"]B cells[\/pb_glossary]. Both types must be activated by foreign [pb_glossary id=\"2704\"]antigens[\/pb_glossary]\u00a0to become functioning immune cells.<\/li>\r\n \t
- Most activated T cells become either [pb_glossary id=\"4893\"]killer T cells[\/pb_glossary]\u00a0or [pb_glossary id=\"5537\"]helper T cells[\/pb_glossary]. Killer T cells destroy cells that are infected with pathogens or are cancerous. Helper T cells manage immune responses by releasing [pb_glossary id=\"4865\"]cytokines[\/pb_glossary]\u00a0that control other types of leukocytes.<\/li>\r\n \t
- Activated B cells form [pb_glossary id=\"4902\"]plasma cells[\/pb_glossary]\u00a0that secrete [pb_glossary id=\"5659\"]antibodies[\/pb_glossary], which bind to specific antigens on pathogens or infected cells. The antibody-antigen complexes generally lead to the destruction of the cells, for example, by attracting phagocytes or triggering the [pb_glossary id=\"4868\"]complement system[\/pb_glossary].<\/li>\r\n \t
- After an adaptive immune response occurs, long-lasting [pb_glossary id=\"4904\"]memory B cells and memory T cells[\/pb_glossary] may remain to confer immunity to the specific pathogen that caused the adaptive immune response. These memory cells are ready to activate an immediate response if they are exposed to the same antigen again in the future.<\/li>\r\n \t
- Immunity may be active or passive. Active immunity occurs when the immune system has been presented with antigens that elicit an adaptive immune response. This may occur naturally as the result of an infection, or artificially as the result of immunization. Active immunity may last for years or even for life.<\/li>\r\n \t
- Passive immunity occurs without an adaptive immune response by the transfer of antibodies or activated T cells. This may occur naturally between a mother and her fetus or her nursing infant, or it may occur artificially by injection. Passive immunity lasts only as long as the antibodies or activated T cells remain alive in the body, generally just weeks or months.<\/li>\r\n \t
- Many pathogens have evolved mechanisms to evade the adaptive immune system. For example, human immunodeficiency virus ([pb_glossary id=\"4501\"]HIV[\/pb_glossary]) evades the adaptive immune system by frequently changing its antigens and by forming its outer envelope from the host\u2019s cell membrane.<\/li>\r\n<\/ul>\r\n<\/div>\r\n<\/div>\r\n
\r\n17.5 Review Questions<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- What is the adaptive immune system?<\/li>\r\n \t
- Define immunity.<\/li>\r\n \t
- [h5p id=\"639\"]<\/li>\r\n \t
- How are lymphocytes activated?<\/li>\r\n \t
- Identify two common types of T cells and their functions.<\/li>\r\n \t
- How do activated B cells help defend against pathogens?<\/li>\r\n \t
- How does passive immunity differ from active immunity?\u00a0How may passive immunity occur?<\/li>\r\n \t
- What are two ways that active immunity may come about?<\/li>\r\n \t
- What ways of evading the human adaptive immune system evolved in human immunodeficiency virus (HIV)?<\/li>\r\n \t
- Why do vaccinations expose\u00a0a person to a version of a pathogen?<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n
\r\n17.5 Explore More<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\nhttps:\/\/youtu.be\/rb7TVW77ZCs\r\nHow do vaccines work? - Kelwalin Dhanasarnsombut, TED-Ed, 2015.<\/p>\r\nhttps:\/\/youtu.be\/yqUFy-t4MlQ\r\n
How we conquered the deadly smallpox virus - Simona Zompi, TED-Ed, 2013.<\/p>\r\nhttps:\/\/youtu.be\/5THf6gTNqO8\r\n
Why Do We Need A New Flu Shot Every Year? Seeker, 2015.<\/p>\r\nhttps:\/\/youtu.be\/X-rC78MKZvw\r\n
An HIV Vaccine: Mapping Uncharted Territory, NIAID, 2016.<\/p>\r\n\r\n<\/div>\r\n<\/div>\r\n \r\n
Attributions<\/h2>\r\nFigure 17.5.1<\/strong>\r\n\r\nKiller_T_cells_surround_a_cancer_cell<\/a> by Alex Ritter, Jennifer Lippincott Schwartz and Gillian Griffiths at the National Institutes of Health\/ Visuals Online<\/a> on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).\r\n\r\nFigure 17.5.2<\/strong>\r\n\r\nT_cell_activation.svg<\/a> by Rehua<\/a> (derivative work) on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).\u00a0(Original image: T_cell_activation.png<\/a>:\u00a0 from The Immune System - NIH Publication No. 03\u20135423<\/a>)\r\n\r\nFigure 17.5.3<\/strong>\r\n\r\nCytotoxic T Cell function<\/a>\u00a0by 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 17.5.4<\/strong>\r\n\r\nB_cell_activation.svg<\/a> by Fred the Oyster<\/a> on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain). (Original from The Immune System - NIH Publication No. 03\u20135423<\/a>)\r\n\r\nFigure 17.5.5<\/strong>\r\n\r\nAntibody.svg<\/a> by Fvasconcellos<\/a>\u00a0 on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain). (Original black and white image from the National Human Genome Research Institute's Talking Genetics Glossary<\/a>)\r\n\r\nFigure 17.5.7<\/strong>\r\n\r\nimmunizations<\/a> by U.S. Air Force photo by Airman 1st Class Destinee Dougherty from Military Health System website, Health.mil, is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).\r\n\r\n \r\nReferences<\/h2>\r\n
HealthLinkBC. (2018). B.C. immunization schedules. \u00a0Gov.BC.CA. \u00a0https:\/\/www.healthlinkbc.ca\/tools-videos\/bc-immunization-schedules<\/p>\r\n
Mayo Clinic Staff. (n.d.). Measles [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/measles\/symptoms-causes\/syc-20374857<\/p>\r\n
Mayo Clinic Staff. (n.d.). Mumps [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/mumps\/symptoms-causes\/syc-20375361<\/p>\r\n
Mayo Clinic Staff. (n.d.). Polio [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/polio\/symptoms-causes\/syc-20376512<\/p>\r\n
Mayo Clinic Staff. (n.d.). Smallpox [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/smallpox\/symptoms-causes\/syc-20353027<\/p>\r\n
NIAID. (2016, August 11). An HIV vaccine: Mapping uncharted territory. YouTube. https:\/\/www.youtube.com\/watch?v=X-rC78MKZvw&feature=youtu.be<\/p>\r\n
OpenStax. (2016, March 23). Figure 4 Na\u00efve\u00a0CD4<\/mtext>+<\/sup>\u00a0T cells engage MHC II molecules on antigen-presenting cells (APCs) and become activated. Clones of the activated helper T cell, in turn, activate B cells and CD8+<\/sup> T cells, which become cytotoxic T cells. Cytotoxic T cells kill infected cells [digital image].\u00a0In OpenStax, Biology<\/em> (Section 42.2). OpenStax CNX. https:\/\/cnx.org\/contents\/GFy_h8cu@10.53:etZobsU-@6\/Adaptive-Immune-Response<\/p>\r\nSeeker. (2015, September 2). Why do we need a new flu shot every year? YouTube. https:\/\/www.youtube.com\/watch?v=5THf6gTNqO8<\/p>\r\n
TED-Ed. (2015, January 12). How do vaccines work? - Kelwalin Dhanasarnsombut. YouTube. https:\/\/www.youtube.com\/watch?v=rb7TVW77ZCs&feature=youtu.be<\/p>\r\n
TED-Ed. (2013, October 28). How we conquered the deadly smallpox virus - Simona Zompi. YouTube. https:\/\/www.youtube.com\/watch?v=yqUFy-t4MlQ&feature=youtu.be<\/p>","rendered":"
<\/p>\nFigure 17.5.1 Kill the cancer.<\/em><\/figcaption><\/figure>\n\nThe Kiss of Death<\/h1>\n<\/div>\n
The photomicrograph in Figure 17.5.1 shows a group of killer T cells (green and red) surrounding a cancer<\/a> cell (blue, centre). When a killer T cell makes contact with the cancer cell, it attaches to and spreads over the dangerous target. The killer T cell then uses special chemicals stored in vesicles<\/span><\/a><\/span>\u00a0(red) to deliver the killing blow. This event has thus been nicknamed \u201cthe kiss of death.\u201d After the target cell is killed, the killer T cells move on to find the next victim. Killer T cells like these are important players in the adaptive immune system.<\/span><\/p>\n\nWhat Is the Adaptive Immune System?<\/h1>\n<\/div>\n
The\u00a0adaptive immune system<\/a><\/strong>\u00a0is a subsystem of the overall immune system. It is composed of highly specialized cells and processes that eliminate specific pathogens<\/span><\/a><\/span>\u00a0and tumor cells. An adaptive immune response is set in motion by antigen<\/span>s<\/span><\/a>\u00a0that the immune system recognizes as foreign. Unlike an innate immune response, an adaptive immune response is highly specific to a particular pathogen (or its antigen). An important function of the adaptive immune system that is\u00a0<\/span>not<\/em>\u00a0shared by the innate immune system is the creation of immunological memory \u2014 or immunity<\/a> \u2014 which occurs after the initial response to a specific pathogen. It allows for a faster, stronger response on subsequent encounters with the same pathogen, usually before the pathogen can even cause symptoms of illness.<\/span><\/p>\nLymphocytes<\/span><\/a><\/span>\u00a0are the main cells of the adaptive immune system. They are leukocyte<\/span>s<\/span><\/a><\/span>\u00a0that arise and mature in organs of the lymphatic system, including the bone marrow<\/a> and thymus<\/a>. The human body normally has about 2 trillion lymphocytes, which constitute about 1\/3 of all leukocytes. Most of the lymphocytes are normally sequestered within tissue fluid or organs of the lymphatic system, including the tonsil<\/span>s<\/span><\/a><\/span>, spleen<\/a>, and lymph node<\/span>s<\/span><\/a><\/span>. Only about 2% of the lymphocytes are normally circulating in the blood. There are two main types of lymphocytes involved in adaptive immune responses, called T cells and B cells. T cells destroy infected cells or release chemicals that regulate immune responses. B cells secrete antibodies<\/a> that bind with antigen<\/span>s<\/span>[<\/span>\/pb_glossary]<\/span>\u00a0of [pb_glossary id=\"271\"]pathogen<\/span>s<\/span><\/a><\/span>\u00a0so they can be removed by other immune cells or processes.<\/span><\/p>\n\nPathways of the Adaptive Immune Response<\/h1>\n
There are some general similarities in the way in which the separate adaptive immune responses occur in T cell and B cell responses.\u00a0 In both pathways, a foreign antigen is recognized by the B or T cell.\u00a0 From there, cytokines produced by helper T-cells promote clonal expansion of lymphocytes.\u00a0 From this clonal expansion, two types of B or T cells are produced- cells that directly fight the pathogen invasion and cells that remain behind to provide long-term immunity<\/a>.\u00a0 Finally, once the pathogen invasion has been eradicated, the plasma cells and killer T cells go through apoptosis (programmed cell death).<\/p>\nT Cells<\/h1>\n<\/div>\n
There are multiple types of\u00a0T cells<\/a>,<\/strong>\u00a0or T lymphocytes. Major types are killer (or cytotoxic) T cells<\/a> and helper T cells<\/a>. Both types develop from immature T cells that become activated by exposure to an antigen<\/a>.<\/p>\nT Cell Activation (or Cell-Mediated Immunity)<\/h2>\n
T cells must be activated to become either killer T cells or helper T cells. This requires presentation of a foreign antigen by antigen-presenting cells<\/a>, as shown in Figure 17.5.2. Antigen-presenting cells may be dendritic cells<\/span><\/a><\/span>, macrophage<\/span>s<\/span><\/a><\/span>, or B cell<\/span>s<\/span><\/a><\/span>. Activation occurs when T cells are presented with a foreign antigen coupled with an MHC self antigen. Helper T cells are more easily activated than killer T cells. Activation of killer T cells is strongly regulated and may require additional stimulation from helper T cells.<\/span><\/p>\nFigure 17.5.2 Exposure to a foreign antigen on an antigen-presenting cell is necessary to activate T cells to become killer T cells or helper T cells.<\/em><\/figcaption><\/figure>\nKiller T Cells<\/h2>\n
Activated killer T cells induce the death of cells that bear a specific non-self<\/a> antigen because they are infected with pathogens or are cancerous. The antigen targets the cell for destruction by killer T cells, which travel through the bloodstream searching for target cells to kill. Killer T cells may use various mechanisms to kill target cells. One way is by releasing toxins in granules that enter and kill infected or cancerous cells (see Figure 17.5.3).<\/p>\nFigure 17.5.3 A killer T cell releases toxins that destroy an infected body cell and the viruses it contains.<\/em><\/figcaption><\/figure>\nHelper T Cells<\/h2>\n
Activated helper T cells<\/span><\/a><\/span>\u00a0do not kill infected or cancerous cells. Instead, their role is to \u201cmanage\u201d both innate<\/a> and adaptive immune<\/a> responses by directing other cells to perform these tasks. They control other cells by releasing cytokine<\/span>s<\/span><\/a><\/span>, which are proteins that can influence the activity of many cell types, including killer T cell<\/span>s<\/span><\/a><\/span>, B cells<\/a>, and macrophages<\/a>.\u00a0Some cytokines released by helper T cells assist with the activation of killer T cells.<\/span><\/p>\n\nB Cells<\/h1>\n<\/div>\n
B cell<\/strong>s<\/strong><\/a><\/strong>, or B lymphocytes, are the major cells involved in the creation of antibodies<\/a> that circulate in blood plasma<\/a> and lymph<\/a>. Antibodies are large, Y-shaped proteins used by the immune system to identify and neutralize foreign invaders. Besides producing antibodies, B cells may also function as antigen-presenting cell<\/span>s<\/span><\/a><\/span>, or secrete cytokines that help control other immune cells and responses.<\/span><\/p>\nB Cell Activation (or Antibody-Mediated Immunity)<\/h2>\n
Before B cells can actively function to defend the host, they must be activated. As shown in Figure 17.5.4, B cell activation begins when a B cell engulfs and digests an antigen. The antigen may be either free floating in the lymph, or it may be presented by an antigen-presenting cell, such as a dendritic cell or macrophage. In either case, the B cell then displays antigen fragments bound to its own MHC antigens. The MHC-antigen complex on the B cell attracts helper T cells. The helper T cells, in turn, secrete cytokines that help the B cell to multiply, and the daughter cells to mature into plasma cells.<\/p>\nFigure 17.5.4 B cells are activated and become antibody-producing plasma cells with the help of helper T cells.<\/em><\/figcaption><\/figure>\nPlasma Cells<\/h2>\nFigure 17.5.5 Each antibody fits its antigen like a lock fits a key.<\/em><\/figcaption><\/figure>\n
Figure 17.5.4 B cells are activated and become antibody-producing plasma cells with the help of helper T cells.<\/em>[\/caption]\r\nPlasma Cells<\/h2>\r\n[caption id=\"attachment_4903\" align=\"alignright\" width=\"325\"] Figure 17.5.5 Each antibody fits its antigen like a lock fits a key.<\/em>[\/caption]\r\n\r\n[pb_glossary id=\"4902\"]Plasma cell<\/strong>s<\/strong>[\/pb_glossary]<\/strong> are antibody-secreting cells that form from activated B cells. Each plasma cell is like a tiny antibody factory. It may secrete millions of copies of an antibody, each of which can bind to the specific antigen that activated the original B cell. The specificity of an antibody to a specific antigen is illustrated in Figure 17.5.5. When antibodies bind with antigens, it makes the cells bearing them easier targets for phagocytes to find and destroy. Antibody-antigen complexes may also trigger the complement system of the innate immune system, which destroys the cells in a cascade of protein enzymes. In addition, the complexes are likely to clump together (agglutinate). If this occurs, they are filtered out of the blood in the spleen or liver.<\/span>\r\n\r\nImmunity<\/h1>\r\n<\/div>\r\nOnce a pathogen has been cleared from the body, most activated T cells and B cells die within a few days.\u00a0A\u00a0few of the cells, however, survive and remain in the body as memory T cells or memory B cells. These [pb_glossary id=\"4904\"]memory cell<\/strong>s[\/pb_glossary] are ready to activate an immediate response if they are exposed to the same antigen again in the future. This is the basis of\u00a0[pb_glossary id=\"4884\"]immunity[\/pb_glossary].<\/strong>\r\n\r\nThe earliest known reference to the concept of immunity relates to the bubonic plaque (see Figure 17.5.6). In 430 B.C., a Greek historian and general named Thucydides noted that people who had recovered from a previous bout of the plague could nurse people who were sick with the plague without contracting the illness a second time. We now know that this is true of many diseases, and that it occurs because of active immunity.\r\n\r\n[caption id=\"attachment_4905\" align=\"aligncenter\" width=\"332\"] Figure 17.5.6 Dead, blackened tissues at the finger tips and other extremities are a sign of the bubonic plague, giving rise to its other name, the black death.<\/em>[\/caption]\r\nActive Immunity<\/h2>\r\n[pb_glossary id=\"4906\"]Active immunity[\/pb_glossary]<\/strong>\u00a0is the ability of the adaptive immune system to resist a specific pathogen because it has formed an immunological memory of the pathogen. Active immunity is adaptive, because it occurs during the lifetime of an individual as an adaptation to infection with a specific pathogen, and prepares the immune system for future challenges from that pathogen. Active immunity can come about naturally or artificially.\r\nNaturally Acquired Active Immunity<\/h3>\r\nActive immunity is acquired naturally when a pathogen invades the body and activates the adaptive immune system. When the initial infection is over, memory B cells and memory T cells remain, providing immunological memory of the pathogen. As long as the memory cells are alive, the immune system is ready to mount an immediate response if the same pathogen tries to infect the body again.\r\nArtificially Acquired Active Immunity<\/h3>\r\nActive immunity can also be acquired artificially through immunization.\u00a0[pb_glossary id=\"4908\"]Immunization[\/pb_glossary]<\/strong> is the deliberate exposure of a person to a pathogen in order to provoke an adaptive immune response and the formation of memory cells specific to that pathogen. The pathogen is introduced in a vaccine \u2014 usually by injection, sometimes by nose or mouth (see Figure 17.5.7) \u2014 so immunization is also called vaccination.\r\n\r\n[caption id=\"attachment_4909\" align=\"alignleft\" width=\"511\"] Figure 17.5.7 This young child is receiving a vaccine. Vaccines are a safe way to create immunity against life-threatening diseases.<\/em>[\/caption]\r\n\r\nTypically, only part of a pathogen, a weakened form of the pathogen, or a dead pathogen is used in a vaccine, which causes an adaptive immune response without making the immunized person sick. This is how you most likely became immune to diseases such as measles<\/a>, mumps<\/a>, and chicken pox. Immunizations may last for a lifetime, or they may require periodic booster shots to maintain immunity. While immunization generally has long-lasting effects, it usually takes several weeks to develop full immunity.\r\n\r\nImmunization is the most effective method ever discovered of preventing infectious diseases. As many as 3 million deaths are prevented each year because of vaccinations. Widespread immunity from vaccinations is largely responsible for the worldwide eradication of smallpox<\/a>, and the near elimination of several other infectious diseases from many populations, including polio<\/a> and measles. Immunization is so successful because it exploits the natural specificity and inducibility of the adaptive immune system.\r\nPassive Immunity<\/h2>\r\n[pb_glossary id=\"4910\"]Passive immunity[\/pb_glossary]<\/strong>\u00a0results when pathogen-specific antibodies or activated T cells are transferred to a person who has never been exposed to the pathogen. Passive immunity provides immediate protection from a pathogen, but the adaptive immune system does not develop immunological memory to protect the host from the same pathogen in the future. Unlike active immunity, passive immunity lasts only as long as the transferred antibodies or T cells survive in the blood \u2014 usually between a few days and a few months. However, like active immunity, passive immunity can be acquired both naturally and artificially.\r\nNaturally Acquired Passive Immunity<\/h3>\r\nPassive immunity is acquired naturally by a fetus through its mother\u2019s blood. [pb_glossary id=\"5659\"]Antibodies[\/pb_glossary] are transported from mother to fetus across the placenta, so babies have high levels of antibodies at birth. Their antibodies have the same range of antigen specificities as their mother\u2019s. Passive immunity may also be acquired by an infant through the mother\u2019s breast milk. This gives young infants protection from common pathogens in their environment while their own immune system matures.\r\nArtificially Acquired Passive Immunity<\/h3>\r\nOlder children and adults can acquire passive immunity artificially through the injection of antibodies or activated T cells, which may be done when there is a high risk of infection and insufficient time for the body to develop active immunity through vaccination. It may also be done to reduce symptoms of ongoing disease, or to compensate for immunodeficiency diseases.\r\n\r\nAdaptive Immune Evasion<\/h1>\r\n<\/div>\r\nMany pathogens have been around for a long time, living with human populations for generations. To persist, some have evolved mechanisms to evade the adaptive immune system of human hosts. One way they have done this is by rapidly changing their non-essential antigens. This is called antigenic variation. An example of a pathogen that takes this approach is human immunodeficiency virus (HIV). It mutates rapidly so the proteins on its viral envelope are constantly changing. By the time the adaptive immune system responds, the virus\u2019s antigens have changed. Antigenic variation is the main reason that efforts to develop a vaccine against HIV have not yet been successful.\r\n\r\nAnother evasion approach that some pathogens may take is to mask pathogen antigens with host molecules so the host\u2019s immune system cannot detect the antigens. HIV takes this approach, as well. The envelope that covers the virus is formed from the outermost membrane of the host cell.\r\n\r\nFeature: My Human Body<\/h1>\r\n<\/div>\r\nIf you think that immunizations are just for kids, think again. There are several vaccines recommended by HealthLinkBC<\/a> for people over the age of 18. The tables below from HealthLinkBC show the vaccine schedules recommended for infants and children, school-aged children, and adults and senior. Additional vaccines may be recommended for certain adults based on specific travel plans, medical conditions or other indications. Are you up to date with your vaccines? You can check with your doctor to be sure.\r\n\r\n\r\n\r\n\r\n<\/div>\r\n\r\n\r\n \r\n\r\n\r\n\r\n17.5 Summary<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- The [pb_glossary id=\"4822\"]adaptive immune system[\/pb_glossary] is a subsystem of the overall immune system that recognizes and makes a tailored attack against specific [pb_glossary id=\"5799\"]pathogens[\/pb_glossary]\u00a0or tumor cells. It is a slower, but more effective response than the [pb_glossary id=\"4820\"]innate immune response[\/pb_glossary], and also leads to immunity to particular pathogens.<\/li>\r\n \t
