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The immune system provides various defenses or immunity against infectious agents ranging from viruses to multicellular parasites. Histologically, this system consists of a large, diverse population of leukocytes located within every tissue of the body and lymphoid organs interconnected only by the blood and lymphatic circulation. Immunity obviously has tremendous medical importance, one part of which focuses on autoimmune diseases in which immune cells function abnormally and attack molecular components of the body’s own organs.
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Immunologists recognize two partially overlapping lines of defense against invaders and/or other abnormal, potentially harmful cells: innate immunity and adaptive immunity. Innate immunity encompasses a wide variety of nonspecific effector mechanisms which evolved much earlier than adaptive defenses. Important cells mediating innate immunity include most of the granulocytes and certain other leukocytes described in Chapters 12 and 13. Conversely, adaptive immunity develops to target specific microbial invaders, depends on lymphocytes and antigen-presenting cells (APCs) discussed in this chapter, and produces memory cells that permit a very rapid response if that specific microbe appears again.
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The lymphocytes and APCs for adaptive immunity become localized throughout the body in the blood, lymph, epithelia and connective tissues. Lymphocytes develop initially in primary lymphoid organs (the thymus and bone marrow), but most lymphocyte activation and proliferation occur in secondary lymphoid organs (the lymph nodes, the spleen, and diffuse lymphoid tissue found in the mucosa of the digestive system, including the tonsils, Peyer patches, and appendix). The immune cells located diffusely in the digestive, respiratory, or urogenital mucosae collectively comprise the mucosa-associated lymphoid tissue (MALT). Proliferating B lymphocytes in the secondary structures of MALT arrange themselves in small spherical lymphoid nodules. The wide distribution of immune system cells and the constant traffic of lymphocytes through the blood, lymph, connective tissues, and secondary lymphoid structures provide the body with an elaborate and efficient system of surveillance and defense (Figure 14–1).
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INNATE & ADAPTIVE IMMUNITY
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The system of defenses termed innate immunity involves immediate, nonspecific actions, including physical barriers such as the skin and mucous membranes of the gastrointestinal, respiratory, and urogenital tracts that prevent easy microbial penetration of the host body. Bacteria, fungi, and parasites that manage to breach these barriers quickly get removed by neutrophils and other leukocytes in the epithelia or adjacent connective tissue. Toll-like receptors (TLRs) on leukocytes allow the recognition ...