{"id":56,"date":"2021-01-25T19:26:08","date_gmt":"2021-01-25T19:26:08","guid":{"rendered":"https:\/\/courses.lumenlearning.com\/suny-hvcc-healthpsychology\/?post_type=chapter&p=56"},"modified":"2021-01-25T19:28:45","modified_gmt":"2021-01-25T19:28:45","slug":"introduction-to-the-immune-system","status":"publish","type":"chapter","link":"https:\/\/courses.lumenlearning.com\/suny-hvcc-healthpsychology\/chapter\/introduction-to-the-immune-system\/","title":{"raw":"Introduction to the Immune System","rendered":"Introduction to the Immune System"},"content":{"raw":"
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What is the Immune System?<\/h2>\r\n

The\u00a0immune system<\/strong>\u00a0is a host defense system. It comprises many biological structures \u2014ranging from individual white blood cells to entire organs \u2014 as well as many complex biological processes. The function of the immune system is to protect the host from pathogens and other causes of disease such as tumor cells. To function properly, the immune system must be able to detect a wide variety of pathogens. It also must be able to distinguish the cells of pathogens from the host\u2019s own cells and also to distinguish cancerous or damaged host cells from healthy cells. In humans and most other vertebrates, the immune system consists of layered defenses that have increased specificity for particular pathogens or tumor cells. The layered defenses of the human immune system are usually classified into two subsystems called the innate immune system and the adaptive immune system.<\/p>\r\n\r\n

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Innate Immune System<\/h3>\r\n
\"Phagocytosis
Figure\u00a020.2.<\/span>2<\/span><\/span><\/span><\/span><\/span>20.2.2<\/span><\/span>: A leukocyte called a macrophage phagocytizes bacteria in the series of steps shown here. First, the bacterium is engulfed. Then it is digested. And, finally, the components of the bacterium are expelled.<\/figcaption><\/figure>\r\n

The innate immune system provides a very quick but non-specific response to pathogens. It responds the same way regardless of the type of pathogen that is attacking the host. It includes barriers \u2014 such as the skin and mucous membranes \u2014 that normally keep pathogens out of the body. It also includes general responses to pathogens that manage to breach these barriers, including chemicals and cells that attack the pathogens inside the human host. For example, certain leukocytes (white blood cells) engulf and destroy pathogens they encounter in the process called phagocytosis, which is illustrated in Figure\u00a020.2.<\/span>2<\/span><\/span><\/span><\/span><\/span>20.2.2<\/span><\/span>. Exposure to pathogens leads to an immediate maximal response from the innate immune system.<\/p>\r\n\r\n<\/div>\r\n

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Adaptive Immune System<\/h3>\r\n

The adaptive immune system is activated if pathogens successfully enter the body and manage to evade the general defenses of the innate immune system. An adaptive response is specific to the particular type of pathogen that has invaded the body or to cancerous cells. It takes longer to launch a specific attack, but once it is underway, its specificity makes it very effective. An adaptive response also usually leads to immunity. This is a state of resistance to a specific pathogen due to the ability of the adaptive immune system to \u201cremember\u201d the pathogen and immediately mount a strong attack tailored to that particular pathogen if it invades again in the future.<\/p>\r\n\r\n<\/div>\r\n<\/div>\r\n

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Self vs. Non-Self<\/h2>\r\n

Both innate and adaptive immune responses depend on the ability of the immune system to distinguish between self and non-self molecules. Self molecules are those components of an organism\u2019s body that can be distinguished from foreign substances by the immune system. Virtually all body cells have surface proteins that are part of a complex called the\u00a0major histocompatibility complex (MHC)<\/strong>. These proteins are one way the immune system recognizes body cells as self. Non-self proteins, in contrast, are recognized as foreign because they are different from self-proteins.<\/p>\r\n\r\n

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Antigens and Antibodies<\/h3>\r\n

Many non-self molecules comprise a class of compounds called antigens. Antigens, which are usually proteins, bind to specific receptors on immune system cells and elicit an adaptive immune response. Some adaptive immune system cells (B cells) respond to foreign antigens by producing antibodies. An antibody is a molecule that precisely matches and binds to a specific antigen. This may target the antigen (and the pathogen displaying it) for destruction by other immune cells.<\/p>\r\n

Antigens on the surface of pathogens are how the adaptive immune system recognizes specific pathogens. Antigen specificity allows for the generation of responses tailored to the specific pathogen. It is also how the adaptive immune system \u201dremembers\u201d the same pathogen in the future.<\/p>\r\n\r\n<\/div>\r\n

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Immune Surveillance<\/h3>\r\n

Another important role of the immune system is to identify and eliminate tumor cells. This is called\u00a0immune surveillance.<\/strong>\u00a0The transformed cells of tumors express antigens that are not found on normal body cells. The main response of the immune system to tumor cells is to destroy them. This is carried out primarily by aptly named killer T cells of the adaptive immune system.<\/p>\r\n\r\n<\/div>\r\n<\/div>\r\n

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Lymphatic System<\/h2>\r\n

The lymphatic system is a human organ system that is a vital part of the adaptive immune system. It is also part of the cardiovascular system and plays a major role in the digestive system (see the concept\u00a0Lymphatic System<\/a>). The major structures of the lymphatic system are shown in Figure\u00a020.2.<\/span>3<\/span><\/span><\/span><\/span><\/span>20.2.3<\/span><\/span>.<\/p>\r\n

The lymphatic system consists of several lymphatic organs and a body-wide network of lymphatic vessels that transport the fluid called lymph. Lymph is essentially blood plasma that has leaked from capillaries into tissue spaces. It includes many leukocytes, especially lymphocytes, which are the major cells of the lymphatic system. Like other leukocytes, lymphocytes defend the body. There are several different types of lymphocytes that fight pathogens or cancer cells as part of the adaptive immune system.<\/p>\r\n

Major lymphatic organs include the thymus and bone marrow. Their function is to form and\/or mature lymphocytes. Other lymphatic organs include the spleen, tonsils, and lymph nodes, which are small clumps of lymphoid tissue clustered along lymphatic vessels. These other lymphatic organs harbor mature lymphocytes and filter lymph. They are sites where pathogens collect and adaptive immune responses generally begin.<\/p>\r\n\r\n

\"Lymphatic
Figure\u00a020.2.<\/span>3<\/span><\/span><\/span><\/span><\/span>20.2.3<\/span><\/span>: The lymphatic system includes the organs and vessels illustrated here.<\/figcaption><\/figure>\r\n<\/div>\r\n
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Neuroimmune System vs. Peripheral Immune System<\/h2>\r\n

The brain and spinal cord are normally protected from pathogens in the blood by the selectively permeable blood-brain and blood-spinal cord barriers. These barriers are part of the\u00a0neuroimmune system.<\/strong>\u00a0The neuroimmune system has traditionally been considered to be distinct from the rest of the immune system, which is called the\u00a0peripheral immune system<\/strong>, although that view may be changing (see the following Feature: Human Biology in the News). Unlike the peripheral system, in which leukocytes are the main cells, the main cells of the neuroimmune system are thought to be nervous system cells called glial cells. These cells are able to recognize and respond to pathogens, debris, and other potential dangers. Types of glial cells involved in neuroimmune responses include microglial cells and astrocytes.<\/p>\r\n\r\n