- [pb_glossary id=\"4832\"]Lymphocytes[\/pb_glossary]\u00a0produced by the [pb_glossary id=\"2911\"]lymphatic system[\/pb_glossary] are the main cells of the adaptive immune system. There are two major types of lymphocytes: [pb_glossary id=\"4892\"]T cells[\/pb_glossary]\u00a0and [pb_glossary id=\"5539\"]B cells[\/pb_glossary]. Both types must be activated by foreign [pb_glossary id=\"2704\"]antigens[\/pb_glossary]\u00a0to become functioning immune cells.<\/li>\r\n \t
- Most activated T cells become either [pb_glossary id=\"4893\"]killer T cells[\/pb_glossary]\u00a0or [pb_glossary id=\"5537\"]helper T cells[\/pb_glossary]. Killer T cells destroy cells that are infected with pathogens or are cancerous. Helper T cells manage immune responses by releasing [pb_glossary id=\"4865\"]cytokines[\/pb_glossary]\u00a0that control other types of leukocytes.<\/li>\r\n \t
- Activated B cells form [pb_glossary id=\"4902\"]plasma cells[\/pb_glossary]\u00a0that secrete [pb_glossary id=\"5659\"]antibodies[\/pb_glossary], which bind to specific antigens on pathogens or infected cells. The antibody-antigen complexes generally lead to the destruction of the cells, for example, by attracting phagocytes or triggering the [pb_glossary id=\"4868\"]complement system[\/pb_glossary].<\/li>\r\n \t
- After an adaptive immune response occurs, long-lasting [pb_glossary id=\"4904\"]memory B cells and memory T cells[\/pb_glossary] may remain to confer immunity to the specific pathogen that caused the adaptive immune response. These memory cells are ready to activate an immediate response if they are exposed to the same antigen again in the future.<\/li>\r\n \t
- Immunity may be active or passive. Active immunity occurs when the immune system has been presented with antigens that elicit an adaptive immune response. This may occur naturally as the result of an infection, or artificially as the result of immunization. Active immunity may last for years or even for life.<\/li>\r\n \t
- Passive immunity occurs without an adaptive immune response by the transfer of antibodies or activated T cells. This may occur naturally between a mother and her fetus or her nursing infant, or it may occur artificially by injection. Passive immunity lasts only as long as the antibodies or activated T cells remain alive in the body, generally just weeks or months.<\/li>\r\n \t
- Many pathogens have evolved mechanisms to evade the adaptive immune system. For example, human immunodeficiency virus ([pb_glossary id=\"4501\"]HIV[\/pb_glossary]) evades the adaptive immune system by frequently changing its antigens and by forming its outer envelope from the host\u2019s cell membrane.<\/li>\r\n<\/ul>\r\n<\/div>\r\n<\/div>\r\n
\r\n17.5 Review Questions<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- What is the adaptive immune system?<\/li>\r\n \t
- Define immunity.<\/li>\r\n \t
- [h5p id=\"639\"]<\/li>\r\n \t
- How are lymphocytes activated?<\/li>\r\n \t
- Identify two common types of T cells and their functions.<\/li>\r\n \t
- How do activated B cells help defend against pathogens?<\/li>\r\n \t
- How does passive immunity differ from active immunity?\u00a0How may passive immunity occur?<\/li>\r\n \t
- What are two ways that active immunity may come about?<\/li>\r\n \t
- What ways of evading the human adaptive immune system evolved in human immunodeficiency virus (HIV)?<\/li>\r\n \t
- Why do vaccinations expose\u00a0a person to a version of a pathogen?<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n
\r\n17.5 Explore More<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\nhttps:\/\/youtu.be\/rb7TVW77ZCs\r\nHow do vaccines work? - Kelwalin Dhanasarnsombut, TED-Ed, 2015.<\/p>\r\nhttps:\/\/youtu.be\/yqUFy-t4MlQ\r\n
How we conquered the deadly smallpox virus - Simona Zompi, TED-Ed, 2013.<\/p>\r\nhttps:\/\/youtu.be\/5THf6gTNqO8\r\n
Why Do We Need A New Flu Shot Every Year? Seeker, 2015.<\/p>\r\nhttps:\/\/youtu.be\/X-rC78MKZvw\r\n
An HIV Vaccine: Mapping Uncharted Territory, NIAID, 2016.<\/p>\r\n\r\n<\/div>\r\n<\/div>\r\n \r\n
Attributions<\/h2>\r\nFigure 17.5.1<\/strong>\r\n\r\nKiller_T_cells_surround_a_cancer_cell<\/a> by Alex Ritter, Jennifer Lippincott Schwartz and Gillian Griffiths at the National Institutes of Health\/ Visuals Online<\/a> on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).\r\n\r\nFigure 17.5.2<\/strong>\r\n\r\nT_cell_activation.svg<\/a> by Rehua<\/a> (derivative work) on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).\u00a0(Original image: T_cell_activation.png<\/a>:\u00a0 from The Immune System - NIH Publication No. 03\u20135423<\/a>)\r\n\r\nFigure 17.5.3<\/strong>\r\n\r\nCytotoxic T Cell function<\/a>\u00a0by 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 17.5.4<\/strong>\r\n\r\nB_cell_activation.svg<\/a> by Fred the Oyster<\/a> on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain). (Original from The Immune System - NIH Publication No. 03\u20135423<\/a>)\r\n\r\nFigure 17.5.5<\/strong>\r\n\r\nAntibody.svg<\/a> by Fvasconcellos<\/a>\u00a0 on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain). (Original black and white image from the National Human Genome Research Institute's Talking Genetics Glossary<\/a>)\r\n\r\nFigure 17.5.7<\/strong>\r\n\r\nimmunizations<\/a> by U.S. Air Force photo by Airman 1st Class Destinee Dougherty from Military Health System website, Health.mil, is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).\r\n\r\n \r\nReferences<\/h2>\r\n
HealthLinkBC. (2018). B.C. immunization schedules. \u00a0Gov.BC.CA. \u00a0https:\/\/www.healthlinkbc.ca\/tools-videos\/bc-immunization-schedules<\/p>\r\n
Mayo Clinic Staff. (n.d.). Measles [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/measles\/symptoms-causes\/syc-20374857<\/p>\r\n
Mayo Clinic Staff. (n.d.). Mumps [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/mumps\/symptoms-causes\/syc-20375361<\/p>\r\n
Mayo Clinic Staff. (n.d.). Polio [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/polio\/symptoms-causes\/syc-20376512<\/p>\r\n
Mayo Clinic Staff. (n.d.). Smallpox [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/smallpox\/symptoms-causes\/syc-20353027<\/p>\r\n
NIAID. (2016, August 11). An HIV vaccine: Mapping uncharted territory. YouTube. https:\/\/www.youtube.com\/watch?v=X-rC78MKZvw&feature=youtu.be<\/p>\r\n
OpenStax. (2016, March 23). Figure 4 Na\u00efve\u00a0CD4<\/mtext>+<\/sup>\u00a0T cells engage MHC II molecules on antigen-presenting cells (APCs) and become activated. Clones of the activated helper T cell, in turn, activate B cells and CD8+<\/sup> T cells, which become cytotoxic T cells. Cytotoxic T cells kill infected cells [digital image].\u00a0In OpenStax, Biology<\/em> (Section 42.2). OpenStax CNX. https:\/\/cnx.org\/contents\/GFy_h8cu@10.53:etZobsU-@6\/Adaptive-Immune-Response<\/p>\r\nSeeker. (2015, September 2). Why do we need a new flu shot every year? YouTube. https:\/\/www.youtube.com\/watch?v=5THf6gTNqO8<\/p>\r\n
TED-Ed. (2015, January 12). How do vaccines work? - Kelwalin Dhanasarnsombut. YouTube. https:\/\/www.youtube.com\/watch?v=rb7TVW77ZCs&feature=youtu.be<\/p>\r\n
TED-Ed. (2013, October 28). How we conquered the deadly smallpox virus - Simona Zompi. YouTube. https:\/\/www.youtube.com\/watch?v=yqUFy-t4MlQ&feature=youtu.be<\/p>","rendered":"
<\/p>\nFigure 17.5.1 Kill the cancer.<\/em><\/figcaption><\/figure>\n\nThe Kiss of Death<\/h1>\n<\/div>\n
The photomicrograph in Figure 17.5.1 shows a group of killer T cells (green and red) surrounding a cancer<\/a> cell (blue, centre). When a killer T cell makes contact with the cancer cell, it attaches to and spreads over the dangerous target. The killer T cell then uses special chemicals stored in vesicles<\/span><\/a><\/span>\u00a0(red) to deliver the killing blow. This event has thus been nicknamed \u201cthe kiss of death.\u201d After the target cell is killed, the killer T cells move on to find the next victim. Killer T cells like these are important players in the adaptive immune system.<\/span><\/p>\n\nWhat Is the Adaptive Immune System?<\/h1>\n<\/div>\n
The\u00a0adaptive immune system<\/a><\/strong>\u00a0is a subsystem of the overall immune system. It is composed of highly specialized cells and processes that eliminate specific pathogens<\/span><\/a><\/span>\u00a0and tumor cells. An adaptive immune response is set in motion by antigen<\/span>s<\/span><\/a>\u00a0that the immune system recognizes as foreign. Unlike an innate immune response, an adaptive immune response is highly specific to a particular pathogen (or its antigen). An important function of the adaptive immune system that is\u00a0<\/span>not<\/em>\u00a0shared by the innate immune system is the creation of immunological memory \u2014 or immunity<\/a> \u2014 which occurs after the initial response to a specific pathogen. It allows for a faster, stronger response on subsequent encounters with the same pathogen, usually before the pathogen can even cause symptoms of illness.<\/span><\/p>\nLymphocytes<\/span><\/a><\/span>\u00a0are the main cells of the adaptive immune system. They are leukocyte<\/span>s<\/span><\/a><\/span>\u00a0that arise and mature in organs of the lymphatic system, including the bone marrow<\/a> and thymus<\/a>. The human body normally has about 2 trillion lymphocytes, which constitute about 1\/3 of all leukocytes. Most of the lymphocytes are normally sequestered within tissue fluid or organs of the lymphatic system, including the tonsil<\/span>s<\/span><\/a><\/span>, spleen<\/a>, and lymph node<\/span>s<\/span><\/a><\/span>. Only about 2% of the lymphocytes are normally circulating in the blood. There are two main types of lymphocytes involved in adaptive immune responses, called T cells and B cells. T cells destroy infected cells or release chemicals that regulate immune responses. B cells secrete antibodies<\/a> that bind with antigen<\/span>s<\/span>[<\/span>\/pb_glossary]<\/span>\u00a0of [pb_glossary id=\"271\"]pathogen<\/span>s<\/span><\/a><\/span>\u00a0so they can be removed by other immune cells or processes.<\/span><\/p>\n\nPathways of the Adaptive Immune Response<\/h1>\n
There are some general similarities in the way in which the separate adaptive immune responses occur in T cell and B cell responses.\u00a0 In both pathways, a foreign antigen is recognized by the B or T cell.\u00a0 From there, cytokines produced by helper T-cells promote clonal expansion of lymphocytes.\u00a0 From this clonal expansion, two types of B or T cells are produced- cells that directly fight the pathogen invasion and cells that remain behind to provide long-term immunity<\/a>.\u00a0 Finally, once the pathogen invasion has been eradicated, the plasma cells and killer T cells go through apoptosis (programmed cell death).<\/p>\nT Cells<\/h1>\n<\/div>\n
There are multiple types of\u00a0T cells<\/a>,<\/strong>\u00a0or T lymphocytes. Major types are killer (or cytotoxic) T cells<\/a> and helper T cells<\/a>. Both types develop from immature T cells that become activated by exposure to an antigen<\/a>.<\/p>\nT Cell Activation (or Cell-Mediated Immunity)<\/h2>\n
T cells must be activated to become either killer T cells or helper T cells. This requires presentation of a foreign antigen by antigen-presenting cells<\/a>, as shown in Figure 17.5.2. Antigen-presenting cells may be dendritic cells<\/span><\/a><\/span>, macrophage<\/span>s<\/span><\/a><\/span>, or B cell<\/span>s<\/span><\/a><\/span>. Activation occurs when T cells are presented with a foreign antigen coupled with an MHC self antigen. Helper T cells are more easily activated than killer T cells. Activation of killer T cells is strongly regulated and may require additional stimulation from helper T cells.<\/span><\/p>\nFigure 17.5.2 Exposure to a foreign antigen on an antigen-presenting cell is necessary to activate T cells to become killer T cells or helper T cells.<\/em><\/figcaption><\/figure>\nKiller T Cells<\/h2>\n
Activated killer T cells induce the death of cells that bear a specific non-self<\/a> antigen because they are infected with pathogens or are cancerous. The antigen targets the cell for destruction by killer T cells, which travel through the bloodstream searching for target cells to kill. Killer T cells may use various mechanisms to kill target cells. One way is by releasing toxins in granules that enter and kill infected or cancerous cells (see Figure 17.5.3).<\/p>\nFigure 17.5.3 A killer T cell releases toxins that destroy an infected body cell and the viruses it contains.<\/em><\/figcaption><\/figure>\nHelper T Cells<\/h2>\n
Activated helper T cells<\/span><\/a><\/span>\u00a0do not kill infected or cancerous cells. Instead, their role is to \u201cmanage\u201d both innate<\/a> and adaptive immune<\/a> responses by directing other cells to perform these tasks. They control other cells by releasing cytokine<\/span>s<\/span><\/a><\/span>, which are proteins that can influence the activity of many cell types, including killer T cell<\/span>s<\/span><\/a><\/span>, B cells<\/a>, and macrophages<\/a>.\u00a0Some cytokines released by helper T cells assist with the activation of killer T cells.<\/span><\/p>\n\nB Cells<\/h1>\n<\/div>\n
B cell<\/strong>s<\/strong><\/a><\/strong>, or B lymphocytes, are the major cells involved in the creation of antibodies<\/a> that circulate in blood plasma<\/a> and lymph<\/a>. Antibodies are large, Y-shaped proteins used by the immune system to identify and neutralize foreign invaders. Besides producing antibodies, B cells may also function as antigen-presenting cell<\/span>s<\/span><\/a><\/span>, or secrete cytokines that help control other immune cells and responses.<\/span><\/p>\nB Cell Activation (or Antibody-Mediated Immunity)<\/h2>\n
Before B cells can actively function to defend the host, they must be activated. As shown in Figure 17.5.4, B cell activation begins when a B cell engulfs and digests an antigen. The antigen may be either free floating in the lymph, or it may be presented by an antigen-presenting cell, such as a dendritic cell or macrophage. In either case, the B cell then displays antigen fragments bound to its own MHC antigens. The MHC-antigen complex on the B cell attracts helper T cells. The helper T cells, in turn, secrete cytokines that help the B cell to multiply, and the daughter cells to mature into plasma cells.<\/p>\nFigure 17.5.4 B cells are activated and become antibody-producing plasma cells with the help of helper T cells.<\/em><\/figcaption><\/figure>\nPlasma Cells<\/h2>\nFigure 17.5.5 Each antibody fits its antigen like a lock fits a key.<\/em><\/figcaption><\/figure>\n
Immunity<\/h1>\r\n<\/div>\r\nOnce a pathogen has been cleared from the body, most activated T cells and B cells die within a few days.\u00a0A\u00a0few of the cells, however, survive and remain in the body as memory T cells or memory B cells. These [pb_glossary id=\"4904\"]memory cell<\/strong>s[\/pb_glossary] are ready to activate an immediate response if they are exposed to the same antigen again in the future. This is the basis of\u00a0[pb_glossary id=\"4884\"]immunity[\/pb_glossary].<\/strong>\r\n\r\nThe earliest known reference to the concept of immunity relates to the bubonic plaque (see Figure 17.5.6). In 430 B.C., a Greek historian and general named Thucydides noted that people who had recovered from a previous bout of the plague could nurse people who were sick with the plague without contracting the illness a second time. We now know that this is true of many diseases, and that it occurs because of active immunity.\r\n\r\n[caption id=\"attachment_4905\" align=\"aligncenter\" width=\"332\"] Figure 17.5.6 Dead, blackened tissues at the finger tips and other extremities are a sign of the bubonic plague, giving rise to its other name, the black death.<\/em>[\/caption]\r\nActive Immunity<\/h2>\r\n[pb_glossary id=\"4906\"]Active immunity[\/pb_glossary]<\/strong>\u00a0is the ability of the adaptive immune system to resist a specific pathogen because it has formed an immunological memory of the pathogen. Active immunity is adaptive, because it occurs during the lifetime of an individual as an adaptation to infection with a specific pathogen, and prepares the immune system for future challenges from that pathogen. Active immunity can come about naturally or artificially.\r\nNaturally Acquired Active Immunity<\/h3>\r\nActive immunity is acquired naturally when a pathogen invades the body and activates the adaptive immune system. When the initial infection is over, memory B cells and memory T cells remain, providing immunological memory of the pathogen. As long as the memory cells are alive, the immune system is ready to mount an immediate response if the same pathogen tries to infect the body again.\r\nArtificially Acquired Active Immunity<\/h3>\r\nActive immunity can also be acquired artificially through immunization.\u00a0[pb_glossary id=\"4908\"]Immunization[\/pb_glossary]<\/strong> is the deliberate exposure of a person to a pathogen in order to provoke an adaptive immune response and the formation of memory cells specific to that pathogen. The pathogen is introduced in a vaccine \u2014 usually by injection, sometimes by nose or mouth (see Figure 17.5.7) \u2014 so immunization is also called vaccination.\r\n\r\n[caption id=\"attachment_4909\" align=\"alignleft\" width=\"511\"] Figure 17.5.7 This young child is receiving a vaccine. Vaccines are a safe way to create immunity against life-threatening diseases.<\/em>[\/caption]\r\n\r\nTypically, only part of a pathogen, a weakened form of the pathogen, or a dead pathogen is used in a vaccine, which causes an adaptive immune response without making the immunized person sick. This is how you most likely became immune to diseases such as measles<\/a>, mumps<\/a>, and chicken pox. Immunizations may last for a lifetime, or they may require periodic booster shots to maintain immunity. While immunization generally has long-lasting effects, it usually takes several weeks to develop full immunity.\r\n\r\nImmunization is the most effective method ever discovered of preventing infectious diseases. As many as 3 million deaths are prevented each year because of vaccinations. Widespread immunity from vaccinations is largely responsible for the worldwide eradication of smallpox<\/a>, and the near elimination of several other infectious diseases from many populations, including polio<\/a> and measles. Immunization is so successful because it exploits the natural specificity and inducibility of the adaptive immune system.\r\nPassive Immunity<\/h2>\r\n[pb_glossary id=\"4910\"]Passive immunity[\/pb_glossary]<\/strong>\u00a0results when pathogen-specific antibodies or activated T cells are transferred to a person who has never been exposed to the pathogen. Passive immunity provides immediate protection from a pathogen, but the adaptive immune system does not develop immunological memory to protect the host from the same pathogen in the future. Unlike active immunity, passive immunity lasts only as long as the transferred antibodies or T cells survive in the blood \u2014 usually between a few days and a few months. However, like active immunity, passive immunity can be acquired both naturally and artificially.\r\nNaturally Acquired Passive Immunity<\/h3>\r\nPassive immunity is acquired naturally by a fetus through its mother\u2019s blood. [pb_glossary id=\"5659\"]Antibodies[\/pb_glossary] are transported from mother to fetus across the placenta, so babies have high levels of antibodies at birth. Their antibodies have the same range of antigen specificities as their mother\u2019s. Passive immunity may also be acquired by an infant through the mother\u2019s breast milk. This gives young infants protection from common pathogens in their environment while their own immune system matures.\r\nArtificially Acquired Passive Immunity<\/h3>\r\nOlder children and adults can acquire passive immunity artificially through the injection of antibodies or activated T cells, which may be done when there is a high risk of infection and insufficient time for the body to develop active immunity through vaccination. It may also be done to reduce symptoms of ongoing disease, or to compensate for immunodeficiency diseases.\r\n\r\nAdaptive Immune Evasion<\/h1>\r\n<\/div>\r\nMany pathogens have been around for a long time, living with human populations for generations. To persist, some have evolved mechanisms to evade the adaptive immune system of human hosts. One way they have done this is by rapidly changing their non-essential antigens. This is called antigenic variation. An example of a pathogen that takes this approach is human immunodeficiency virus (HIV). It mutates rapidly so the proteins on its viral envelope are constantly changing. By the time the adaptive immune system responds, the virus\u2019s antigens have changed. Antigenic variation is the main reason that efforts to develop a vaccine against HIV have not yet been successful.\r\n\r\nAnother evasion approach that some pathogens may take is to mask pathogen antigens with host molecules so the host\u2019s immune system cannot detect the antigens. HIV takes this approach, as well. The envelope that covers the virus is formed from the outermost membrane of the host cell.\r\n\r\nFeature: My Human Body<\/h1>\r\n<\/div>\r\nIf you think that immunizations are just for kids, think again. There are several vaccines recommended by HealthLinkBC<\/a> for people over the age of 18. The tables below from HealthLinkBC show the vaccine schedules recommended for infants and children, school-aged children, and adults and senior. Additional vaccines may be recommended for certain adults based on specific travel plans, medical conditions or other indications. Are you up to date with your vaccines? You can check with your doctor to be sure.\r\n\r\n\r\n\r\n\r\n<\/div>\r\n\r\n\r\n \r\n\r\n\r\n\r\n17.5 Summary<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- The [pb_glossary id=\"4822\"]adaptive immune system[\/pb_glossary] is a subsystem of the overall immune system that recognizes and makes a tailored attack against specific [pb_glossary id=\"5799\"]pathogens[\/pb_glossary]\u00a0or tumor cells. It is a slower, but more effective response than the [pb_glossary id=\"4820\"]innate immune response[\/pb_glossary], and also leads to immunity to particular pathogens.<\/li>\r\n \t
