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The immune system provides defense 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 begin to function abnormally and attack molecular components of the body’s own organs.

Immunologists recognize two partially overlapping lines of defense against invaders and/or other abnormal, potentially harmful cells: innate immunity and adaptive immunity. The first of these is nonspecific, involves a wide variety of effector mechanisms, and is evolutionarily older than the second type. Among the cells mediating innate immunity are most of the granulocytes and other leukocytes described in Chapters 12 and 13. Conversely, adaptive immunity aims at specific microbial invaders, is mediated by lymphocytes and antigen-presenting cells (APCs) discussed in this chapter, and produces memory cells that permit a similar, very rapid response if that specific microbe appears again.

The lymphocytes and APCs for adaptive immunity are distributed throughout the body in the blood, lymph, and epithelial and connective tissues. Lymphocytes are formed 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 comprise what is collectively known as mucosa-associated lymphoid tissue (MALT). Proliferating B lymphocytes in the secondary structures of MALT are arranged 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).


The lymphoid organs and main paths of lymphatic vessels.

The lymphatic system is composed of lymphatic vessels that transport interstitial fluid (as lymph) back to the blood circulation, and the lymphoid organs that house lymphocytes and other cells of the body’s immune defense system. Primary lymphoid organs are the bone marrow and thymus, where B and T lymphocytes are formed, respectively. The secondary lymphoid organs include the lymph nodes, mucosa-associated lymphoid tissue (MALT), and spleen.


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 infections or penetration of the host body. Bacteria, fungi, and parasites that manage to penetrate these barriers are quickly removed by neutrophils and other leukocytes in the ...

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