- [pb_glossary id=\"4832\"]Lymphocytes[\/pb_glossary]\u00a0produced by the [pb_glossary id=\"2911\"]lymphatic system[\/pb_glossary] are the main cells of the adaptive immune system. There are two major types of lymphocytes: [pb_glossary id=\"4892\"]T cells[\/pb_glossary]\u00a0and [pb_glossary id=\"5539\"]B cells[\/pb_glossary]. Both types must be activated by foreign [pb_glossary id=\"2704\"]antigens[\/pb_glossary]\u00a0to become functioning immune cells.<\/li>\r\n \t
- Most activated T cells become either [pb_glossary id=\"4893\"]killer T cells[\/pb_glossary]\u00a0or [pb_glossary id=\"5537\"]helper T cells[\/pb_glossary]. Killer T cells destroy cells that are infected with pathogens or are cancerous. Helper T cells manage immune responses by releasing [pb_glossary id=\"4865\"]cytokines[\/pb_glossary]\u00a0that control other types of leukocytes.<\/li>\r\n \t
- Activated B cells form [pb_glossary id=\"4902\"]plasma cells[\/pb_glossary]\u00a0that secrete [pb_glossary id=\"5659\"]antibodies[\/pb_glossary], which bind to specific antigens on pathogens or infected cells. The antibody-antigen complexes generally lead to the destruction of the cells, for example, by attracting phagocytes or triggering the [pb_glossary id=\"4868\"]complement system[\/pb_glossary].<\/li>\r\n \t
- After an adaptive immune response occurs, long-lasting [pb_glossary id=\"4904\"]memory B cells and memory T cells[\/pb_glossary] may remain to confer immunity to the specific pathogen that caused the adaptive immune response. These memory cells are ready to activate an immediate response if they are exposed to the same antigen again in the future.<\/li>\r\n \t
- Immunity may be active or passive. Active immunity occurs when the immune system has been presented with antigens that elicit an adaptive immune response. This may occur naturally as the result of an infection, or artificially as the result of immunization. Active immunity may last for years or even for life.<\/li>\r\n \t
- Passive immunity occurs without an adaptive immune response by the transfer of antibodies or activated T cells. This may occur naturally between a mother and her fetus or her nursing infant, or it may occur artificially by injection. Passive immunity lasts only as long as the antibodies or activated T cells remain alive in the body, generally just weeks or months.<\/li>\r\n \t
- Many pathogens have evolved mechanisms to evade the adaptive immune system. For example, human immunodeficiency virus ([pb_glossary id=\"4501\"]HIV[\/pb_glossary]) evades the adaptive immune system by frequently changing its antigens and by forming its outer envelope from the host\u2019s cell membrane.<\/li>\r\n<\/ul>\r\n<\/div>\r\n<\/div>\r\n
\r\n17.5 Review Questions<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- What is the adaptive immune system?<\/li>\r\n \t
- Define immunity.<\/li>\r\n \t
- [h5p id=\"639\"]<\/li>\r\n \t
- How are lymphocytes activated?<\/li>\r\n \t
- Identify two common types of T cells and their functions.<\/li>\r\n \t
- How do activated B cells help defend against pathogens?<\/li>\r\n \t
- How does passive immunity differ from active immunity?\u00a0How may passive immunity occur?<\/li>\r\n \t
- What are two ways that active immunity may come about?<\/li>\r\n \t
- What ways of evading the human adaptive immune system evolved in human immunodeficiency virus (HIV)?<\/li>\r\n \t
- Why do vaccinations expose\u00a0a person to a version of a pathogen?<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n
\r\n17.5 Explore More<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\nhttps:\/\/youtu.be\/rb7TVW77ZCs\r\nHow do vaccines work? - Kelwalin Dhanasarnsombut, TED-Ed, 2015.<\/p>\r\nhttps:\/\/youtu.be\/yqUFy-t4MlQ\r\n
How we conquered the deadly smallpox virus - Simona Zompi, TED-Ed, 2013.<\/p>\r\nhttps:\/\/youtu.be\/5THf6gTNqO8\r\n
Why Do We Need A New Flu Shot Every Year? Seeker, 2015.<\/p>\r\nhttps:\/\/youtu.be\/X-rC78MKZvw\r\n
An HIV Vaccine: Mapping Uncharted Territory, NIAID, 2016.<\/p>\r\n\r\n<\/div>\r\n<\/div>\r\n \r\n
Attributions<\/h2>\r\nFigure 17.5.1<\/strong>\r\n\r\nKiller_T_cells_surround_a_cancer_cell<\/a> by Alex Ritter, Jennifer Lippincott Schwartz and Gillian Griffiths at the National Institutes of Health\/ Visuals Online<\/a> on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).\r\n\r\nFigure 17.5.2<\/strong>\r\n\r\nT_cell_activation.svg<\/a> by Rehua<\/a> (derivative work) on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).\u00a0(Original image: T_cell_activation.png<\/a>:\u00a0 from The Immune System - NIH Publication No. 03\u20135423<\/a>)\r\n\r\nFigure 17.5.3<\/strong>\r\n\r\nCytotoxic T Cell function<\/a>\u00a0by 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 17.5.4<\/strong>\r\n\r\nB_cell_activation.svg<\/a> by Fred the Oyster<\/a> on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain). (Original from The Immune System - NIH Publication No. 03\u20135423<\/a>)\r\n\r\nFigure 17.5.5<\/strong>\r\n\r\nAntibody.svg<\/a> by Fvasconcellos<\/a>\u00a0 on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain). (Original black and white image from the National Human Genome Research Institute's Talking Genetics Glossary<\/a>)\r\n\r\nFigure 17.5.7<\/strong>\r\n\r\nimmunizations<\/a> by U.S. Air Force photo by Airman 1st Class Destinee Dougherty from Military Health System website, Health.mil, is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).\r\n\r\n \r\nReferences<\/h2>\r\n
HealthLinkBC. (2018). B.C. immunization schedules. \u00a0Gov.BC.CA. \u00a0https:\/\/www.healthlinkbc.ca\/tools-videos\/bc-immunization-schedules<\/p>\r\n
Mayo Clinic Staff. (n.d.). Measles [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/measles\/symptoms-causes\/syc-20374857<\/p>\r\n
Mayo Clinic Staff. (n.d.). Mumps [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/mumps\/symptoms-causes\/syc-20375361<\/p>\r\n
Mayo Clinic Staff. (n.d.). Polio [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/polio\/symptoms-causes\/syc-20376512<\/p>\r\n
Mayo Clinic Staff. (n.d.). Smallpox [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/smallpox\/symptoms-causes\/syc-20353027<\/p>\r\n
NIAID. (2016, August 11). An HIV vaccine: Mapping uncharted territory. YouTube. https:\/\/www.youtube.com\/watch?v=X-rC78MKZvw&feature=youtu.be<\/p>\r\n
OpenStax. (2016, March 23). Figure 4 Na\u00efve\u00a0CD4<\/mtext>+<\/sup>\u00a0T cells engage MHC II molecules on antigen-presenting cells (APCs) and become activated. Clones of the activated helper T cell, in turn, activate B cells and CD8+<\/sup> T cells, which become cytotoxic T cells. Cytotoxic T cells kill infected cells [digital image].\u00a0In OpenStax, Biology<\/em> (Section 42.2). OpenStax CNX. https:\/\/cnx.org\/contents\/GFy_h8cu@10.53:etZobsU-@6\/Adaptive-Immune-Response<\/p>\r\nSeeker. (2015, September 2). Why do we need a new flu shot every year? YouTube. https:\/\/www.youtube.com\/watch?v=5THf6gTNqO8<\/p>\r\n
TED-Ed. (2015, January 12). How do vaccines work? - Kelwalin Dhanasarnsombut. YouTube. https:\/\/www.youtube.com\/watch?v=rb7TVW77ZCs&feature=youtu.be<\/p>\r\n
TED-Ed. (2013, October 28). How we conquered the deadly smallpox virus - Simona Zompi. YouTube. https:\/\/www.youtube.com\/watch?v=yqUFy-t4MlQ&feature=youtu.be<\/p>","rendered":"
<\/p>\nFigure 17.5.1 Kill the cancer.<\/em><\/figcaption><\/figure>\n\nThe Kiss of Death<\/h1>\n<\/div>\n
The photomicrograph in Figure 17.5.1 shows a group of killer T cells (green and red) surrounding a cancer<\/a> cell (blue, centre). When a killer T cell makes contact with the cancer cell, it attaches to and spreads over the dangerous target. The killer T cell then uses special chemicals stored in vesicles<\/span><\/a><\/span>\u00a0(red) to deliver the killing blow. This event has thus been nicknamed \u201cthe kiss of death.\u201d After the target cell is killed, the killer T cells move on to find the next victim. Killer T cells like these are important players in the adaptive immune system.<\/span><\/p>\n\nWhat Is the Adaptive Immune System?<\/h1>\n<\/div>\n
The\u00a0adaptive immune system<\/a><\/strong>\u00a0is a subsystem of the overall immune system. It is composed of highly specialized cells and processes that eliminate specific pathogens<\/span><\/a><\/span>\u00a0and tumor cells. An adaptive immune response is set in motion by antigen<\/span>s<\/span><\/a>\u00a0that the immune system recognizes as foreign. Unlike an innate immune response, an adaptive immune response is highly specific to a particular pathogen (or its antigen). An important function of the adaptive immune system that is\u00a0<\/span>not<\/em>\u00a0shared by the innate immune system is the creation of immunological memory \u2014 or immunity<\/a> \u2014 which occurs after the initial response to a specific pathogen. It allows for a faster, stronger response on subsequent encounters with the same pathogen, usually before the pathogen can even cause symptoms of illness.<\/span><\/p>\nLymphocytes<\/span><\/a><\/span>\u00a0are the main cells of the adaptive immune system. They are leukocyte<\/span>s<\/span><\/a><\/span>\u00a0that arise and mature in organs of the lymphatic system, including the bone marrow<\/a> and thymus<\/a>. The human body normally has about 2 trillion lymphocytes, which constitute about 1\/3 of all leukocytes. Most of the lymphocytes are normally sequestered within tissue fluid or organs of the lymphatic system, including the tonsil<\/span>s<\/span><\/a><\/span>, spleen<\/a>, and lymph node<\/span>s<\/span><\/a><\/span>. Only about 2% of the lymphocytes are normally circulating in the blood. There are two main types of lymphocytes involved in adaptive immune responses, called T cells and B cells. T cells destroy infected cells or release chemicals that regulate immune responses. B cells secrete antibodies<\/a> that bind with antigen<\/span>s<\/span>[<\/span>\/pb_glossary]<\/span>\u00a0of [pb_glossary id=\"271\"]pathogen<\/span>s<\/span><\/a><\/span>\u00a0so they can be removed by other immune cells or processes.<\/span><\/p>\n\nPathways of the Adaptive Immune Response<\/h1>\n
There are some general similarities in the way in which the separate adaptive immune responses occur in T cell and B cell responses.\u00a0 In both pathways, a foreign antigen is recognized by the B or T cell.\u00a0 From there, cytokines produced by helper T-cells promote clonal expansion of lymphocytes.\u00a0 From this clonal expansion, two types of B or T cells are produced- cells that directly fight the pathogen invasion and cells that remain behind to provide long-term immunity<\/a>.\u00a0 Finally, once the pathogen invasion has been eradicated, the plasma cells and killer T cells go through apoptosis (programmed cell death).<\/p>\nT Cells<\/h1>\n<\/div>\n
There are multiple types of\u00a0T cells<\/a>,<\/strong>\u00a0or T lymphocytes. Major types are killer (or cytotoxic) T cells<\/a> and helper T cells<\/a>. Both types develop from immature T cells that become activated by exposure to an antigen<\/a>.<\/p>\nT Cell Activation (or Cell-Mediated Immunity)<\/h2>\n
T cells must be activated to become either killer T cells or helper T cells. This requires presentation of a foreign antigen by antigen-presenting cells<\/a>, as shown in Figure 17.5.2. Antigen-presenting cells may be dendritic cells<\/span><\/a><\/span>, macrophage<\/span>s<\/span><\/a><\/span>, or B cell<\/span>s<\/span><\/a><\/span>. Activation occurs when T cells are presented with a foreign antigen coupled with an MHC self antigen. Helper T cells are more easily activated than killer T cells. Activation of killer T cells is strongly regulated and may require additional stimulation from helper T cells.<\/span><\/p>\nFigure 17.5.2 Exposure to a foreign antigen on an antigen-presenting cell is necessary to activate T cells to become killer T cells or helper T cells.<\/em><\/figcaption><\/figure>\nKiller T Cells<\/h2>\n
Activated killer T cells induce the death of cells that bear a specific non-self<\/a> antigen because they are infected with pathogens or are cancerous. The antigen targets the cell for destruction by killer T cells, which travel through the bloodstream searching for target cells to kill. Killer T cells may use various mechanisms to kill target cells. One way is by releasing toxins in granules that enter and kill infected or cancerous cells (see Figure 17.5.3).<\/p>\nFigure 17.5.3 A killer T cell releases toxins that destroy an infected body cell and the viruses it contains.<\/em><\/figcaption><\/figure>\nHelper T Cells<\/h2>\n
Activated helper T cells<\/span><\/a><\/span>\u00a0do not kill infected or cancerous cells. Instead, their role is to \u201cmanage\u201d both innate<\/a> and adaptive immune<\/a> responses by directing other cells to perform these tasks. They control other cells by releasing cytokine<\/span>s<\/span><\/a><\/span>, which are proteins that can influence the activity of many cell types, including killer T cell<\/span>s<\/span><\/a><\/span>, B cells<\/a>, and macrophages<\/a>.\u00a0Some cytokines released by helper T cells assist with the activation of killer T cells.<\/span><\/p>\n\nB Cells<\/h1>\n<\/div>\n
B cell<\/strong>s<\/strong><\/a><\/strong>, or B lymphocytes, are the major cells involved in the creation of antibodies<\/a> that circulate in blood plasma<\/a> and lymph<\/a>. Antibodies are large, Y-shaped proteins used by the immune system to identify and neutralize foreign invaders. Besides producing antibodies, B cells may also function as antigen-presenting cell<\/span>s<\/span><\/a><\/span>, or secrete cytokines that help control other immune cells and responses.<\/span><\/p>\nB Cell Activation (or Antibody-Mediated Immunity)<\/h2>\n
Before B cells can actively function to defend the host, they must be activated. As shown in Figure 17.5.4, B cell activation begins when a B cell engulfs and digests an antigen. The antigen may be either free floating in the lymph, or it may be presented by an antigen-presenting cell, such as a dendritic cell or macrophage. In either case, the B cell then displays antigen fragments bound to its own MHC antigens. The MHC-antigen complex on the B cell attracts helper T cells. The helper T cells, in turn, secrete cytokines that help the B cell to multiply, and the daughter cells to mature into plasma cells.<\/p>\nFigure 17.5.4 B cells are activated and become antibody-producing plasma cells with the help of helper T cells.<\/em><\/figcaption><\/figure>\nPlasma Cells<\/h2>\nFigure 17.5.5 Each antibody fits its antigen like a lock fits a key.<\/em><\/figcaption><\/figure>\n
Naturally Acquired Active Immunity<\/h3>\r\nActive immunity is acquired naturally when a pathogen invades the body and activates the adaptive immune system. When the initial infection is over, memory B cells and memory T cells remain, providing immunological memory of the pathogen. As long as the memory cells are alive, the immune system is ready to mount an immediate response if the same pathogen tries to infect the body again.\r\nArtificially Acquired Active Immunity<\/h3>\r\nActive immunity can also be acquired artificially through immunization.\u00a0[pb_glossary id=\"4908\"]Immunization[\/pb_glossary]<\/strong> is the deliberate exposure of a person to a pathogen in order to provoke an adaptive immune response and the formation of memory cells specific to that pathogen. The pathogen is introduced in a vaccine \u2014 usually by injection, sometimes by nose or mouth (see Figure 17.5.7) \u2014 so immunization is also called vaccination.\r\n\r\n[caption id=\"attachment_4909\" align=\"alignleft\" width=\"511\"] Figure 17.5.7 This young child is receiving a vaccine. Vaccines are a safe way to create immunity against life-threatening diseases.<\/em>[\/caption]\r\n\r\nTypically, only part of a pathogen, a weakened form of the pathogen, or a dead pathogen is used in a vaccine, which causes an adaptive immune response without making the immunized person sick. This is how you most likely became immune to diseases such as measles<\/a>, mumps<\/a>, and chicken pox. Immunizations may last for a lifetime, or they may require periodic booster shots to maintain immunity. While immunization generally has long-lasting effects, it usually takes several weeks to develop full immunity.\r\n\r\nImmunization is the most effective method ever discovered of preventing infectious diseases. As many as 3 million deaths are prevented each year because of vaccinations. Widespread immunity from vaccinations is largely responsible for the worldwide eradication of smallpox<\/a>, and the near elimination of several other infectious diseases from many populations, including polio<\/a> and measles. Immunization is so successful because it exploits the natural specificity and inducibility of the adaptive immune system.\r\nPassive Immunity<\/h2>\r\n[pb_glossary id=\"4910\"]Passive immunity[\/pb_glossary]<\/strong>\u00a0results when pathogen-specific antibodies or activated T cells are transferred to a person who has never been exposed to the pathogen. Passive immunity provides immediate protection from a pathogen, but the adaptive immune system does not develop immunological memory to protect the host from the same pathogen in the future. Unlike active immunity, passive immunity lasts only as long as the transferred antibodies or T cells survive in the blood \u2014 usually between a few days and a few months. However, like active immunity, passive immunity can be acquired both naturally and artificially.\r\nNaturally Acquired Passive Immunity<\/h3>\r\nPassive immunity is acquired naturally by a fetus through its mother\u2019s blood. [pb_glossary id=\"5659\"]Antibodies[\/pb_glossary] are transported from mother to fetus across the placenta, so babies have high levels of antibodies at birth. Their antibodies have the same range of antigen specificities as their mother\u2019s. Passive immunity may also be acquired by an infant through the mother\u2019s breast milk. This gives young infants protection from common pathogens in their environment while their own immune system matures.\r\nArtificially Acquired Passive Immunity<\/h3>\r\nOlder children and adults can acquire passive immunity artificially through the injection of antibodies or activated T cells, which may be done when there is a high risk of infection and insufficient time for the body to develop active immunity through vaccination. It may also be done to reduce symptoms of ongoing disease, or to compensate for immunodeficiency diseases.\r\n\r\nAdaptive Immune Evasion<\/h1>\r\n<\/div>\r\nMany pathogens have been around for a long time, living with human populations for generations. To persist, some have evolved mechanisms to evade the adaptive immune system of human hosts. One way they have done this is by rapidly changing their non-essential antigens. This is called antigenic variation. An example of a pathogen that takes this approach is human immunodeficiency virus (HIV). It mutates rapidly so the proteins on its viral envelope are constantly changing. By the time the adaptive immune system responds, the virus\u2019s antigens have changed. Antigenic variation is the main reason that efforts to develop a vaccine against HIV have not yet been successful.\r\n\r\nAnother evasion approach that some pathogens may take is to mask pathogen antigens with host molecules so the host\u2019s immune system cannot detect the antigens. HIV takes this approach, as well. The envelope that covers the virus is formed from the outermost membrane of the host cell.\r\n\r\nFeature: My Human Body<\/h1>\r\n<\/div>\r\nIf you think that immunizations are just for kids, think again. There are several vaccines recommended by HealthLinkBC<\/a> for people over the age of 18. The tables below from HealthLinkBC show the vaccine schedules recommended for infants and children, school-aged children, and adults and senior. Additional vaccines may be recommended for certain adults based on specific travel plans, medical conditions or other indications. Are you up to date with your vaccines? You can check with your doctor to be sure.\r\n\r\n\r\n\r\n\r\n<\/div>\r\n\r\n\r\n \r\n\r\n\r\n\r\n17.5 Summary<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- The [pb_glossary id=\"4822\"]adaptive immune system[\/pb_glossary] is a subsystem of the overall immune system that recognizes and makes a tailored attack against specific [pb_glossary id=\"5799\"]pathogens[\/pb_glossary]\u00a0or tumor cells. It is a slower, but more effective response than the [pb_glossary id=\"4820\"]innate immune response[\/pb_glossary], and also leads to immunity to particular pathogens.<\/li>\r\n \t
- [pb_glossary id=\"4832\"]Lymphocytes[\/pb_glossary]\u00a0produced by the [pb_glossary id=\"2911\"]lymphatic system[\/pb_glossary] are the main cells of the adaptive immune system. There are two major types of lymphocytes: [pb_glossary id=\"4892\"]T cells[\/pb_glossary]\u00a0and [pb_glossary id=\"5539\"]B cells[\/pb_glossary]. Both types must be activated by foreign [pb_glossary id=\"2704\"]antigens[\/pb_glossary]\u00a0to become functioning immune cells.<\/li>\r\n \t
- Most activated T cells become either [pb_glossary id=\"4893\"]killer T cells[\/pb_glossary]\u00a0or [pb_glossary id=\"5537\"]helper T cells[\/pb_glossary]. Killer T cells destroy cells that are infected with pathogens or are cancerous. Helper T cells manage immune responses by releasing [pb_glossary id=\"4865\"]cytokines[\/pb_glossary]\u00a0that control other types of leukocytes.<\/li>\r\n \t
- Activated B cells form [pb_glossary id=\"4902\"]plasma cells[\/pb_glossary]\u00a0that secrete [pb_glossary id=\"5659\"]antibodies[\/pb_glossary], which bind to specific antigens on pathogens or infected cells. The antibody-antigen complexes generally lead to the destruction of the cells, for example, by attracting phagocytes or triggering the [pb_glossary id=\"4868\"]complement system[\/pb_glossary].<\/li>\r\n \t
- After an adaptive immune response occurs, long-lasting [pb_glossary id=\"4904\"]memory B cells and memory T cells[\/pb_glossary] may remain to confer immunity to the specific pathogen that caused the adaptive immune response. These memory cells are ready to activate an immediate response if they are exposed to the same antigen again in the future.<\/li>\r\n \t
- Immunity may be active or passive. Active immunity occurs when the immune system has been presented with antigens that elicit an adaptive immune response. This may occur naturally as the result of an infection, or artificially as the result of immunization. Active immunity may last for years or even for life.<\/li>\r\n \t
- Passive immunity occurs without an adaptive immune response by the transfer of antibodies or activated T cells. This may occur naturally between a mother and her fetus or her nursing infant, or it may occur artificially by injection. Passive immunity lasts only as long as the antibodies or activated T cells remain alive in the body, generally just weeks or months.<\/li>\r\n \t
- Many pathogens have evolved mechanisms to evade the adaptive immune system. For example, human immunodeficiency virus ([pb_glossary id=\"4501\"]HIV[\/pb_glossary]) evades the adaptive immune system by frequently changing its antigens and by forming its outer envelope from the host\u2019s cell membrane.<\/li>\r\n<\/ul>\r\n<\/div>\r\n<\/div>\r\n
\r\n17.5 Review Questions<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- What is the adaptive immune system?<\/li>\r\n \t
- Define immunity.<\/li>\r\n \t
- [h5p id=\"639\"]<\/li>\r\n \t
- How are lymphocytes activated?<\/li>\r\n \t
- Identify two common types of T cells and their functions.<\/li>\r\n \t
- How do activated B cells help defend against pathogens?<\/li>\r\n \t
- How does passive immunity differ from active immunity?\u00a0How may passive immunity occur?<\/li>\r\n \t
- What are two ways that active immunity may come about?<\/li>\r\n \t
- What ways of evading the human adaptive immune system evolved in human immunodeficiency virus (HIV)?<\/li>\r\n \t
- Why do vaccinations expose\u00a0a person to a version of a pathogen?<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n
\r\n17.5 Explore More<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\nhttps:\/\/youtu.be\/rb7TVW77ZCs\r\nHow do vaccines work? - Kelwalin Dhanasarnsombut, TED-Ed, 2015.<\/p>\r\nhttps:\/\/youtu.be\/yqUFy-t4MlQ\r\n
How we conquered the deadly smallpox virus - Simona Zompi, TED-Ed, 2013.<\/p>\r\nhttps:\/\/youtu.be\/5THf6gTNqO8\r\n
Why Do We Need A New Flu Shot Every Year? Seeker, 2015.<\/p>\r\nhttps:\/\/youtu.be\/X-rC78MKZvw\r\n
An HIV Vaccine: Mapping Uncharted Territory, NIAID, 2016.<\/p>\r\n\r\n<\/div>\r\n<\/div>\r\n \r\n
Attributions<\/h2>\r\nFigure 17.5.1<\/strong>\r\n\r\nKiller_T_cells_surround_a_cancer_cell<\/a> by Alex Ritter, Jennifer Lippincott Schwartz and Gillian Griffiths at the National Institutes of Health\/ Visuals Online<\/a> on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).\r\n\r\nFigure 17.5.2<\/strong>\r\n\r\nT_cell_activation.svg<\/a> by Rehua<\/a> (derivative work) on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).\u00a0(Original image: T_cell_activation.png<\/a>:\u00a0 from The Immune System - NIH Publication No. 03\u20135423<\/a>)\r\n\r\nFigure 17.5.3<\/strong>\r\n\r\nCytotoxic T Cell function<\/a>\u00a0by 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 17.5.4<\/strong>\r\n\r\nB_cell_activation.svg<\/a> by Fred the Oyster<\/a> on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain). (Original from The Immune System - NIH Publication No. 03\u20135423<\/a>)\r\n\r\nFigure 17.5.5<\/strong>\r\n\r\nAntibody.svg<\/a> by Fvasconcellos<\/a>\u00a0 on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain). (Original black and white image from the National Human Genome Research Institute's Talking Genetics Glossary<\/a>)\r\n\r\nFigure 17.5.7<\/strong>\r\n\r\nimmunizations<\/a> by U.S. Air Force photo by Airman 1st Class Destinee Dougherty from Military Health System website, Health.mil, is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).\r\n\r\n \r\nReferences<\/h2>\r\n
HealthLinkBC. (2018). B.C. immunization schedules. \u00a0Gov.BC.CA. \u00a0https:\/\/www.healthlinkbc.ca\/tools-videos\/bc-immunization-schedules<\/p>\r\n
Mayo Clinic Staff. (n.d.). Measles [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/measles\/symptoms-causes\/syc-20374857<\/p>\r\n
Mayo Clinic Staff. (n.d.). Mumps [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/mumps\/symptoms-causes\/syc-20375361<\/p>\r\n
Mayo Clinic Staff. (n.d.). Polio [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/polio\/symptoms-causes\/syc-20376512<\/p>\r\n
Mayo Clinic Staff. (n.d.). Smallpox [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/smallpox\/symptoms-causes\/syc-20353027<\/p>\r\n
NIAID. (2016, August 11). An HIV vaccine: Mapping uncharted territory. YouTube. https:\/\/www.youtube.com\/watch?v=X-rC78MKZvw&feature=youtu.be<\/p>\r\n
OpenStax. (2016, March 23). Figure 4 Na\u00efve\u00a0CD4<\/mtext>+<\/sup>\u00a0T cells engage MHC II molecules on antigen-presenting cells (APCs) and become activated. Clones of the activated helper T cell, in turn, activate B cells and CD8+<\/sup> T cells, which become cytotoxic T cells. Cytotoxic T cells kill infected cells [digital image].\u00a0In OpenStax, Biology<\/em> (Section 42.2). OpenStax CNX. https:\/\/cnx.org\/contents\/GFy_h8cu@10.53:etZobsU-@6\/Adaptive-Immune-Response<\/p>\r\nSeeker. (2015, September 2). Why do we need a new flu shot every year? YouTube. https:\/\/www.youtube.com\/watch?v=5THf6gTNqO8<\/p>\r\n
TED-Ed. (2015, January 12). How do vaccines work? - Kelwalin Dhanasarnsombut. YouTube. https:\/\/www.youtube.com\/watch?v=rb7TVW77ZCs&feature=youtu.be<\/p>\r\n
TED-Ed. (2013, October 28). How we conquered the deadly smallpox virus - Simona Zompi. YouTube. https:\/\/www.youtube.com\/watch?v=yqUFy-t4MlQ&feature=youtu.be<\/p>","rendered":"
<\/p>\nFigure 17.5.1 Kill the cancer.<\/em><\/figcaption><\/figure>\n\nThe Kiss of Death<\/h1>\n<\/div>\n
The photomicrograph in Figure 17.5.1 shows a group of killer T cells (green and red) surrounding a cancer<\/a> cell (blue, centre). When a killer T cell makes contact with the cancer cell, it attaches to and spreads over the dangerous target. The killer T cell then uses special chemicals stored in vesicles<\/span><\/a><\/span>\u00a0(red) to deliver the killing blow. This event has thus been nicknamed \u201cthe kiss of death.\u201d After the target cell is killed, the killer T cells move on to find the next victim. Killer T cells like these are important players in the adaptive immune system.<\/span><\/p>\n\nWhat Is the Adaptive Immune System?<\/h1>\n<\/div>\n
The\u00a0adaptive immune system<\/a><\/strong>\u00a0is a subsystem of the overall immune system. It is composed of highly specialized cells and processes that eliminate specific pathogens<\/span><\/a><\/span>\u00a0and tumor cells. An adaptive immune response is set in motion by antigen<\/span>s<\/span><\/a>\u00a0that the immune system recognizes as foreign. Unlike an innate immune response, an adaptive immune response is highly specific to a particular pathogen (or its antigen). An important function of the adaptive immune system that is\u00a0<\/span>not<\/em>\u00a0shared by the innate immune system is the creation of immunological memory \u2014 or immunity<\/a> \u2014 which occurs after the initial response to a specific pathogen. It allows for a faster, stronger response on subsequent encounters with the same pathogen, usually before the pathogen can even cause symptoms of illness.<\/span><\/p>\nLymphocytes<\/span><\/a><\/span>\u00a0are the main cells of the adaptive immune system. They are leukocyte<\/span>s<\/span><\/a><\/span>\u00a0that arise and mature in organs of the lymphatic system, including the bone marrow<\/a> and thymus<\/a>. The human body normally has about 2 trillion lymphocytes, which constitute about 1\/3 of all leukocytes. Most of the lymphocytes are normally sequestered within tissue fluid or organs of the lymphatic system, including the tonsil<\/span>s<\/span><\/a><\/span>, spleen<\/a>, and lymph node<\/span>s<\/span><\/a><\/span>. Only about 2% of the lymphocytes are normally circulating in the blood. There are two main types of lymphocytes involved in adaptive immune responses, called T cells and B cells. T cells destroy infected cells or release chemicals that regulate immune responses. B cells secrete antibodies<\/a> that bind with antigen<\/span>s<\/span>[<\/span>\/pb_glossary]<\/span>\u00a0of [pb_glossary id=\"271\"]pathogen<\/span>s<\/span><\/a><\/span>\u00a0so they can be removed by other immune cells or processes.<\/span><\/p>\n\nPathways of the Adaptive Immune Response<\/h1>\n
There are some general similarities in the way in which the separate adaptive immune responses occur in T cell and B cell responses.\u00a0 In both pathways, a foreign antigen is recognized by the B or T cell.\u00a0 From there, cytokines produced by helper T-cells promote clonal expansion of lymphocytes.\u00a0 From this clonal expansion, two types of B or T cells are produced- cells that directly fight the pathogen invasion and cells that remain behind to provide long-term immunity<\/a>.\u00a0 Finally, once the pathogen invasion has been eradicated, the plasma cells and killer T cells go through apoptosis (programmed cell death).<\/p>\nT Cells<\/h1>\n<\/div>\n
There are multiple types of\u00a0T cells<\/a>,<\/strong>\u00a0or T lymphocytes. Major types are killer (or cytotoxic) T cells<\/a> and helper T cells<\/a>. Both types develop from immature T cells that become activated by exposure to an antigen<\/a>.<\/p>\nT Cell Activation (or Cell-Mediated Immunity)<\/h2>\n
T cells must be activated to become either killer T cells or helper T cells. This requires presentation of a foreign antigen by antigen-presenting cells<\/a>, as shown in Figure 17.5.2. Antigen-presenting cells may be dendritic cells<\/span><\/a><\/span>, macrophage<\/span>s<\/span><\/a><\/span>, or B cell<\/span>s<\/span><\/a><\/span>. Activation occurs when T cells are presented with a foreign antigen coupled with an MHC self antigen. Helper T cells are more easily activated than killer T cells. Activation of killer T cells is strongly regulated and may require additional stimulation from helper T cells.<\/span><\/p>\nFigure 17.5.2 Exposure to a foreign antigen on an antigen-presenting cell is necessary to activate T cells to become killer T cells or helper T cells.<\/em><\/figcaption><\/figure>\nKiller T Cells<\/h2>\n
Activated killer T cells induce the death of cells that bear a specific non-self<\/a> antigen because they are infected with pathogens or are cancerous. The antigen targets the cell for destruction by killer T cells, which travel through the bloodstream searching for target cells to kill. Killer T cells may use various mechanisms to kill target cells. One way is by releasing toxins in granules that enter and kill infected or cancerous cells (see Figure 17.5.3).<\/p>\nFigure 17.5.3 A killer T cell releases toxins that destroy an infected body cell and the viruses it contains.<\/em><\/figcaption><\/figure>\nHelper T Cells<\/h2>\n
Activated helper T cells<\/span><\/a><\/span>\u00a0do not kill infected or cancerous cells. Instead, their role is to \u201cmanage\u201d both innate<\/a> and adaptive immune<\/a> responses by directing other cells to perform these tasks. They control other cells by releasing cytokine<\/span>s<\/span><\/a><\/span>, which are proteins that can influence the activity of many cell types, including killer T cell<\/span>s<\/span><\/a><\/span>, B cells<\/a>, and macrophages<\/a>.\u00a0Some cytokines released by helper T cells assist with the activation of killer T cells.<\/span><\/p>\n\nB Cells<\/h1>\n<\/div>\n
B cell<\/strong>s<\/strong><\/a><\/strong>, or B lymphocytes, are the major cells involved in the creation of antibodies<\/a> that circulate in blood plasma<\/a> and lymph<\/a>. Antibodies are large, Y-shaped proteins used by the immune system to identify and neutralize foreign invaders. Besides producing antibodies, B cells may also function as antigen-presenting cell<\/span>s<\/span><\/a><\/span>, or secrete cytokines that help control other immune cells and responses.<\/span><\/p>\nB Cell Activation (or Antibody-Mediated Immunity)<\/h2>\n
Before B cells can actively function to defend the host, they must be activated. As shown in Figure 17.5.4, B cell activation begins when a B cell engulfs and digests an antigen. The antigen may be either free floating in the lymph, or it may be presented by an antigen-presenting cell, such as a dendritic cell or macrophage. In either case, the B cell then displays antigen fragments bound to its own MHC antigens. The MHC-antigen complex on the B cell attracts helper T cells. The helper T cells, in turn, secrete cytokines that help the B cell to multiply, and the daughter cells to mature into plasma cells.<\/p>\nFigure 17.5.4 B cells are activated and become antibody-producing plasma cells with the help of helper T cells.<\/em><\/figcaption><\/figure>\nPlasma Cells<\/h2>\nFigure 17.5.5 Each antibody fits its antigen like a lock fits a key.<\/em><\/figcaption><\/figure>\n
Figure 17.5.7 This young child is receiving a vaccine. Vaccines are a safe way to create immunity against life-threatening diseases.<\/em>[\/caption]\r\n\r\nTypically, only part of a pathogen, a weakened form of the pathogen, or a dead pathogen is used in a vaccine, which causes an adaptive immune response without making the immunized person sick. This is how you most likely became immune to diseases such as measles<\/a>, mumps<\/a>, and chicken pox. Immunizations may last for a lifetime, or they may require periodic booster shots to maintain immunity. While immunization generally has long-lasting effects, it usually takes several weeks to develop full immunity.\r\n\r\nImmunization is the most effective method ever discovered of preventing infectious diseases. As many as 3 million deaths are prevented each year because of vaccinations. Widespread immunity from vaccinations is largely responsible for the worldwide eradication of smallpox<\/a>, and the near elimination of several other infectious diseases from many populations, including polio<\/a> and measles. Immunization is so successful because it exploits the natural specificity and inducibility of the adaptive immune system.\r\nPassive Immunity<\/h2>\r\n[pb_glossary id=\"4910\"]Passive immunity[\/pb_glossary]<\/strong>\u00a0results when pathogen-specific antibodies or activated T cells are transferred to a person who has never been exposed to the pathogen. Passive immunity provides immediate protection from a pathogen, but the adaptive immune system does not develop immunological memory to protect the host from the same pathogen in the future. Unlike active immunity, passive immunity lasts only as long as the transferred antibodies or T cells survive in the blood \u2014 usually between a few days and a few months. However, like active immunity, passive immunity can be acquired both naturally and artificially.\r\nNaturally Acquired Passive Immunity<\/h3>\r\nPassive immunity is acquired naturally by a fetus through its mother\u2019s blood. [pb_glossary id=\"5659\"]Antibodies[\/pb_glossary] are transported from mother to fetus across the placenta, so babies have high levels of antibodies at birth. Their antibodies have the same range of antigen specificities as their mother\u2019s. Passive immunity may also be acquired by an infant through the mother\u2019s breast milk. This gives young infants protection from common pathogens in their environment while their own immune system matures.\r\nArtificially Acquired Passive Immunity<\/h3>\r\nOlder children and adults can acquire passive immunity artificially through the injection of antibodies or activated T cells, which may be done when there is a high risk of infection and insufficient time for the body to develop active immunity through vaccination. It may also be done to reduce symptoms of ongoing disease, or to compensate for immunodeficiency diseases.\r\n\r\nAdaptive Immune Evasion<\/h1>\r\n<\/div>\r\nMany pathogens have been around for a long time, living with human populations for generations. To persist, some have evolved mechanisms to evade the adaptive immune system of human hosts. One way they have done this is by rapidly changing their non-essential antigens. This is called antigenic variation. An example of a pathogen that takes this approach is human immunodeficiency virus (HIV). It mutates rapidly so the proteins on its viral envelope are constantly changing. By the time the adaptive immune system responds, the virus\u2019s antigens have changed. Antigenic variation is the main reason that efforts to develop a vaccine against HIV have not yet been successful.\r\n\r\nAnother evasion approach that some pathogens may take is to mask pathogen antigens with host molecules so the host\u2019s immune system cannot detect the antigens. HIV takes this approach, as well. The envelope that covers the virus is formed from the outermost membrane of the host cell.\r\n\r\nFeature: My Human Body<\/h1>\r\n<\/div>\r\nIf you think that immunizations are just for kids, think again. There are several vaccines recommended by HealthLinkBC<\/a> for people over the age of 18. The tables below from HealthLinkBC show the vaccine schedules recommended for infants and children, school-aged children, and adults and senior. Additional vaccines may be recommended for certain adults based on specific travel plans, medical conditions or other indications. Are you up to date with your vaccines? You can check with your doctor to be sure.\r\n\r\n\r\n\r\n\r\n<\/div>\r\n\r\n\r\n \r\n\r\n\r\n\r\n17.5 Summary<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- The [pb_glossary id=\"4822\"]adaptive immune system[\/pb_glossary] is a subsystem of the overall immune system that recognizes and makes a tailored attack against specific [pb_glossary id=\"5799\"]pathogens[\/pb_glossary]\u00a0or tumor cells. It is a slower, but more effective response than the [pb_glossary id=\"4820\"]innate immune response[\/pb_glossary], and also leads to immunity to particular pathogens.<\/li>\r\n \t
- [pb_glossary id=\"4832\"]Lymphocytes[\/pb_glossary]\u00a0produced by the [pb_glossary id=\"2911\"]lymphatic system[\/pb_glossary] are the main cells of the adaptive immune system. There are two major types of lymphocytes: [pb_glossary id=\"4892\"]T cells[\/pb_glossary]\u00a0and [pb_glossary id=\"5539\"]B cells[\/pb_glossary]. Both types must be activated by foreign [pb_glossary id=\"2704\"]antigens[\/pb_glossary]\u00a0to become functioning immune cells.<\/li>\r\n \t
- Most activated T cells become either [pb_glossary id=\"4893\"]killer T cells[\/pb_glossary]\u00a0or [pb_glossary id=\"5537\"]helper T cells[\/pb_glossary]. Killer T cells destroy cells that are infected with pathogens or are cancerous. Helper T cells manage immune responses by releasing [pb_glossary id=\"4865\"]cytokines[\/pb_glossary]\u00a0that control other types of leukocytes.<\/li>\r\n \t
- Activated B cells form [pb_glossary id=\"4902\"]plasma cells[\/pb_glossary]\u00a0that secrete [pb_glossary id=\"5659\"]antibodies[\/pb_glossary], which bind to specific antigens on pathogens or infected cells. The antibody-antigen complexes generally lead to the destruction of the cells, for example, by attracting phagocytes or triggering the [pb_glossary id=\"4868\"]complement system[\/pb_glossary].<\/li>\r\n \t
- After an adaptive immune response occurs, long-lasting [pb_glossary id=\"4904\"]memory B cells and memory T cells[\/pb_glossary] may remain to confer immunity to the specific pathogen that caused the adaptive immune response. These memory cells are ready to activate an immediate response if they are exposed to the same antigen again in the future.<\/li>\r\n \t
- Immunity may be active or passive. Active immunity occurs when the immune system has been presented with antigens that elicit an adaptive immune response. This may occur naturally as the result of an infection, or artificially as the result of immunization. Active immunity may last for years or even for life.<\/li>\r\n \t
- Passive immunity occurs without an adaptive immune response by the transfer of antibodies or activated T cells. This may occur naturally between a mother and her fetus or her nursing infant, or it may occur artificially by injection. Passive immunity lasts only as long as the antibodies or activated T cells remain alive in the body, generally just weeks or months.<\/li>\r\n \t
- Many pathogens have evolved mechanisms to evade the adaptive immune system. For example, human immunodeficiency virus ([pb_glossary id=\"4501\"]HIV[\/pb_glossary]) evades the adaptive immune system by frequently changing its antigens and by forming its outer envelope from the host\u2019s cell membrane.<\/li>\r\n<\/ul>\r\n<\/div>\r\n<\/div>\r\n
\r\n17.5 Review Questions<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- What is the adaptive immune system?<\/li>\r\n \t
- Define immunity.<\/li>\r\n \t
- [h5p id=\"639\"]<\/li>\r\n \t
- How are lymphocytes activated?<\/li>\r\n \t
- Identify two common types of T cells and their functions.<\/li>\r\n \t
- How do activated B cells help defend against pathogens?<\/li>\r\n \t
- How does passive immunity differ from active immunity?\u00a0How may passive immunity occur?<\/li>\r\n \t
- What are two ways that active immunity may come about?<\/li>\r\n \t
- What ways of evading the human adaptive immune system evolved in human immunodeficiency virus (HIV)?<\/li>\r\n \t
- Why do vaccinations expose\u00a0a person to a version of a pathogen?<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n
\r\n17.5 Explore More<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\nhttps:\/\/youtu.be\/rb7TVW77ZCs\r\nHow do vaccines work? - Kelwalin Dhanasarnsombut, TED-Ed, 2015.<\/p>\r\nhttps:\/\/youtu.be\/yqUFy-t4MlQ\r\n
How we conquered the deadly smallpox virus - Simona Zompi, TED-Ed, 2013.<\/p>\r\nhttps:\/\/youtu.be\/5THf6gTNqO8\r\n
Why Do We Need A New Flu Shot Every Year? Seeker, 2015.<\/p>\r\nhttps:\/\/youtu.be\/X-rC78MKZvw\r\n
An HIV Vaccine: Mapping Uncharted Territory, NIAID, 2016.<\/p>\r\n\r\n<\/div>\r\n<\/div>\r\n \r\n
Attributions<\/h2>\r\nFigure 17.5.1<\/strong>\r\n\r\nKiller_T_cells_surround_a_cancer_cell<\/a> by Alex Ritter, Jennifer Lippincott Schwartz and Gillian Griffiths at the National Institutes of Health\/ Visuals Online<\/a> on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).\r\n\r\nFigure 17.5.2<\/strong>\r\n\r\nT_cell_activation.svg<\/a> by Rehua<\/a> (derivative work) on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).\u00a0(Original image: T_cell_activation.png<\/a>:\u00a0 from The Immune System - NIH Publication No. 03\u20135423<\/a>)\r\n\r\nFigure 17.5.3<\/strong>\r\n\r\nCytotoxic T Cell function<\/a>\u00a0by 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 17.5.4<\/strong>\r\n\r\nB_cell_activation.svg<\/a> by Fred the Oyster<\/a> on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain). (Original from The Immune System - NIH Publication No. 03\u20135423<\/a>)\r\n\r\nFigure 17.5.5<\/strong>\r\n\r\nAntibody.svg<\/a> by Fvasconcellos<\/a>\u00a0 on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain). (Original black and white image from the National Human Genome Research Institute's Talking Genetics Glossary<\/a>)\r\n\r\nFigure 17.5.7<\/strong>\r\n\r\nimmunizations<\/a> by U.S. Air Force photo by Airman 1st Class Destinee Dougherty from Military Health System website, Health.mil, is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).\r\n\r\n \r\nReferences<\/h2>\r\n
HealthLinkBC. (2018). B.C. immunization schedules. \u00a0Gov.BC.CA. \u00a0https:\/\/www.healthlinkbc.ca\/tools-videos\/bc-immunization-schedules<\/p>\r\n
Mayo Clinic Staff. (n.d.). Measles [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/measles\/symptoms-causes\/syc-20374857<\/p>\r\n
Mayo Clinic Staff. (n.d.). Mumps [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/mumps\/symptoms-causes\/syc-20375361<\/p>\r\n
Mayo Clinic Staff. (n.d.). Polio [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/polio\/symptoms-causes\/syc-20376512<\/p>\r\n
Mayo Clinic Staff. (n.d.). Smallpox [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/smallpox\/symptoms-causes\/syc-20353027<\/p>\r\n
NIAID. (2016, August 11). An HIV vaccine: Mapping uncharted territory. YouTube. https:\/\/www.youtube.com\/watch?v=X-rC78MKZvw&feature=youtu.be<\/p>\r\n
OpenStax. (2016, March 23). Figure 4 Na\u00efve\u00a0CD4<\/mtext>+<\/sup>\u00a0T cells engage MHC II molecules on antigen-presenting cells (APCs) and become activated. Clones of the activated helper T cell, in turn, activate B cells and CD8+<\/sup> T cells, which become cytotoxic T cells. Cytotoxic T cells kill infected cells [digital image].\u00a0In OpenStax, Biology<\/em> (Section 42.2). OpenStax CNX. https:\/\/cnx.org\/contents\/GFy_h8cu@10.53:etZobsU-@6\/Adaptive-Immune-Response<\/p>\r\nSeeker. (2015, September 2). Why do we need a new flu shot every year? YouTube. https:\/\/www.youtube.com\/watch?v=5THf6gTNqO8<\/p>\r\n
TED-Ed. (2015, January 12). How do vaccines work? - Kelwalin Dhanasarnsombut. YouTube. https:\/\/www.youtube.com\/watch?v=rb7TVW77ZCs&feature=youtu.be<\/p>\r\n
TED-Ed. (2013, October 28). How we conquered the deadly smallpox virus - Simona Zompi. YouTube. https:\/\/www.youtube.com\/watch?v=yqUFy-t4MlQ&feature=youtu.be<\/p>","rendered":"
<\/p>\nFigure 17.5.1 Kill the cancer.<\/em><\/figcaption><\/figure>\n\nThe Kiss of Death<\/h1>\n<\/div>\n
The photomicrograph in Figure 17.5.1 shows a group of killer T cells (green and red) surrounding a cancer<\/a> cell (blue, centre). When a killer T cell makes contact with the cancer cell, it attaches to and spreads over the dangerous target. The killer T cell then uses special chemicals stored in vesicles<\/span><\/a><\/span>\u00a0(red) to deliver the killing blow. This event has thus been nicknamed \u201cthe kiss of death.\u201d After the target cell is killed, the killer T cells move on to find the next victim. Killer T cells like these are important players in the adaptive immune system.<\/span><\/p>\n\nWhat Is the Adaptive Immune System?<\/h1>\n<\/div>\n
The\u00a0adaptive immune system<\/a><\/strong>\u00a0is a subsystem of the overall immune system. It is composed of highly specialized cells and processes that eliminate specific pathogens<\/span><\/a><\/span>\u00a0and tumor cells. An adaptive immune response is set in motion by antigen<\/span>s<\/span><\/a>\u00a0that the immune system recognizes as foreign. Unlike an innate immune response, an adaptive immune response is highly specific to a particular pathogen (or its antigen). An important function of the adaptive immune system that is\u00a0<\/span>not<\/em>\u00a0shared by the innate immune system is the creation of immunological memory \u2014 or immunity<\/a> \u2014 which occurs after the initial response to a specific pathogen. It allows for a faster, stronger response on subsequent encounters with the same pathogen, usually before the pathogen can even cause symptoms of illness.<\/span><\/p>\nLymphocytes<\/span><\/a><\/span>\u00a0are the main cells of the adaptive immune system. They are leukocyte<\/span>s<\/span><\/a><\/span>\u00a0that arise and mature in organs of the lymphatic system, including the bone marrow<\/a> and thymus<\/a>. The human body normally has about 2 trillion lymphocytes, which constitute about 1\/3 of all leukocytes. Most of the lymphocytes are normally sequestered within tissue fluid or organs of the lymphatic system, including the tonsil<\/span>s<\/span><\/a><\/span>, spleen<\/a>, and lymph node<\/span>s<\/span><\/a><\/span>. Only about 2% of the lymphocytes are normally circulating in the blood. There are two main types of lymphocytes involved in adaptive immune responses, called T cells and B cells. T cells destroy infected cells or release chemicals that regulate immune responses. B cells secrete antibodies<\/a> that bind with antigen<\/span>s<\/span>[<\/span>\/pb_glossary]<\/span>\u00a0of [pb_glossary id=\"271\"]pathogen<\/span>s<\/span><\/a><\/span>\u00a0so they can be removed by other immune cells or processes.<\/span><\/p>\n\nPathways of the Adaptive Immune Response<\/h1>\n
There are some general similarities in the way in which the separate adaptive immune responses occur in T cell and B cell responses.\u00a0 In both pathways, a foreign antigen is recognized by the B or T cell.\u00a0 From there, cytokines produced by helper T-cells promote clonal expansion of lymphocytes.\u00a0 From this clonal expansion, two types of B or T cells are produced- cells that directly fight the pathogen invasion and cells that remain behind to provide long-term immunity<\/a>.\u00a0 Finally, once the pathogen invasion has been eradicated, the plasma cells and killer T cells go through apoptosis (programmed cell death).<\/p>\nT Cells<\/h1>\n<\/div>\n
There are multiple types of\u00a0T cells<\/a>,<\/strong>\u00a0or T lymphocytes. Major types are killer (or cytotoxic) T cells<\/a> and helper T cells<\/a>. Both types develop from immature T cells that become activated by exposure to an antigen<\/a>.<\/p>\nT Cell Activation (or Cell-Mediated Immunity)<\/h2>\n
T cells must be activated to become either killer T cells or helper T cells. This requires presentation of a foreign antigen by antigen-presenting cells<\/a>, as shown in Figure 17.5.2. Antigen-presenting cells may be dendritic cells<\/span><\/a><\/span>, macrophage<\/span>s<\/span><\/a><\/span>, or B cell<\/span>s<\/span><\/a><\/span>. Activation occurs when T cells are presented with a foreign antigen coupled with an MHC self antigen. Helper T cells are more easily activated than killer T cells. Activation of killer T cells is strongly regulated and may require additional stimulation from helper T cells.<\/span><\/p>\nFigure 17.5.2 Exposure to a foreign antigen on an antigen-presenting cell is necessary to activate T cells to become killer T cells or helper T cells.<\/em><\/figcaption><\/figure>\nKiller T Cells<\/h2>\n
Activated killer T cells induce the death of cells that bear a specific non-self<\/a> antigen because they are infected with pathogens or are cancerous. The antigen targets the cell for destruction by killer T cells, which travel through the bloodstream searching for target cells to kill. Killer T cells may use various mechanisms to kill target cells. One way is by releasing toxins in granules that enter and kill infected or cancerous cells (see Figure 17.5.3).<\/p>\nFigure 17.5.3 A killer T cell releases toxins that destroy an infected body cell and the viruses it contains.<\/em><\/figcaption><\/figure>\nHelper T Cells<\/h2>\n
Activated helper T cells<\/span><\/a><\/span>\u00a0do not kill infected or cancerous cells. Instead, their role is to \u201cmanage\u201d both innate<\/a> and adaptive immune<\/a> responses by directing other cells to perform these tasks. They control other cells by releasing cytokine<\/span>s<\/span><\/a><\/span>, which are proteins that can influence the activity of many cell types, including killer T cell<\/span>s<\/span><\/a><\/span>, B cells<\/a>, and macrophages<\/a>.\u00a0Some cytokines released by helper T cells assist with the activation of killer T cells.<\/span><\/p>\n\nB Cells<\/h1>\n<\/div>\n
B cell<\/strong>s<\/strong><\/a><\/strong>, or B lymphocytes, are the major cells involved in the creation of antibodies<\/a> that circulate in blood plasma<\/a> and lymph<\/a>. Antibodies are large, Y-shaped proteins used by the immune system to identify and neutralize foreign invaders. Besides producing antibodies, B cells may also function as antigen-presenting cell<\/span>s<\/span><\/a><\/span>, or secrete cytokines that help control other immune cells and responses.<\/span><\/p>\nB Cell Activation (or Antibody-Mediated Immunity)<\/h2>\n
Before B cells can actively function to defend the host, they must be activated. As shown in Figure 17.5.4, B cell activation begins when a B cell engulfs and digests an antigen. The antigen may be either free floating in the lymph, or it may be presented by an antigen-presenting cell, such as a dendritic cell or macrophage. In either case, the B cell then displays antigen fragments bound to its own MHC antigens. The MHC-antigen complex on the B cell attracts helper T cells. The helper T cells, in turn, secrete cytokines that help the B cell to multiply, and the daughter cells to mature into plasma cells.<\/p>\nFigure 17.5.4 B cells are activated and become antibody-producing plasma cells with the help of helper T cells.<\/em><\/figcaption><\/figure>\nPlasma Cells<\/h2>\nFigure 17.5.5 Each antibody fits its antigen like a lock fits a key.<\/em><\/figcaption><\/figure>\n
Artificially Acquired Passive Immunity<\/h3>\r\nOlder children and adults can acquire passive immunity artificially through the injection of antibodies or activated T cells, which may be done when there is a high risk of infection and insufficient time for the body to develop active immunity through vaccination. It may also be done to reduce symptoms of ongoing disease, or to compensate for immunodeficiency diseases.\r\n\r\nAdaptive Immune Evasion<\/h1>\r\n<\/div>\r\nMany pathogens have been around for a long time, living with human populations for generations. To persist, some have evolved mechanisms to evade the adaptive immune system of human hosts. One way they have done this is by rapidly changing their non-essential antigens. This is called antigenic variation. An example of a pathogen that takes this approach is human immunodeficiency virus (HIV). It mutates rapidly so the proteins on its viral envelope are constantly changing. By the time the adaptive immune system responds, the virus\u2019s antigens have changed. Antigenic variation is the main reason that efforts to develop a vaccine against HIV have not yet been successful.\r\n\r\nAnother evasion approach that some pathogens may take is to mask pathogen antigens with host molecules so the host\u2019s immune system cannot detect the antigens. HIV takes this approach, as well. The envelope that covers the virus is formed from the outermost membrane of the host cell.\r\n\r\nFeature: My Human Body<\/h1>\r\n<\/div>\r\nIf you think that immunizations are just for kids, think again. There are several vaccines recommended by HealthLinkBC<\/a> for people over the age of 18. The tables below from HealthLinkBC show the vaccine schedules recommended for infants and children, school-aged children, and adults and senior. Additional vaccines may be recommended for certain adults based on specific travel plans, medical conditions or other indications. Are you up to date with your vaccines? You can check with your doctor to be sure.\r\n\r\n\r\n\r\n\r\n<\/div>\r\n\r\n\r\n \r\n\r\n\r\n\r\n17.5 Summary<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- The [pb_glossary id=\"4822\"]adaptive immune system[\/pb_glossary] is a subsystem of the overall immune system that recognizes and makes a tailored attack against specific [pb_glossary id=\"5799\"]pathogens[\/pb_glossary]\u00a0or tumor cells. It is a slower, but more effective response than the [pb_glossary id=\"4820\"]innate immune response[\/pb_glossary], and also leads to immunity to particular pathogens.<\/li>\r\n \t
- [pb_glossary id=\"4832\"]Lymphocytes[\/pb_glossary]\u00a0produced by the [pb_glossary id=\"2911\"]lymphatic system[\/pb_glossary] are the main cells of the adaptive immune system. There are two major types of lymphocytes: [pb_glossary id=\"4892\"]T cells[\/pb_glossary]\u00a0and [pb_glossary id=\"5539\"]B cells[\/pb_glossary]. Both types must be activated by foreign [pb_glossary id=\"2704\"]antigens[\/pb_glossary]\u00a0to become functioning immune cells.<\/li>\r\n \t
- Most activated T cells become either [pb_glossary id=\"4893\"]killer T cells[\/pb_glossary]\u00a0or [pb_glossary id=\"5537\"]helper T cells[\/pb_glossary]. Killer T cells destroy cells that are infected with pathogens or are cancerous. Helper T cells manage immune responses by releasing [pb_glossary id=\"4865\"]cytokines[\/pb_glossary]\u00a0that control other types of leukocytes.<\/li>\r\n \t
- Activated B cells form [pb_glossary id=\"4902\"]plasma cells[\/pb_glossary]\u00a0that secrete [pb_glossary id=\"5659\"]antibodies[\/pb_glossary], which bind to specific antigens on pathogens or infected cells. The antibody-antigen complexes generally lead to the destruction of the cells, for example, by attracting phagocytes or triggering the [pb_glossary id=\"4868\"]complement system[\/pb_glossary].<\/li>\r\n \t
- After an adaptive immune response occurs, long-lasting [pb_glossary id=\"4904\"]memory B cells and memory T cells[\/pb_glossary] may remain to confer immunity to the specific pathogen that caused the adaptive immune response. These memory cells are ready to activate an immediate response if they are exposed to the same antigen again in the future.<\/li>\r\n \t
- Immunity may be active or passive. Active immunity occurs when the immune system has been presented with antigens that elicit an adaptive immune response. This may occur naturally as the result of an infection, or artificially as the result of immunization. Active immunity may last for years or even for life.<\/li>\r\n \t
- Passive immunity occurs without an adaptive immune response by the transfer of antibodies or activated T cells. This may occur naturally between a mother and her fetus or her nursing infant, or it may occur artificially by injection. Passive immunity lasts only as long as the antibodies or activated T cells remain alive in the body, generally just weeks or months.<\/li>\r\n \t
- Many pathogens have evolved mechanisms to evade the adaptive immune system. For example, human immunodeficiency virus ([pb_glossary id=\"4501\"]HIV[\/pb_glossary]) evades the adaptive immune system by frequently changing its antigens and by forming its outer envelope from the host\u2019s cell membrane.<\/li>\r\n<\/ul>\r\n<\/div>\r\n<\/div>\r\n
\r\n17.5 Review Questions<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- What is the adaptive immune system?<\/li>\r\n \t
- Define immunity.<\/li>\r\n \t
- [h5p id=\"639\"]<\/li>\r\n \t
- How are lymphocytes activated?<\/li>\r\n \t
- Identify two common types of T cells and their functions.<\/li>\r\n \t
- How do activated B cells help defend against pathogens?<\/li>\r\n \t
- How does passive immunity differ from active immunity?\u00a0How may passive immunity occur?<\/li>\r\n \t
- What are two ways that active immunity may come about?<\/li>\r\n \t
- What ways of evading the human adaptive immune system evolved in human immunodeficiency virus (HIV)?<\/li>\r\n \t
- Why do vaccinations expose\u00a0a person to a version of a pathogen?<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n
\r\n17.5 Explore More<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\nhttps:\/\/youtu.be\/rb7TVW77ZCs\r\nHow do vaccines work? - Kelwalin Dhanasarnsombut, TED-Ed, 2015.<\/p>\r\nhttps:\/\/youtu.be\/yqUFy-t4MlQ\r\n
How we conquered the deadly smallpox virus - Simona Zompi, TED-Ed, 2013.<\/p>\r\nhttps:\/\/youtu.be\/5THf6gTNqO8\r\n
Why Do We Need A New Flu Shot Every Year? Seeker, 2015.<\/p>\r\nhttps:\/\/youtu.be\/X-rC78MKZvw\r\n
An HIV Vaccine: Mapping Uncharted Territory, NIAID, 2016.<\/p>\r\n\r\n<\/div>\r\n<\/div>\r\n \r\n
Attributions<\/h2>\r\nFigure 17.5.1<\/strong>\r\n\r\nKiller_T_cells_surround_a_cancer_cell<\/a> by Alex Ritter, Jennifer Lippincott Schwartz and Gillian Griffiths at the National Institutes of Health\/ Visuals Online<\/a> on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).\r\n\r\nFigure 17.5.2<\/strong>\r\n\r\nT_cell_activation.svg<\/a> by Rehua<\/a> (derivative work) on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).\u00a0(Original image: T_cell_activation.png<\/a>:\u00a0 from The Immune System - NIH Publication No. 03\u20135423<\/a>)\r\n\r\nFigure 17.5.3<\/strong>\r\n\r\nCytotoxic T Cell function<\/a>\u00a0by 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 17.5.4<\/strong>\r\n\r\nB_cell_activation.svg<\/a> by Fred the Oyster<\/a> on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain). (Original from The Immune System - NIH Publication No. 03\u20135423<\/a>)\r\n\r\nFigure 17.5.5<\/strong>\r\n\r\nAntibody.svg<\/a> by Fvasconcellos<\/a>\u00a0 on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain). (Original black and white image from the National Human Genome Research Institute's Talking Genetics Glossary<\/a>)\r\n\r\nFigure 17.5.7<\/strong>\r\n\r\nimmunizations<\/a> by U.S. Air Force photo by Airman 1st Class Destinee Dougherty from Military Health System website, Health.mil, is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).\r\n\r\n \r\nReferences<\/h2>\r\n
HealthLinkBC. (2018). B.C. immunization schedules. \u00a0Gov.BC.CA. \u00a0https:\/\/www.healthlinkbc.ca\/tools-videos\/bc-immunization-schedules<\/p>\r\n
Mayo Clinic Staff. (n.d.). Measles [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/measles\/symptoms-causes\/syc-20374857<\/p>\r\n
Mayo Clinic Staff. (n.d.). Mumps [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/mumps\/symptoms-causes\/syc-20375361<\/p>\r\n
Mayo Clinic Staff. (n.d.). Polio [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/polio\/symptoms-causes\/syc-20376512<\/p>\r\n
Mayo Clinic Staff. (n.d.). Smallpox [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/smallpox\/symptoms-causes\/syc-20353027<\/p>\r\n
NIAID. (2016, August 11). An HIV vaccine: Mapping uncharted territory. YouTube. https:\/\/www.youtube.com\/watch?v=X-rC78MKZvw&feature=youtu.be<\/p>\r\n
OpenStax. (2016, March 23). Figure 4 Na\u00efve\u00a0CD4<\/mtext>+<\/sup>\u00a0T cells engage MHC II molecules on antigen-presenting cells (APCs) and become activated. Clones of the activated helper T cell, in turn, activate B cells and CD8+<\/sup> T cells, which become cytotoxic T cells. Cytotoxic T cells kill infected cells [digital image].\u00a0In OpenStax, Biology<\/em> (Section 42.2). OpenStax CNX. https:\/\/cnx.org\/contents\/GFy_h8cu@10.53:etZobsU-@6\/Adaptive-Immune-Response<\/p>\r\nSeeker. (2015, September 2). Why do we need a new flu shot every year? YouTube. https:\/\/www.youtube.com\/watch?v=5THf6gTNqO8<\/p>\r\n
TED-Ed. (2015, January 12). How do vaccines work? - Kelwalin Dhanasarnsombut. YouTube. https:\/\/www.youtube.com\/watch?v=rb7TVW77ZCs&feature=youtu.be<\/p>\r\n
TED-Ed. (2013, October 28). How we conquered the deadly smallpox virus - Simona Zompi. YouTube. https:\/\/www.youtube.com\/watch?v=yqUFy-t4MlQ&feature=youtu.be<\/p>","rendered":"
<\/p>\nFigure 17.5.1 Kill the cancer.<\/em><\/figcaption><\/figure>\n\nThe Kiss of Death<\/h1>\n<\/div>\n
The photomicrograph in Figure 17.5.1 shows a group of killer T cells (green and red) surrounding a cancer<\/a> cell (blue, centre). When a killer T cell makes contact with the cancer cell, it attaches to and spreads over the dangerous target. The killer T cell then uses special chemicals stored in vesicles<\/span><\/a><\/span>\u00a0(red) to deliver the killing blow. This event has thus been nicknamed \u201cthe kiss of death.\u201d After the target cell is killed, the killer T cells move on to find the next victim. Killer T cells like these are important players in the adaptive immune system.<\/span><\/p>\n\nWhat Is the Adaptive Immune System?<\/h1>\n<\/div>\n
The\u00a0adaptive immune system<\/a><\/strong>\u00a0is a subsystem of the overall immune system. It is composed of highly specialized cells and processes that eliminate specific pathogens<\/span><\/a><\/span>\u00a0and tumor cells. An adaptive immune response is set in motion by antigen<\/span>s<\/span><\/a>\u00a0that the immune system recognizes as foreign. Unlike an innate immune response, an adaptive immune response is highly specific to a particular pathogen (or its antigen). An important function of the adaptive immune system that is\u00a0<\/span>not<\/em>\u00a0shared by the innate immune system is the creation of immunological memory \u2014 or immunity<\/a> \u2014 which occurs after the initial response to a specific pathogen. It allows for a faster, stronger response on subsequent encounters with the same pathogen, usually before the pathogen can even cause symptoms of illness.<\/span><\/p>\nLymphocytes<\/span><\/a><\/span>\u00a0are the main cells of the adaptive immune system. They are leukocyte<\/span>s<\/span><\/a><\/span>\u00a0that arise and mature in organs of the lymphatic system, including the bone marrow<\/a> and thymus<\/a>. The human body normally has about 2 trillion lymphocytes, which constitute about 1\/3 of all leukocytes. Most of the lymphocytes are normally sequestered within tissue fluid or organs of the lymphatic system, including the tonsil<\/span>s<\/span><\/a><\/span>, spleen<\/a>, and lymph node<\/span>s<\/span><\/a><\/span>. Only about 2% of the lymphocytes are normally circulating in the blood. There are two main types of lymphocytes involved in adaptive immune responses, called T cells and B cells. T cells destroy infected cells or release chemicals that regulate immune responses. B cells secrete antibodies<\/a> that bind with antigen<\/span>s<\/span>[<\/span>\/pb_glossary]<\/span>\u00a0of [pb_glossary id=\"271\"]pathogen<\/span>s<\/span><\/a><\/span>\u00a0so they can be removed by other immune cells or processes.<\/span><\/p>\n\nPathways of the Adaptive Immune Response<\/h1>\n
There are some general similarities in the way in which the separate adaptive immune responses occur in T cell and B cell responses.\u00a0 In both pathways, a foreign antigen is recognized by the B or T cell.\u00a0 From there, cytokines produced by helper T-cells promote clonal expansion of lymphocytes.\u00a0 From this clonal expansion, two types of B or T cells are produced- cells that directly fight the pathogen invasion and cells that remain behind to provide long-term immunity<\/a>.\u00a0 Finally, once the pathogen invasion has been eradicated, the plasma cells and killer T cells go through apoptosis (programmed cell death).<\/p>\nT Cells<\/h1>\n<\/div>\n
There are multiple types of\u00a0T cells<\/a>,<\/strong>\u00a0or T lymphocytes. Major types are killer (or cytotoxic) T cells<\/a> and helper T cells<\/a>. Both types develop from immature T cells that become activated by exposure to an antigen<\/a>.<\/p>\nT Cell Activation (or Cell-Mediated Immunity)<\/h2>\n
T cells must be activated to become either killer T cells or helper T cells. This requires presentation of a foreign antigen by antigen-presenting cells<\/a>, as shown in Figure 17.5.2. Antigen-presenting cells may be dendritic cells<\/span><\/a><\/span>, macrophage<\/span>s<\/span><\/a><\/span>, or B cell<\/span>s<\/span><\/a><\/span>. Activation occurs when T cells are presented with a foreign antigen coupled with an MHC self antigen. Helper T cells are more easily activated than killer T cells. Activation of killer T cells is strongly regulated and may require additional stimulation from helper T cells.<\/span><\/p>\nFigure 17.5.2 Exposure to a foreign antigen on an antigen-presenting cell is necessary to activate T cells to become killer T cells or helper T cells.<\/em><\/figcaption><\/figure>\nKiller T Cells<\/h2>\n
Activated killer T cells induce the death of cells that bear a specific non-self<\/a> antigen because they are infected with pathogens or are cancerous. The antigen targets the cell for destruction by killer T cells, which travel through the bloodstream searching for target cells to kill. Killer T cells may use various mechanisms to kill target cells. One way is by releasing toxins in granules that enter and kill infected or cancerous cells (see Figure 17.5.3).<\/p>\nFigure 17.5.3 A killer T cell releases toxins that destroy an infected body cell and the viruses it contains.<\/em><\/figcaption><\/figure>\nHelper T Cells<\/h2>\n
Activated helper T cells<\/span><\/a><\/span>\u00a0do not kill infected or cancerous cells. Instead, their role is to \u201cmanage\u201d both innate<\/a> and adaptive immune<\/a> responses by directing other cells to perform these tasks. They control other cells by releasing cytokine<\/span>s<\/span><\/a><\/span>, which are proteins that can influence the activity of many cell types, including killer T cell<\/span>s<\/span><\/a><\/span>, B cells<\/a>, and macrophages<\/a>.\u00a0Some cytokines released by helper T cells assist with the activation of killer T cells.<\/span><\/p>\n\nB Cells<\/h1>\n<\/div>\n
B cell<\/strong>s<\/strong><\/a><\/strong>, or B lymphocytes, are the major cells involved in the creation of antibodies<\/a> that circulate in blood plasma<\/a> and lymph<\/a>. Antibodies are large, Y-shaped proteins used by the immune system to identify and neutralize foreign invaders. Besides producing antibodies, B cells may also function as antigen-presenting cell<\/span>s<\/span><\/a><\/span>, or secrete cytokines that help control other immune cells and responses.<\/span><\/p>\nB Cell Activation (or Antibody-Mediated Immunity)<\/h2>\n
Before B cells can actively function to defend the host, they must be activated. As shown in Figure 17.5.4, B cell activation begins when a B cell engulfs and digests an antigen. The antigen may be either free floating in the lymph, or it may be presented by an antigen-presenting cell, such as a dendritic cell or macrophage. In either case, the B cell then displays antigen fragments bound to its own MHC antigens. The MHC-antigen complex on the B cell attracts helper T cells. The helper T cells, in turn, secrete cytokines that help the B cell to multiply, and the daughter cells to mature into plasma cells.<\/p>\nFigure 17.5.4 B cells are activated and become antibody-producing plasma cells with the help of helper T cells.<\/em><\/figcaption><\/figure>\nPlasma Cells<\/h2>\nFigure 17.5.5 Each antibody fits its antigen like a lock fits a key.<\/em><\/figcaption><\/figure>\n
Adaptive Immune Evasion<\/h1>\r\n<\/div>\r\nMany pathogens have been around for a long time, living with human populations for generations. To persist, some have evolved mechanisms to evade the adaptive immune system of human hosts. One way they have done this is by rapidly changing their non-essential antigens. This is called antigenic variation. An example of a pathogen that takes this approach is human immunodeficiency virus (HIV). It mutates rapidly so the proteins on its viral envelope are constantly changing. By the time the adaptive immune system responds, the virus\u2019s antigens have changed. Antigenic variation is the main reason that efforts to develop a vaccine against HIV have not yet been successful.\r\n\r\nAnother evasion approach that some pathogens may take is to mask pathogen antigens with host molecules so the host\u2019s immune system cannot detect the antigens. HIV takes this approach, as well. The envelope that covers the virus is formed from the outermost membrane of the host cell.\r\n\r\nFeature: My Human Body<\/h1>\r\n<\/div>\r\nIf you think that immunizations are just for kids, think again. There are several vaccines recommended by HealthLinkBC<\/a> for people over the age of 18. The tables below from HealthLinkBC show the vaccine schedules recommended for infants and children, school-aged children, and adults and senior. Additional vaccines may be recommended for certain adults based on specific travel plans, medical conditions or other indications. Are you up to date with your vaccines? You can check with your doctor to be sure.\r\n\r\n\r\n\r\n\r\n<\/div>\r\n\r\n\r\n \r\n\r\n\r\n\r\n17.5 Summary<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- The [pb_glossary id=\"4822\"]adaptive immune system[\/pb_glossary] is a subsystem of the overall immune system that recognizes and makes a tailored attack against specific [pb_glossary id=\"5799\"]pathogens[\/pb_glossary]\u00a0or tumor cells. It is a slower, but more effective response than the [pb_glossary id=\"4820\"]innate immune response[\/pb_glossary], and also leads to immunity to particular pathogens.<\/li>\r\n \t
- [pb_glossary id=\"4832\"]Lymphocytes[\/pb_glossary]\u00a0produced by the [pb_glossary id=\"2911\"]lymphatic system[\/pb_glossary] are the main cells of the adaptive immune system. There are two major types of lymphocytes: [pb_glossary id=\"4892\"]T cells[\/pb_glossary]\u00a0and [pb_glossary id=\"5539\"]B cells[\/pb_glossary]. Both types must be activated by foreign [pb_glossary id=\"2704\"]antigens[\/pb_glossary]\u00a0to become functioning immune cells.<\/li>\r\n \t
- Most activated T cells become either [pb_glossary id=\"4893\"]killer T cells[\/pb_glossary]\u00a0or [pb_glossary id=\"5537\"]helper T cells[\/pb_glossary]. Killer T cells destroy cells that are infected with pathogens or are cancerous. Helper T cells manage immune responses by releasing [pb_glossary id=\"4865\"]cytokines[\/pb_glossary]\u00a0that control other types of leukocytes.<\/li>\r\n \t
- Activated B cells form [pb_glossary id=\"4902\"]plasma cells[\/pb_glossary]\u00a0that secrete [pb_glossary id=\"5659\"]antibodies[\/pb_glossary], which bind to specific antigens on pathogens or infected cells. The antibody-antigen complexes generally lead to the destruction of the cells, for example, by attracting phagocytes or triggering the [pb_glossary id=\"4868\"]complement system[\/pb_glossary].<\/li>\r\n \t
- After an adaptive immune response occurs, long-lasting [pb_glossary id=\"4904\"]memory B cells and memory T cells[\/pb_glossary] may remain to confer immunity to the specific pathogen that caused the adaptive immune response. These memory cells are ready to activate an immediate response if they are exposed to the same antigen again in the future.<\/li>\r\n \t
- Immunity may be active or passive. Active immunity occurs when the immune system has been presented with antigens that elicit an adaptive immune response. This may occur naturally as the result of an infection, or artificially as the result of immunization. Active immunity may last for years or even for life.<\/li>\r\n \t
- Passive immunity occurs without an adaptive immune response by the transfer of antibodies or activated T cells. This may occur naturally between a mother and her fetus or her nursing infant, or it may occur artificially by injection. Passive immunity lasts only as long as the antibodies or activated T cells remain alive in the body, generally just weeks or months.<\/li>\r\n \t
- Many pathogens have evolved mechanisms to evade the adaptive immune system. For example, human immunodeficiency virus ([pb_glossary id=\"4501\"]HIV[\/pb_glossary]) evades the adaptive immune system by frequently changing its antigens and by forming its outer envelope from the host\u2019s cell membrane.<\/li>\r\n<\/ul>\r\n<\/div>\r\n<\/div>\r\n
\r\n17.5 Review Questions<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- What is the adaptive immune system?<\/li>\r\n \t
- Define immunity.<\/li>\r\n \t
- [h5p id=\"639\"]<\/li>\r\n \t
- How are lymphocytes activated?<\/li>\r\n \t
- Identify two common types of T cells and their functions.<\/li>\r\n \t
- How do activated B cells help defend against pathogens?<\/li>\r\n \t
- How does passive immunity differ from active immunity?\u00a0How may passive immunity occur?<\/li>\r\n \t
- What are two ways that active immunity may come about?<\/li>\r\n \t
- What ways of evading the human adaptive immune system evolved in human immunodeficiency virus (HIV)?<\/li>\r\n \t
- Why do vaccinations expose\u00a0a person to a version of a pathogen?<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n
\r\n17.5 Explore More<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\nhttps:\/\/youtu.be\/rb7TVW77ZCs\r\nHow do vaccines work? - Kelwalin Dhanasarnsombut, TED-Ed, 2015.<\/p>\r\nhttps:\/\/youtu.be\/yqUFy-t4MlQ\r\n
How we conquered the deadly smallpox virus - Simona Zompi, TED-Ed, 2013.<\/p>\r\nhttps:\/\/youtu.be\/5THf6gTNqO8\r\n
Why Do We Need A New Flu Shot Every Year? Seeker, 2015.<\/p>\r\nhttps:\/\/youtu.be\/X-rC78MKZvw\r\n
An HIV Vaccine: Mapping Uncharted Territory, NIAID, 2016.<\/p>\r\n\r\n<\/div>\r\n<\/div>\r\n \r\n
Attributions<\/h2>\r\nFigure 17.5.1<\/strong>\r\n\r\nKiller_T_cells_surround_a_cancer_cell<\/a> by Alex Ritter, Jennifer Lippincott Schwartz and Gillian Griffiths at the National Institutes of Health\/ Visuals Online<\/a> on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).\r\n\r\nFigure 17.5.2<\/strong>\r\n\r\nT_cell_activation.svg<\/a> by Rehua<\/a> (derivative work) on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).\u00a0(Original image: T_cell_activation.png<\/a>:\u00a0 from The Immune System - NIH Publication No. 03\u20135423<\/a>)\r\n\r\nFigure 17.5.3<\/strong>\r\n\r\nCytotoxic T Cell function<\/a>\u00a0by 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 17.5.4<\/strong>\r\n\r\nB_cell_activation.svg<\/a> by Fred the Oyster<\/a> on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain). (Original from The Immune System - NIH Publication No. 03\u20135423<\/a>)\r\n\r\nFigure 17.5.5<\/strong>\r\n\r\nAntibody.svg<\/a> by Fvasconcellos<\/a>\u00a0 on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain). (Original black and white image from the National Human Genome Research Institute's Talking Genetics Glossary<\/a>)\r\n\r\nFigure 17.5.7<\/strong>\r\n\r\nimmunizations<\/a> by U.S. Air Force photo by Airman 1st Class Destinee Dougherty from Military Health System website, Health.mil, is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).\r\n\r\n \r\nReferences<\/h2>\r\n
HealthLinkBC. (2018). B.C. immunization schedules. \u00a0Gov.BC.CA. \u00a0https:\/\/www.healthlinkbc.ca\/tools-videos\/bc-immunization-schedules<\/p>\r\n
Mayo Clinic Staff. (n.d.). Measles [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/measles\/symptoms-causes\/syc-20374857<\/p>\r\n
Mayo Clinic Staff. (n.d.). Mumps [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/mumps\/symptoms-causes\/syc-20375361<\/p>\r\n
Mayo Clinic Staff. (n.d.). Polio [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/polio\/symptoms-causes\/syc-20376512<\/p>\r\n
Mayo Clinic Staff. (n.d.). Smallpox [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/smallpox\/symptoms-causes\/syc-20353027<\/p>\r\n
NIAID. (2016, August 11). An HIV vaccine: Mapping uncharted territory. YouTube. https:\/\/www.youtube.com\/watch?v=X-rC78MKZvw&feature=youtu.be<\/p>\r\n
OpenStax. (2016, March 23). Figure 4 Na\u00efve\u00a0CD4<\/mtext>+<\/sup>\u00a0T cells engage MHC II molecules on antigen-presenting cells (APCs) and become activated. Clones of the activated helper T cell, in turn, activate B cells and CD8+<\/sup> T cells, which become cytotoxic T cells. Cytotoxic T cells kill infected cells [digital image].\u00a0In OpenStax, Biology<\/em> (Section 42.2). OpenStax CNX. https:\/\/cnx.org\/contents\/GFy_h8cu@10.53:etZobsU-@6\/Adaptive-Immune-Response<\/p>\r\nSeeker. (2015, September 2). Why do we need a new flu shot every year? YouTube. https:\/\/www.youtube.com\/watch?v=5THf6gTNqO8<\/p>\r\n
TED-Ed. (2015, January 12). How do vaccines work? - Kelwalin Dhanasarnsombut. YouTube. https:\/\/www.youtube.com\/watch?v=rb7TVW77ZCs&feature=youtu.be<\/p>\r\n
TED-Ed. (2013, October 28). How we conquered the deadly smallpox virus - Simona Zompi. YouTube. https:\/\/www.youtube.com\/watch?v=yqUFy-t4MlQ&feature=youtu.be<\/p>","rendered":"
<\/p>\nFigure 17.5.1 Kill the cancer.<\/em><\/figcaption><\/figure>\n\nThe Kiss of Death<\/h1>\n<\/div>\n
The photomicrograph in Figure 17.5.1 shows a group of killer T cells (green and red) surrounding a cancer<\/a> cell (blue, centre). When a killer T cell makes contact with the cancer cell, it attaches to and spreads over the dangerous target. The killer T cell then uses special chemicals stored in vesicles<\/span><\/a><\/span>\u00a0(red) to deliver the killing blow. This event has thus been nicknamed \u201cthe kiss of death.\u201d After the target cell is killed, the killer T cells move on to find the next victim. Killer T cells like these are important players in the adaptive immune system.<\/span><\/p>\n\nWhat Is the Adaptive Immune System?<\/h1>\n<\/div>\n
The\u00a0adaptive immune system<\/a><\/strong>\u00a0is a subsystem of the overall immune system. It is composed of highly specialized cells and processes that eliminate specific pathogens<\/span><\/a><\/span>\u00a0and tumor cells. An adaptive immune response is set in motion by antigen<\/span>s<\/span><\/a>\u00a0that the immune system recognizes as foreign. Unlike an innate immune response, an adaptive immune response is highly specific to a particular pathogen (or its antigen). An important function of the adaptive immune system that is\u00a0<\/span>not<\/em>\u00a0shared by the innate immune system is the creation of immunological memory \u2014 or immunity<\/a> \u2014 which occurs after the initial response to a specific pathogen. It allows for a faster, stronger response on subsequent encounters with the same pathogen, usually before the pathogen can even cause symptoms of illness.<\/span><\/p>\nLymphocytes<\/span><\/a><\/span>\u00a0are the main cells of the adaptive immune system. They are leukocyte<\/span>s<\/span><\/a><\/span>\u00a0that arise and mature in organs of the lymphatic system, including the bone marrow<\/a> and thymus<\/a>. The human body normally has about 2 trillion lymphocytes, which constitute about 1\/3 of all leukocytes. Most of the lymphocytes are normally sequestered within tissue fluid or organs of the lymphatic system, including the tonsil<\/span>s<\/span><\/a><\/span>, spleen<\/a>, and lymph node<\/span>s<\/span><\/a><\/span>. Only about 2% of the lymphocytes are normally circulating in the blood. There are two main types of lymphocytes involved in adaptive immune responses, called T cells and B cells. T cells destroy infected cells or release chemicals that regulate immune responses. B cells secrete antibodies<\/a> that bind with antigen<\/span>s<\/span>[<\/span>\/pb_glossary]<\/span>\u00a0of [pb_glossary id=\"271\"]pathogen<\/span>s<\/span><\/a><\/span>\u00a0so they can be removed by other immune cells or processes.<\/span><\/p>\n\nPathways of the Adaptive Immune Response<\/h1>\n
There are some general similarities in the way in which the separate adaptive immune responses occur in T cell and B cell responses.\u00a0 In both pathways, a foreign antigen is recognized by the B or T cell.\u00a0 From there, cytokines produced by helper T-cells promote clonal expansion of lymphocytes.\u00a0 From this clonal expansion, two types of B or T cells are produced- cells that directly fight the pathogen invasion and cells that remain behind to provide long-term immunity<\/a>.\u00a0 Finally, once the pathogen invasion has been eradicated, the plasma cells and killer T cells go through apoptosis (programmed cell death).<\/p>\nT Cells<\/h1>\n<\/div>\n
There are multiple types of\u00a0T cells<\/a>,<\/strong>\u00a0or T lymphocytes. Major types are killer (or cytotoxic) T cells<\/a> and helper T cells<\/a>. Both types develop from immature T cells that become activated by exposure to an antigen<\/a>.<\/p>\nT Cell Activation (or Cell-Mediated Immunity)<\/h2>\n
T cells must be activated to become either killer T cells or helper T cells. This requires presentation of a foreign antigen by antigen-presenting cells<\/a>, as shown in Figure 17.5.2. Antigen-presenting cells may be dendritic cells<\/span><\/a><\/span>, macrophage<\/span>s<\/span><\/a><\/span>, or B cell<\/span>s<\/span><\/a><\/span>. Activation occurs when T cells are presented with a foreign antigen coupled with an MHC self antigen. Helper T cells are more easily activated than killer T cells. Activation of killer T cells is strongly regulated and may require additional stimulation from helper T cells.<\/span><\/p>\nFigure 17.5.2 Exposure to a foreign antigen on an antigen-presenting cell is necessary to activate T cells to become killer T cells or helper T cells.<\/em><\/figcaption><\/figure>\nKiller T Cells<\/h2>\n
Activated killer T cells induce the death of cells that bear a specific non-self<\/a> antigen because they are infected with pathogens or are cancerous. The antigen targets the cell for destruction by killer T cells, which travel through the bloodstream searching for target cells to kill. Killer T cells may use various mechanisms to kill target cells. One way is by releasing toxins in granules that enter and kill infected or cancerous cells (see Figure 17.5.3).<\/p>\nFigure 17.5.3 A killer T cell releases toxins that destroy an infected body cell and the viruses it contains.<\/em><\/figcaption><\/figure>\nHelper T Cells<\/h2>\n
Activated helper T cells<\/span><\/a><\/span>\u00a0do not kill infected or cancerous cells. Instead, their role is to \u201cmanage\u201d both innate<\/a> and adaptive immune<\/a> responses by directing other cells to perform these tasks. They control other cells by releasing cytokine<\/span>s<\/span><\/a><\/span>, which are proteins that can influence the activity of many cell types, including killer T cell<\/span>s<\/span><\/a><\/span>, B cells<\/a>, and macrophages<\/a>.\u00a0Some cytokines released by helper T cells assist with the activation of killer T cells.<\/span><\/p>\n\nB Cells<\/h1>\n<\/div>\n
B cell<\/strong>s<\/strong><\/a><\/strong>, or B lymphocytes, are the major cells involved in the creation of antibodies<\/a> that circulate in blood plasma<\/a> and lymph<\/a>. Antibodies are large, Y-shaped proteins used by the immune system to identify and neutralize foreign invaders. Besides producing antibodies, B cells may also function as antigen-presenting cell<\/span>s<\/span><\/a><\/span>, or secrete cytokines that help control other immune cells and responses.<\/span><\/p>\nB Cell Activation (or Antibody-Mediated Immunity)<\/h2>\n
Before B cells can actively function to defend the host, they must be activated. As shown in Figure 17.5.4, B cell activation begins when a B cell engulfs and digests an antigen. The antigen may be either free floating in the lymph, or it may be presented by an antigen-presenting cell, such as a dendritic cell or macrophage. In either case, the B cell then displays antigen fragments bound to its own MHC antigens. The MHC-antigen complex on the B cell attracts helper T cells. The helper T cells, in turn, secrete cytokines that help the B cell to multiply, and the daughter cells to mature into plasma cells.<\/p>\nFigure 17.5.4 B cells are activated and become antibody-producing plasma cells with the help of helper T cells.<\/em><\/figcaption><\/figure>\nPlasma Cells<\/h2>\nFigure 17.5.5 Each antibody fits its antigen like a lock fits a key.<\/em><\/figcaption><\/figure>\n
Feature: My Human Body<\/h1>\r\n<\/div>\r\nIf you think that immunizations are just for kids, think again. There are several vaccines recommended by HealthLinkBC<\/a> for people over the age of 18. The tables below from HealthLinkBC show the vaccine schedules recommended for infants and children, school-aged children, and adults and senior. Additional vaccines may be recommended for certain adults based on specific travel plans, medical conditions or other indications. Are you up to date with your vaccines? You can check with your doctor to be sure.\r\n\r\n\r\n\r\n\r\n<\/div>\r\n\r\n\r\n \r\n\r\n\r\n\r\n17.5 Summary<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- The [pb_glossary id=\"4822\"]adaptive immune system[\/pb_glossary] is a subsystem of the overall immune system that recognizes and makes a tailored attack against specific [pb_glossary id=\"5799\"]pathogens[\/pb_glossary]\u00a0or tumor cells. It is a slower, but more effective response than the [pb_glossary id=\"4820\"]innate immune response[\/pb_glossary], and also leads to immunity to particular pathogens.<\/li>\r\n \t
- [pb_glossary id=\"4832\"]Lymphocytes[\/pb_glossary]\u00a0produced by the [pb_glossary id=\"2911\"]lymphatic system[\/pb_glossary] are the main cells of the adaptive immune system. There are two major types of lymphocytes: [pb_glossary id=\"4892\"]T cells[\/pb_glossary]\u00a0and [pb_glossary id=\"5539\"]B cells[\/pb_glossary]. Both types must be activated by foreign [pb_glossary id=\"2704\"]antigens[\/pb_glossary]\u00a0to become functioning immune cells.<\/li>\r\n \t
- Most activated T cells become either [pb_glossary id=\"4893\"]killer T cells[\/pb_glossary]\u00a0or [pb_glossary id=\"5537\"]helper T cells[\/pb_glossary]. Killer T cells destroy cells that are infected with pathogens or are cancerous. Helper T cells manage immune responses by releasing [pb_glossary id=\"4865\"]cytokines[\/pb_glossary]\u00a0that control other types of leukocytes.<\/li>\r\n \t
- Activated B cells form [pb_glossary id=\"4902\"]plasma cells[\/pb_glossary]\u00a0that secrete [pb_glossary id=\"5659\"]antibodies[\/pb_glossary], which bind to specific antigens on pathogens or infected cells. The antibody-antigen complexes generally lead to the destruction of the cells, for example, by attracting phagocytes or triggering the [pb_glossary id=\"4868\"]complement system[\/pb_glossary].<\/li>\r\n \t
- After an adaptive immune response occurs, long-lasting [pb_glossary id=\"4904\"]memory B cells and memory T cells[\/pb_glossary] may remain to confer immunity to the specific pathogen that caused the adaptive immune response. These memory cells are ready to activate an immediate response if they are exposed to the same antigen again in the future.<\/li>\r\n \t
- Immunity may be active or passive. Active immunity occurs when the immune system has been presented with antigens that elicit an adaptive immune response. This may occur naturally as the result of an infection, or artificially as the result of immunization. Active immunity may last for years or even for life.<\/li>\r\n \t
- Passive immunity occurs without an adaptive immune response by the transfer of antibodies or activated T cells. This may occur naturally between a mother and her fetus or her nursing infant, or it may occur artificially by injection. Passive immunity lasts only as long as the antibodies or activated T cells remain alive in the body, generally just weeks or months.<\/li>\r\n \t
- Many pathogens have evolved mechanisms to evade the adaptive immune system. For example, human immunodeficiency virus ([pb_glossary id=\"4501\"]HIV[\/pb_glossary]) evades the adaptive immune system by frequently changing its antigens and by forming its outer envelope from the host\u2019s cell membrane.<\/li>\r\n<\/ul>\r\n<\/div>\r\n<\/div>\r\n
\r\n17.5 Review Questions<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- What is the adaptive immune system?<\/li>\r\n \t
- Define immunity.<\/li>\r\n \t
- [h5p id=\"639\"]<\/li>\r\n \t
- How are lymphocytes activated?<\/li>\r\n \t
- Identify two common types of T cells and their functions.<\/li>\r\n \t
- How do activated B cells help defend against pathogens?<\/li>\r\n \t
- How does passive immunity differ from active immunity?\u00a0How may passive immunity occur?<\/li>\r\n \t
- What are two ways that active immunity may come about?<\/li>\r\n \t
- What ways of evading the human adaptive immune system evolved in human immunodeficiency virus (HIV)?<\/li>\r\n \t
- Why do vaccinations expose\u00a0a person to a version of a pathogen?<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n
\r\n17.5 Explore More<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\nhttps:\/\/youtu.be\/rb7TVW77ZCs\r\nHow do vaccines work? - Kelwalin Dhanasarnsombut, TED-Ed, 2015.<\/p>\r\nhttps:\/\/youtu.be\/yqUFy-t4MlQ\r\n
How we conquered the deadly smallpox virus - Simona Zompi, TED-Ed, 2013.<\/p>\r\nhttps:\/\/youtu.be\/5THf6gTNqO8\r\n
Why Do We Need A New Flu Shot Every Year? Seeker, 2015.<\/p>\r\nhttps:\/\/youtu.be\/X-rC78MKZvw\r\n
An HIV Vaccine: Mapping Uncharted Territory, NIAID, 2016.<\/p>\r\n\r\n<\/div>\r\n<\/div>\r\n \r\n
Attributions<\/h2>\r\nFigure 17.5.1<\/strong>\r\n\r\nKiller_T_cells_surround_a_cancer_cell<\/a> by Alex Ritter, Jennifer Lippincott Schwartz and Gillian Griffiths at the National Institutes of Health\/ Visuals Online<\/a> on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).\r\n\r\nFigure 17.5.2<\/strong>\r\n\r\nT_cell_activation.svg<\/a> by Rehua<\/a> (derivative work) on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).\u00a0(Original image: T_cell_activation.png<\/a>:\u00a0 from The Immune System - NIH Publication No. 03\u20135423<\/a>)\r\n\r\nFigure 17.5.3<\/strong>\r\n\r\nCytotoxic T Cell function<\/a>\u00a0by 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 17.5.4<\/strong>\r\n\r\nB_cell_activation.svg<\/a> by Fred the Oyster<\/a> on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain). (Original from The Immune System - NIH Publication No. 03\u20135423<\/a>)\r\n\r\nFigure 17.5.5<\/strong>\r\n\r\nAntibody.svg<\/a> by Fvasconcellos<\/a>\u00a0 on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain). (Original black and white image from the National Human Genome Research Institute's Talking Genetics Glossary<\/a>)\r\n\r\nFigure 17.5.7<\/strong>\r\n\r\nimmunizations<\/a> by U.S. Air Force photo by Airman 1st Class Destinee Dougherty from Military Health System website, Health.mil, is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).\r\n\r\n \r\nReferences<\/h2>\r\n
HealthLinkBC. (2018). B.C. immunization schedules. \u00a0Gov.BC.CA. \u00a0https:\/\/www.healthlinkbc.ca\/tools-videos\/bc-immunization-schedules<\/p>\r\n
Mayo Clinic Staff. (n.d.). Measles [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/measles\/symptoms-causes\/syc-20374857<\/p>\r\n
Mayo Clinic Staff. (n.d.). Mumps [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/mumps\/symptoms-causes\/syc-20375361<\/p>\r\n
Mayo Clinic Staff. (n.d.). Polio [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/polio\/symptoms-causes\/syc-20376512<\/p>\r\n
Mayo Clinic Staff. (n.d.). Smallpox [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/smallpox\/symptoms-causes\/syc-20353027<\/p>\r\n
NIAID. (2016, August 11). An HIV vaccine: Mapping uncharted territory. YouTube. https:\/\/www.youtube.com\/watch?v=X-rC78MKZvw&feature=youtu.be<\/p>\r\n
OpenStax. (2016, March 23). Figure 4 Na\u00efve\u00a0CD4<\/mtext>+<\/sup>\u00a0T cells engage MHC II molecules on antigen-presenting cells (APCs) and become activated. Clones of the activated helper T cell, in turn, activate B cells and CD8+<\/sup> T cells, which become cytotoxic T cells. Cytotoxic T cells kill infected cells [digital image].\u00a0In OpenStax, Biology<\/em> (Section 42.2). OpenStax CNX. https:\/\/cnx.org\/contents\/GFy_h8cu@10.53:etZobsU-@6\/Adaptive-Immune-Response<\/p>\r\nSeeker. (2015, September 2). Why do we need a new flu shot every year? YouTube. https:\/\/www.youtube.com\/watch?v=5THf6gTNqO8<\/p>\r\n
TED-Ed. (2015, January 12). How do vaccines work? - Kelwalin Dhanasarnsombut. YouTube. https:\/\/www.youtube.com\/watch?v=rb7TVW77ZCs&feature=youtu.be<\/p>\r\n
TED-Ed. (2013, October 28). How we conquered the deadly smallpox virus - Simona Zompi. YouTube. https:\/\/www.youtube.com\/watch?v=yqUFy-t4MlQ&feature=youtu.be<\/p>","rendered":"
<\/p>\nFigure 17.5.1 Kill the cancer.<\/em><\/figcaption><\/figure>\n\nThe Kiss of Death<\/h1>\n<\/div>\n
The photomicrograph in Figure 17.5.1 shows a group of killer T cells (green and red) surrounding a cancer<\/a> cell (blue, centre). When a killer T cell makes contact with the cancer cell, it attaches to and spreads over the dangerous target. The killer T cell then uses special chemicals stored in vesicles<\/span><\/a><\/span>\u00a0(red) to deliver the killing blow. This event has thus been nicknamed \u201cthe kiss of death.\u201d After the target cell is killed, the killer T cells move on to find the next victim. Killer T cells like these are important players in the adaptive immune system.<\/span><\/p>\n\nWhat Is the Adaptive Immune System?<\/h1>\n<\/div>\n
The\u00a0adaptive immune system<\/a><\/strong>\u00a0is a subsystem of the overall immune system. It is composed of highly specialized cells and processes that eliminate specific pathogens<\/span><\/a><\/span>\u00a0and tumor cells. An adaptive immune response is set in motion by antigen<\/span>s<\/span><\/a>\u00a0that the immune system recognizes as foreign. Unlike an innate immune response, an adaptive immune response is highly specific to a particular pathogen (or its antigen). An important function of the adaptive immune system that is\u00a0<\/span>not<\/em>\u00a0shared by the innate immune system is the creation of immunological memory \u2014 or immunity<\/a> \u2014 which occurs after the initial response to a specific pathogen. It allows for a faster, stronger response on subsequent encounters with the same pathogen, usually before the pathogen can even cause symptoms of illness.<\/span><\/p>\nLymphocytes<\/span><\/a><\/span>\u00a0are the main cells of the adaptive immune system. They are leukocyte<\/span>s<\/span><\/a><\/span>\u00a0that arise and mature in organs of the lymphatic system, including the bone marrow<\/a> and thymus<\/a>. The human body normally has about 2 trillion lymphocytes, which constitute about 1\/3 of all leukocytes. Most of the lymphocytes are normally sequestered within tissue fluid or organs of the lymphatic system, including the tonsil<\/span>s<\/span><\/a><\/span>, spleen<\/a>, and lymph node<\/span>s<\/span><\/a><\/span>. Only about 2% of the lymphocytes are normally circulating in the blood. There are two main types of lymphocytes involved in adaptive immune responses, called T cells and B cells. T cells destroy infected cells or release chemicals that regulate immune responses. B cells secrete antibodies<\/a> that bind with antigen<\/span>s<\/span>[<\/span>\/pb_glossary]<\/span>\u00a0of [pb_glossary id=\"271\"]pathogen<\/span>s<\/span><\/a><\/span>\u00a0so they can be removed by other immune cells or processes.<\/span><\/p>\n\nPathways of the Adaptive Immune Response<\/h1>\n
There are some general similarities in the way in which the separate adaptive immune responses occur in T cell and B cell responses.\u00a0 In both pathways, a foreign antigen is recognized by the B or T cell.\u00a0 From there, cytokines produced by helper T-cells promote clonal expansion of lymphocytes.\u00a0 From this clonal expansion, two types of B or T cells are produced- cells that directly fight the pathogen invasion and cells that remain behind to provide long-term immunity<\/a>.\u00a0 Finally, once the pathogen invasion has been eradicated, the plasma cells and killer T cells go through apoptosis (programmed cell death).<\/p>\nT Cells<\/h1>\n<\/div>\n
There are multiple types of\u00a0T cells<\/a>,<\/strong>\u00a0or T lymphocytes. Major types are killer (or cytotoxic) T cells<\/a> and helper T cells<\/a>. Both types develop from immature T cells that become activated by exposure to an antigen<\/a>.<\/p>\nT Cell Activation (or Cell-Mediated Immunity)<\/h2>\n
T cells must be activated to become either killer T cells or helper T cells. This requires presentation of a foreign antigen by antigen-presenting cells<\/a>, as shown in Figure 17.5.2. Antigen-presenting cells may be dendritic cells<\/span><\/a><\/span>, macrophage<\/span>s<\/span><\/a><\/span>, or B cell<\/span>s<\/span><\/a><\/span>. Activation occurs when T cells are presented with a foreign antigen coupled with an MHC self antigen. Helper T cells are more easily activated than killer T cells. Activation of killer T cells is strongly regulated and may require additional stimulation from helper T cells.<\/span><\/p>\nFigure 17.5.2 Exposure to a foreign antigen on an antigen-presenting cell is necessary to activate T cells to become killer T cells or helper T cells.<\/em><\/figcaption><\/figure>\nKiller T Cells<\/h2>\n
Activated killer T cells induce the death of cells that bear a specific non-self<\/a> antigen because they are infected with pathogens or are cancerous. The antigen targets the cell for destruction by killer T cells, which travel through the bloodstream searching for target cells to kill. Killer T cells may use various mechanisms to kill target cells. One way is by releasing toxins in granules that enter and kill infected or cancerous cells (see Figure 17.5.3).<\/p>\nFigure 17.5.3 A killer T cell releases toxins that destroy an infected body cell and the viruses it contains.<\/em><\/figcaption><\/figure>\nHelper T Cells<\/h2>\n
Activated helper T cells<\/span><\/a><\/span>\u00a0do not kill infected or cancerous cells. Instead, their role is to \u201cmanage\u201d both innate<\/a> and adaptive immune<\/a> responses by directing other cells to perform these tasks. They control other cells by releasing cytokine<\/span>s<\/span><\/a><\/span>, which are proteins that can influence the activity of many cell types, including killer T cell<\/span>s<\/span><\/a><\/span>, B cells<\/a>, and macrophages<\/a>.\u00a0Some cytokines released by helper T cells assist with the activation of killer T cells.<\/span><\/p>\n\nB Cells<\/h1>\n<\/div>\n
B cell<\/strong>s<\/strong><\/a><\/strong>, or B lymphocytes, are the major cells involved in the creation of antibodies<\/a> that circulate in blood plasma<\/a> and lymph<\/a>. Antibodies are large, Y-shaped proteins used by the immune system to identify and neutralize foreign invaders. Besides producing antibodies, B cells may also function as antigen-presenting cell<\/span>s<\/span><\/a><\/span>, or secrete cytokines that help control other immune cells and responses.<\/span><\/p>\nB Cell Activation (or Antibody-Mediated Immunity)<\/h2>\n
Before B cells can actively function to defend the host, they must be activated. As shown in Figure 17.5.4, B cell activation begins when a B cell engulfs and digests an antigen. The antigen may be either free floating in the lymph, or it may be presented by an antigen-presenting cell, such as a dendritic cell or macrophage. In either case, the B cell then displays antigen fragments bound to its own MHC antigens. The MHC-antigen complex on the B cell attracts helper T cells. The helper T cells, in turn, secrete cytokines that help the B cell to multiply, and the daughter cells to mature into plasma cells.<\/p>\nFigure 17.5.4 B cells are activated and become antibody-producing plasma cells with the help of helper T cells.<\/em><\/figcaption><\/figure>\nPlasma Cells<\/h2>\nFigure 17.5.5 Each antibody fits its antigen like a lock fits a key.<\/em><\/figcaption><\/figure>\n
\r\n\r\n<\/div>\r\n\r\n\r\n \r\n\r\n\r\n17.5 Summary<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- The [pb_glossary id=\"4822\"]adaptive immune system[\/pb_glossary] is a subsystem of the overall immune system that recognizes and makes a tailored attack against specific [pb_glossary id=\"5799\"]pathogens[\/pb_glossary]\u00a0or tumor cells. It is a slower, but more effective response than the [pb_glossary id=\"4820\"]innate immune response[\/pb_glossary], and also leads to immunity to particular pathogens.<\/li>\r\n \t
- [pb_glossary id=\"4832\"]Lymphocytes[\/pb_glossary]\u00a0produced by the [pb_glossary id=\"2911\"]lymphatic system[\/pb_glossary] are the main cells of the adaptive immune system. There are two major types of lymphocytes: [pb_glossary id=\"4892\"]T cells[\/pb_glossary]\u00a0and [pb_glossary id=\"5539\"]B cells[\/pb_glossary]. Both types must be activated by foreign [pb_glossary id=\"2704\"]antigens[\/pb_glossary]\u00a0to become functioning immune cells.<\/li>\r\n \t
- Most activated T cells become either [pb_glossary id=\"4893\"]killer T cells[\/pb_glossary]\u00a0or [pb_glossary id=\"5537\"]helper T cells[\/pb_glossary]. Killer T cells destroy cells that are infected with pathogens or are cancerous. Helper T cells manage immune responses by releasing [pb_glossary id=\"4865\"]cytokines[\/pb_glossary]\u00a0that control other types of leukocytes.<\/li>\r\n \t
- Activated B cells form [pb_glossary id=\"4902\"]plasma cells[\/pb_glossary]\u00a0that secrete [pb_glossary id=\"5659\"]antibodies[\/pb_glossary], which bind to specific antigens on pathogens or infected cells. The antibody-antigen complexes generally lead to the destruction of the cells, for example, by attracting phagocytes or triggering the [pb_glossary id=\"4868\"]complement system[\/pb_glossary].<\/li>\r\n \t
- After an adaptive immune response occurs, long-lasting [pb_glossary id=\"4904\"]memory B cells and memory T cells[\/pb_glossary] may remain to confer immunity to the specific pathogen that caused the adaptive immune response. These memory cells are ready to activate an immediate response if they are exposed to the same antigen again in the future.<\/li>\r\n \t
- Immunity may be active or passive. Active immunity occurs when the immune system has been presented with antigens that elicit an adaptive immune response. This may occur naturally as the result of an infection, or artificially as the result of immunization. Active immunity may last for years or even for life.<\/li>\r\n \t
- Passive immunity occurs without an adaptive immune response by the transfer of antibodies or activated T cells. This may occur naturally between a mother and her fetus or her nursing infant, or it may occur artificially by injection. Passive immunity lasts only as long as the antibodies or activated T cells remain alive in the body, generally just weeks or months.<\/li>\r\n \t
- Many pathogens have evolved mechanisms to evade the adaptive immune system. For example, human immunodeficiency virus ([pb_glossary id=\"4501\"]HIV[\/pb_glossary]) evades the adaptive immune system by frequently changing its antigens and by forming its outer envelope from the host\u2019s cell membrane.<\/li>\r\n<\/ul>\r\n<\/div>\r\n<\/div>\r\n
\r\n17.5 Review Questions<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- What is the adaptive immune system?<\/li>\r\n \t
- Define immunity.<\/li>\r\n \t
- [h5p id=\"639\"]<\/li>\r\n \t
- How are lymphocytes activated?<\/li>\r\n \t
- Identify two common types of T cells and their functions.<\/li>\r\n \t
- How do activated B cells help defend against pathogens?<\/li>\r\n \t
- How does passive immunity differ from active immunity?\u00a0How may passive immunity occur?<\/li>\r\n \t
- What are two ways that active immunity may come about?<\/li>\r\n \t
- What ways of evading the human adaptive immune system evolved in human immunodeficiency virus (HIV)?<\/li>\r\n \t
- Why do vaccinations expose\u00a0a person to a version of a pathogen?<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n
\r\n17.5 Explore More<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\nhttps:\/\/youtu.be\/rb7TVW77ZCs\r\nHow do vaccines work? - Kelwalin Dhanasarnsombut, TED-Ed, 2015.<\/p>\r\nhttps:\/\/youtu.be\/yqUFy-t4MlQ\r\n
How we conquered the deadly smallpox virus - Simona Zompi, TED-Ed, 2013.<\/p>\r\nhttps:\/\/youtu.be\/5THf6gTNqO8\r\n
Why Do We Need A New Flu Shot Every Year? Seeker, 2015.<\/p>\r\nhttps:\/\/youtu.be\/X-rC78MKZvw\r\n
An HIV Vaccine: Mapping Uncharted Territory, NIAID, 2016.<\/p>\r\n\r\n<\/div>\r\n<\/div>\r\n \r\n
Attributions<\/h2>\r\nFigure 17.5.1<\/strong>\r\n\r\nKiller_T_cells_surround_a_cancer_cell<\/a> by Alex Ritter, Jennifer Lippincott Schwartz and Gillian Griffiths at the National Institutes of Health\/ Visuals Online<\/a> on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).\r\n\r\nFigure 17.5.2<\/strong>\r\n\r\nT_cell_activation.svg<\/a> by Rehua<\/a> (derivative work) on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).\u00a0(Original image: T_cell_activation.png<\/a>:\u00a0 from The Immune System - NIH Publication No. 03\u20135423<\/a>)\r\n\r\nFigure 17.5.3<\/strong>\r\n\r\nCytotoxic T Cell function<\/a>\u00a0by 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 17.5.4<\/strong>\r\n\r\nB_cell_activation.svg<\/a> by Fred the Oyster<\/a> on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain). (Original from The Immune System - NIH Publication No. 03\u20135423<\/a>)\r\n\r\nFigure 17.5.5<\/strong>\r\n\r\nAntibody.svg<\/a> by Fvasconcellos<\/a>\u00a0 on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain). (Original black and white image from the National Human Genome Research Institute's Talking Genetics Glossary<\/a>)\r\n\r\nFigure 17.5.7<\/strong>\r\n\r\nimmunizations<\/a> by U.S. Air Force photo by Airman 1st Class Destinee Dougherty from Military Health System website, Health.mil, is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).\r\n\r\n \r\nReferences<\/h2>\r\n
HealthLinkBC. (2018). B.C. immunization schedules. \u00a0Gov.BC.CA. \u00a0https:\/\/www.healthlinkbc.ca\/tools-videos\/bc-immunization-schedules<\/p>\r\n
Mayo Clinic Staff. (n.d.). Measles [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/measles\/symptoms-causes\/syc-20374857<\/p>\r\n
Mayo Clinic Staff. (n.d.). Mumps [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/mumps\/symptoms-causes\/syc-20375361<\/p>\r\n
Mayo Clinic Staff. (n.d.). Polio [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/polio\/symptoms-causes\/syc-20376512<\/p>\r\n
Mayo Clinic Staff. (n.d.). Smallpox [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/smallpox\/symptoms-causes\/syc-20353027<\/p>\r\n
NIAID. (2016, August 11). An HIV vaccine: Mapping uncharted territory. YouTube. https:\/\/www.youtube.com\/watch?v=X-rC78MKZvw&feature=youtu.be<\/p>\r\n
OpenStax. (2016, March 23). Figure 4 Na\u00efve\u00a0CD4<\/mtext>+<\/sup>\u00a0T cells engage MHC II molecules on antigen-presenting cells (APCs) and become activated. Clones of the activated helper T cell, in turn, activate B cells and CD8+<\/sup> T cells, which become cytotoxic T cells. Cytotoxic T cells kill infected cells [digital image].\u00a0In OpenStax, Biology<\/em> (Section 42.2). OpenStax CNX. https:\/\/cnx.org\/contents\/GFy_h8cu@10.53:etZobsU-@6\/Adaptive-Immune-Response<\/p>\r\nSeeker. (2015, September 2). Why do we need a new flu shot every year? YouTube. https:\/\/www.youtube.com\/watch?v=5THf6gTNqO8<\/p>\r\n
TED-Ed. (2015, January 12). How do vaccines work? - Kelwalin Dhanasarnsombut. YouTube. https:\/\/www.youtube.com\/watch?v=rb7TVW77ZCs&feature=youtu.be<\/p>\r\n
TED-Ed. (2013, October 28). How we conquered the deadly smallpox virus - Simona Zompi. YouTube. https:\/\/www.youtube.com\/watch?v=yqUFy-t4MlQ&feature=youtu.be<\/p>","rendered":"
<\/p>\nFigure 17.5.1 Kill the cancer.<\/em><\/figcaption><\/figure>\n\nThe Kiss of Death<\/h1>\n<\/div>\n
The photomicrograph in Figure 17.5.1 shows a group of killer T cells (green and red) surrounding a cancer<\/a> cell (blue, centre). When a killer T cell makes contact with the cancer cell, it attaches to and spreads over the dangerous target. The killer T cell then uses special chemicals stored in vesicles<\/span><\/a><\/span>\u00a0(red) to deliver the killing blow. This event has thus been nicknamed \u201cthe kiss of death.\u201d After the target cell is killed, the killer T cells move on to find the next victim. Killer T cells like these are important players in the adaptive immune system.<\/span><\/p>\n\nWhat Is the Adaptive Immune System?<\/h1>\n<\/div>\n
The\u00a0adaptive immune system<\/a><\/strong>\u00a0is a subsystem of the overall immune system. It is composed of highly specialized cells and processes that eliminate specific pathogens<\/span><\/a><\/span>\u00a0and tumor cells. An adaptive immune response is set in motion by antigen<\/span>s<\/span><\/a>\u00a0that the immune system recognizes as foreign. Unlike an innate immune response, an adaptive immune response is highly specific to a particular pathogen (or its antigen). An important function of the adaptive immune system that is\u00a0<\/span>not<\/em>\u00a0shared by the innate immune system is the creation of immunological memory \u2014 or immunity<\/a> \u2014 which occurs after the initial response to a specific pathogen. It allows for a faster, stronger response on subsequent encounters with the same pathogen, usually before the pathogen can even cause symptoms of illness.<\/span><\/p>\nLymphocytes<\/span><\/a><\/span>\u00a0are the main cells of the adaptive immune system. They are leukocyte<\/span>s<\/span><\/a><\/span>\u00a0that arise and mature in organs of the lymphatic system, including the bone marrow<\/a> and thymus<\/a>. The human body normally has about 2 trillion lymphocytes, which constitute about 1\/3 of all leukocytes. Most of the lymphocytes are normally sequestered within tissue fluid or organs of the lymphatic system, including the tonsil<\/span>s<\/span><\/a><\/span>, spleen<\/a>, and lymph node<\/span>s<\/span><\/a><\/span>. Only about 2% of the lymphocytes are normally circulating in the blood. There are two main types of lymphocytes involved in adaptive immune responses, called T cells and B cells. T cells destroy infected cells or release chemicals that regulate immune responses. B cells secrete antibodies<\/a> that bind with antigen<\/span>s<\/span>[<\/span>\/pb_glossary]<\/span>\u00a0of [pb_glossary id=\"271\"]pathogen<\/span>s<\/span><\/a><\/span>\u00a0so they can be removed by other immune cells or processes.<\/span><\/p>\n\nPathways of the Adaptive Immune Response<\/h1>\n
There are some general similarities in the way in which the separate adaptive immune responses occur in T cell and B cell responses.\u00a0 In both pathways, a foreign antigen is recognized by the B or T cell.\u00a0 From there, cytokines produced by helper T-cells promote clonal expansion of lymphocytes.\u00a0 From this clonal expansion, two types of B or T cells are produced- cells that directly fight the pathogen invasion and cells that remain behind to provide long-term immunity<\/a>.\u00a0 Finally, once the pathogen invasion has been eradicated, the plasma cells and killer T cells go through apoptosis (programmed cell death).<\/p>\nT Cells<\/h1>\n<\/div>\n
There are multiple types of\u00a0T cells<\/a>,<\/strong>\u00a0or T lymphocytes. Major types are killer (or cytotoxic) T cells<\/a> and helper T cells<\/a>. Both types develop from immature T cells that become activated by exposure to an antigen<\/a>.<\/p>\nT Cell Activation (or Cell-Mediated Immunity)<\/h2>\n
T cells must be activated to become either killer T cells or helper T cells. This requires presentation of a foreign antigen by antigen-presenting cells<\/a>, as shown in Figure 17.5.2. Antigen-presenting cells may be dendritic cells<\/span><\/a><\/span>, macrophage<\/span>s<\/span><\/a><\/span>, or B cell<\/span>s<\/span><\/a><\/span>. Activation occurs when T cells are presented with a foreign antigen coupled with an MHC self antigen. Helper T cells are more easily activated than killer T cells. Activation of killer T cells is strongly regulated and may require additional stimulation from helper T cells.<\/span><\/p>\nFigure 17.5.2 Exposure to a foreign antigen on an antigen-presenting cell is necessary to activate T cells to become killer T cells or helper T cells.<\/em><\/figcaption><\/figure>\nKiller T Cells<\/h2>\n
Activated killer T cells induce the death of cells that bear a specific non-self<\/a> antigen because they are infected with pathogens or are cancerous. The antigen targets the cell for destruction by killer T cells, which travel through the bloodstream searching for target cells to kill. Killer T cells may use various mechanisms to kill target cells. One way is by releasing toxins in granules that enter and kill infected or cancerous cells (see Figure 17.5.3).<\/p>\nFigure 17.5.3 A killer T cell releases toxins that destroy an infected body cell and the viruses it contains.<\/em><\/figcaption><\/figure>\nHelper T Cells<\/h2>\n
Activated helper T cells<\/span><\/a><\/span>\u00a0do not kill infected or cancerous cells. Instead, their role is to \u201cmanage\u201d both innate<\/a> and adaptive immune<\/a> responses by directing other cells to perform these tasks. They control other cells by releasing cytokine<\/span>s<\/span><\/a><\/span>, which are proteins that can influence the activity of many cell types, including killer T cell<\/span>s<\/span><\/a><\/span>, B cells<\/a>, and macrophages<\/a>.\u00a0Some cytokines released by helper T cells assist with the activation of killer T cells.<\/span><\/p>\n\nB Cells<\/h1>\n<\/div>\n
B cell<\/strong>s<\/strong><\/a><\/strong>, or B lymphocytes, are the major cells involved in the creation of antibodies<\/a> that circulate in blood plasma<\/a> and lymph<\/a>. Antibodies are large, Y-shaped proteins used by the immune system to identify and neutralize foreign invaders. Besides producing antibodies, B cells may also function as antigen-presenting cell<\/span>s<\/span><\/a><\/span>, or secrete cytokines that help control other immune cells and responses.<\/span><\/p>\nB Cell Activation (or Antibody-Mediated Immunity)<\/h2>\n
Before B cells can actively function to defend the host, they must be activated. As shown in Figure 17.5.4, B cell activation begins when a B cell engulfs and digests an antigen. The antigen may be either free floating in the lymph, or it may be presented by an antigen-presenting cell, such as a dendritic cell or macrophage. In either case, the B cell then displays antigen fragments bound to its own MHC antigens. The MHC-antigen complex on the B cell attracts helper T cells. The helper T cells, in turn, secrete cytokines that help the B cell to multiply, and the daughter cells to mature into plasma cells.<\/p>\nFigure 17.5.4 B cells are activated and become antibody-producing plasma cells with the help of helper T cells.<\/em><\/figcaption><\/figure>\nPlasma Cells<\/h2>\nFigure 17.5.5 Each antibody fits its antigen like a lock fits a key.<\/em><\/figcaption><\/figure>\n
- \r\n \t
- The [pb_glossary id=\"4822\"]adaptive immune system[\/pb_glossary] is a subsystem of the overall immune system that recognizes and makes a tailored attack against specific [pb_glossary id=\"5799\"]pathogens[\/pb_glossary]\u00a0or tumor cells. It is a slower, but more effective response than the [pb_glossary id=\"4820\"]innate immune response[\/pb_glossary], and also leads to immunity to particular pathogens.<\/li>\r\n \t
- [pb_glossary id=\"4832\"]Lymphocytes[\/pb_glossary]\u00a0produced by the [pb_glossary id=\"2911\"]lymphatic system[\/pb_glossary] are the main cells of the adaptive immune system. There are two major types of lymphocytes: [pb_glossary id=\"4892\"]T cells[\/pb_glossary]\u00a0and [pb_glossary id=\"5539\"]B cells[\/pb_glossary]. Both types must be activated by foreign [pb_glossary id=\"2704\"]antigens[\/pb_glossary]\u00a0to become functioning immune cells.<\/li>\r\n \t
- Most activated T cells become either [pb_glossary id=\"4893\"]killer T cells[\/pb_glossary]\u00a0or [pb_glossary id=\"5537\"]helper T cells[\/pb_glossary]. Killer T cells destroy cells that are infected with pathogens or are cancerous. Helper T cells manage immune responses by releasing [pb_glossary id=\"4865\"]cytokines[\/pb_glossary]\u00a0that control other types of leukocytes.<\/li>\r\n \t
- Activated B cells form [pb_glossary id=\"4902\"]plasma cells[\/pb_glossary]\u00a0that secrete [pb_glossary id=\"5659\"]antibodies[\/pb_glossary], which bind to specific antigens on pathogens or infected cells. The antibody-antigen complexes generally lead to the destruction of the cells, for example, by attracting phagocytes or triggering the [pb_glossary id=\"4868\"]complement system[\/pb_glossary].<\/li>\r\n \t
- After an adaptive immune response occurs, long-lasting [pb_glossary id=\"4904\"]memory B cells and memory T cells[\/pb_glossary] may remain to confer immunity to the specific pathogen that caused the adaptive immune response. These memory cells are ready to activate an immediate response if they are exposed to the same antigen again in the future.<\/li>\r\n \t
- Immunity may be active or passive. Active immunity occurs when the immune system has been presented with antigens that elicit an adaptive immune response. This may occur naturally as the result of an infection, or artificially as the result of immunization. Active immunity may last for years or even for life.<\/li>\r\n \t
- Passive immunity occurs without an adaptive immune response by the transfer of antibodies or activated T cells. This may occur naturally between a mother and her fetus or her nursing infant, or it may occur artificially by injection. Passive immunity lasts only as long as the antibodies or activated T cells remain alive in the body, generally just weeks or months.<\/li>\r\n \t
- Many pathogens have evolved mechanisms to evade the adaptive immune system. For example, human immunodeficiency virus ([pb_glossary id=\"4501\"]HIV[\/pb_glossary]) evades the adaptive immune system by frequently changing its antigens and by forming its outer envelope from the host\u2019s cell membrane.<\/li>\r\n<\/ul>\r\n<\/div>\r\n<\/div>\r\n
\r\n 17.5 Review Questions<\/span><\/h1>\r\n<\/header>\r\n
\r\n- \r\n \t
- What is the adaptive immune system?<\/li>\r\n \t
- Define immunity.<\/li>\r\n \t
- [h5p id=\"639\"]<\/li>\r\n \t
- How are lymphocytes activated?<\/li>\r\n \t
- Identify two common types of T cells and their functions.<\/li>\r\n \t
- How do activated B cells help defend against pathogens?<\/li>\r\n \t
- How does passive immunity differ from active immunity?\u00a0How may passive immunity occur?<\/li>\r\n \t
- What are two ways that active immunity may come about?<\/li>\r\n \t
- What ways of evading the human adaptive immune system evolved in human immunodeficiency virus (HIV)?<\/li>\r\n \t
- Why do vaccinations expose\u00a0a person to a version of a pathogen?<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n
\r\n 17.5 Explore More<\/span><\/h1>\r\n<\/header>\r\n
\r\n\r\nhttps:\/\/youtu.be\/rb7TVW77ZCs\r\nHow do vaccines work? - Kelwalin Dhanasarnsombut, TED-Ed, 2015.<\/p>\r\nhttps:\/\/youtu.be\/yqUFy-t4MlQ\r\n
How we conquered the deadly smallpox virus - Simona Zompi, TED-Ed, 2013.<\/p>\r\nhttps:\/\/youtu.be\/5THf6gTNqO8\r\n
Why Do We Need A New Flu Shot Every Year? Seeker, 2015.<\/p>\r\nhttps:\/\/youtu.be\/X-rC78MKZvw\r\n
An HIV Vaccine: Mapping Uncharted Territory, NIAID, 2016.<\/p>\r\n\r\n<\/div>\r\n<\/div>\r\n \r\n
Attributions<\/h2>\r\nFigure 17.5.1<\/strong>\r\n\r\nKiller_T_cells_surround_a_cancer_cell<\/a> by Alex Ritter, Jennifer Lippincott Schwartz and Gillian Griffiths at the National Institutes of Health\/ Visuals Online<\/a> on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).\r\n\r\nFigure 17.5.2<\/strong>\r\n\r\nT_cell_activation.svg<\/a> by Rehua<\/a> (derivative work) on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).\u00a0(Original image: T_cell_activation.png<\/a>:\u00a0 from The Immune System - NIH Publication No. 03\u20135423<\/a>)\r\n\r\nFigure 17.5.3<\/strong>\r\n\r\nCytotoxic T Cell function<\/a>\u00a0by 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 17.5.4<\/strong>\r\n\r\nB_cell_activation.svg<\/a> by Fred the Oyster<\/a> on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain). (Original from The Immune System - NIH Publication No. 03\u20135423<\/a>)\r\n\r\nFigure 17.5.5<\/strong>\r\n\r\nAntibody.svg<\/a> by Fvasconcellos<\/a>\u00a0 on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain). (Original black and white image from the National Human Genome Research Institute's Talking Genetics Glossary<\/a>)\r\n\r\nFigure 17.5.7<\/strong>\r\n\r\nimmunizations<\/a> by U.S. Air Force photo by Airman 1st Class Destinee Dougherty from Military Health System website, Health.mil, is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).\r\n\r\n \r\n
References<\/h2>\r\n
HealthLinkBC. (2018). B.C. immunization schedules. \u00a0Gov.BC.CA. \u00a0https:\/\/www.healthlinkbc.ca\/tools-videos\/bc-immunization-schedules<\/p>\r\n
Mayo Clinic Staff. (n.d.). Measles [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/measles\/symptoms-causes\/syc-20374857<\/p>\r\n
Mayo Clinic Staff. (n.d.). Mumps [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/mumps\/symptoms-causes\/syc-20375361<\/p>\r\n
Mayo Clinic Staff. (n.d.). Polio [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/polio\/symptoms-causes\/syc-20376512<\/p>\r\n
Mayo Clinic Staff. (n.d.). Smallpox [online article]. MayoClinic.org. https:\/\/www.mayoclinic.org\/diseases-conditions\/smallpox\/symptoms-causes\/syc-20353027<\/p>\r\n
NIAID. (2016, August 11). An HIV vaccine: Mapping uncharted territory. YouTube. https:\/\/www.youtube.com\/watch?v=X-rC78MKZvw&feature=youtu.be<\/p>\r\n
OpenStax. (2016, March 23). Figure 4 Na\u00efve\u00a0
CD4<\/mtext>+<\/sup>\u00a0T cells engage MHC II molecules on antigen-presenting cells (APCs) and become activated. Clones of the activated helper T cell, in turn, activate B cells and CD8+<\/sup> T cells, which become cytotoxic T cells. Cytotoxic T cells kill infected cells [digital image].\u00a0In OpenStax, Biology<\/em> (Section 42.2). OpenStax CNX. https:\/\/cnx.org\/contents\/GFy_h8cu@10.53:etZobsU-@6\/Adaptive-Immune-Response<\/p>\r\n Seeker. (2015, September 2). Why do we need a new flu shot every year? YouTube. https:\/\/www.youtube.com\/watch?v=5THf6gTNqO8<\/p>\r\n
TED-Ed. (2015, January 12). How do vaccines work? - Kelwalin Dhanasarnsombut. YouTube. https:\/\/www.youtube.com\/watch?v=rb7TVW77ZCs&feature=youtu.be<\/p>\r\n
TED-Ed. (2013, October 28). How we conquered the deadly smallpox virus - Simona Zompi. YouTube. https:\/\/www.youtube.com\/watch?v=yqUFy-t4MlQ&feature=youtu.be<\/p>","rendered":"
<\/p>\n
Figure 17.5.1 Kill the cancer.<\/em><\/figcaption><\/figure>\n \nThe Kiss of Death<\/h1>\n<\/div>\n
The photomicrograph in Figure 17.5.1 shows a group of killer T cells (green and red) surrounding a cancer<\/a> cell (blue, centre). When a killer T cell makes contact with the cancer cell, it attaches to and spreads over the dangerous target. The killer T cell then uses special chemicals stored in vesicles<\/span><\/a><\/span>\u00a0(red) to deliver the killing blow. This event has thus been nicknamed \u201cthe kiss of death.\u201d After the target cell is killed, the killer T cells move on to find the next victim. Killer T cells like these are important players in the adaptive immune system.<\/span><\/p>\n
\nWhat Is the Adaptive Immune System?<\/h1>\n<\/div>\n
The\u00a0adaptive immune system<\/a><\/strong>\u00a0is a subsystem of the overall immune system. It is composed of highly specialized cells and processes that eliminate specific pathogens<\/span><\/a><\/span>\u00a0and tumor cells. An adaptive immune response is set in motion by antigen<\/span>s<\/span><\/a>\u00a0that the immune system recognizes as foreign. Unlike an innate immune response, an adaptive immune response is highly specific to a particular pathogen (or its antigen). An important function of the adaptive immune system that is\u00a0<\/span>not<\/em>\u00a0shared by the innate immune system is the creation of immunological memory \u2014 or immunity<\/a> \u2014 which occurs after the initial response to a specific pathogen. It allows for a faster, stronger response on subsequent encounters with the same pathogen, usually before the pathogen can even cause symptoms of illness.<\/span><\/p>\n
Lymphocytes<\/span><\/a><\/span>\u00a0are the main cells of the adaptive immune system. They are leukocyte<\/span>s<\/span><\/a><\/span>\u00a0that arise and mature in organs of the lymphatic system, including the bone marrow<\/a> and thymus<\/a>. The human body normally has about 2 trillion lymphocytes, which constitute about 1\/3 of all leukocytes. Most of the lymphocytes are normally sequestered within tissue fluid or organs of the lymphatic system, including the tonsil<\/span>s<\/span><\/a><\/span>, spleen<\/a>, and lymph node<\/span>s<\/span><\/a><\/span>. Only about 2% of the lymphocytes are normally circulating in the blood. There are two main types of lymphocytes involved in adaptive immune responses, called T cells and B cells. T cells destroy infected cells or release chemicals that regulate immune responses. B cells secrete antibodies<\/a> that bind with antigen<\/span>s<\/span>[<\/span>\/pb_glossary]<\/span>\u00a0of [pb_glossary id=\"271\"]pathogen<\/span>s<\/span><\/a><\/span>\u00a0so they can be removed by other immune cells or processes.<\/span><\/p>\n
\nPathways of the Adaptive Immune Response<\/h1>\n
There are some general similarities in the way in which the separate adaptive immune responses occur in T cell and B cell responses.\u00a0 In both pathways, a foreign antigen is recognized by the B or T cell.\u00a0 From there, cytokines produced by helper T-cells promote clonal expansion of lymphocytes.\u00a0 From this clonal expansion, two types of B or T cells are produced- cells that directly fight the pathogen invasion and cells that remain behind to provide long-term immunity<\/a>.\u00a0 Finally, once the pathogen invasion has been eradicated, the plasma cells and killer T cells go through apoptosis (programmed cell death).<\/p>\n
T Cells<\/h1>\n<\/div>\n
There are multiple types of\u00a0T cells<\/a>,<\/strong>\u00a0or T lymphocytes. Major types are killer (or cytotoxic) T cells<\/a> and helper T cells<\/a>. Both types develop from immature T cells that become activated by exposure to an antigen<\/a>.<\/p>\n
T Cell Activation (or Cell-Mediated Immunity)<\/h2>\n
T cells must be activated to become either killer T cells or helper T cells. This requires presentation of a foreign antigen by antigen-presenting cells<\/a>, as shown in Figure 17.5.2. Antigen-presenting cells may be dendritic cells<\/span><\/a><\/span>, macrophage<\/span>s<\/span><\/a><\/span>, or B cell<\/span>s<\/span><\/a><\/span>. Activation occurs when T cells are presented with a foreign antigen coupled with an MHC self antigen. Helper T cells are more easily activated than killer T cells. Activation of killer T cells is strongly regulated and may require additional stimulation from helper T cells.<\/span><\/p>\n
Figure 17.5.2 Exposure to a foreign antigen on an antigen-presenting cell is necessary to activate T cells to become killer T cells or helper T cells.<\/em><\/figcaption><\/figure>\n Killer T Cells<\/h2>\n
Activated killer T cells induce the death of cells that bear a specific non-self<\/a> antigen because they are infected with pathogens or are cancerous. The antigen targets the cell for destruction by killer T cells, which travel through the bloodstream searching for target cells to kill. Killer T cells may use various mechanisms to kill target cells. One way is by releasing toxins in granules that enter and kill infected or cancerous cells (see Figure 17.5.3).<\/p>\n
Figure 17.5.3 A killer T cell releases toxins that destroy an infected body cell and the viruses it contains.<\/em><\/figcaption><\/figure>\n Helper T Cells<\/h2>\n
Activated helper T cells<\/span><\/a><\/span>\u00a0do not kill infected or cancerous cells. Instead, their role is to \u201cmanage\u201d both innate<\/a> and adaptive immune<\/a> responses by directing other cells to perform these tasks. They control other cells by releasing cytokine<\/span>s<\/span><\/a><\/span>, which are proteins that can influence the activity of many cell types, including killer T cell<\/span>s<\/span><\/a><\/span>, B cells<\/a>, and macrophages<\/a>.\u00a0Some cytokines released by helper T cells assist with the activation of killer T cells.<\/span><\/p>\n
\nB Cells<\/h1>\n<\/div>\n
B cell<\/strong>s<\/strong><\/a><\/strong>, or B lymphocytes, are the major cells involved in the creation of antibodies<\/a> that circulate in blood plasma<\/a> and lymph<\/a>. Antibodies are large, Y-shaped proteins used by the immune system to identify and neutralize foreign invaders. Besides producing antibodies, B cells may also function as antigen-presenting cell<\/span>s<\/span><\/a><\/span>, or secrete cytokines that help control other immune cells and responses.<\/span><\/p>\n
B Cell Activation (or Antibody-Mediated Immunity)<\/h2>\n
Before B cells can actively function to defend the host, they must be activated. As shown in Figure 17.5.4, B cell activation begins when a B cell engulfs and digests an antigen. The antigen may be either free floating in the lymph, or it may be presented by an antigen-presenting cell, such as a dendritic cell or macrophage. In either case, the B cell then displays antigen fragments bound to its own MHC antigens. The MHC-antigen complex on the B cell attracts helper T cells. The helper T cells, in turn, secrete cytokines that help the B cell to multiply, and the daughter cells to mature into plasma cells.<\/p>\n
Figure 17.5.4 B cells are activated and become antibody-producing plasma cells with the help of helper T cells.<\/em><\/figcaption><\/figure>\n Plasma Cells<\/h2>\n
Figure 17.5.5 Each antibody fits its antigen like a lock fits a key.<\/em><\/figcaption><\/figure>\n
![Killer_T_cells_surround_a_cancer_cell<\/a> by Alex Ritter, Jennifer Lippincott Schwartz and Gillian Griffiths at the](\"https:\/\/commons.wikimedia.org\/wiki\/File:Killer_T_cells_surround_a_cancer_cell.png\"){kind=link}
![T_cell_activation.svg<\/a> by](\"https:\/\/commons.wikimedia.org\/wiki\/File:T_cell_activation.svg\"){kind=link}
![T_cell_activation.png<\/a>:\u00a0 from](\"https:\/\/commons.wikimedia.org\/wiki\/File:T_cell_activation.png\"){kind=link}
![Cytotoxic T Cell function<\/a>\u00a0by](\"https:\/\/commons.wikimedia.org\/wiki\/File:Figure_42_02_04.png\"){kind=link}
![B_cell_activation.svg<\/a> by](\"https:\/\/commons.wikimedia.org\/wiki\/File:B_cell_activation.svg\"){kind=link}
![Antibody.svg<\/a> by](\"https:\/\/commons.wikimedia.org\/wiki\/File:Antibody.svg\"){kind=link}