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Circulation of Lymphocytes
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The lymphoid system is the anatomic seat of the complex cellular and molecular interactions that make up the immune response (Chapter 4: The Immune Response). The cells of the lymphoid system must have access to all parts of the body, both for the detection of and the response to antigen (humoral and cellular immunity). As noted in Chapter 4: The Immune Response, lymphocytes bearing antigen receptors, both B and T cells, circulate extensively in the bloodstream. Immune cells that leave the blood in any tissue are collected in lymphatics and passed centrally, often through one or more series of lymph nodes, in which they may pause before reentering the blood. Lymphocytes also enter the lymph nodes directly from the blood through postcapillary venules (Figure 28-1).
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Within the lymphoid system there are several focal concentrations of immune cells (lymph nodes, spleen, tonsils, etc) wherein lymphocytes, macrophages, and other immune cells are arranged in a manner advantageous to the various interactions that make up the immune response. It is no accident that major accumulations of lymphoid tissue occur at portals of antigen entry: the tonsils (mouth and nose), the respiratory and gastrointestinal submucosa (for inhaled and ingested antigens), the lymph nodes (for lymph drainage of skin and organs), and the spleen (as the blood filter).
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The thymus and bone marrow are often termed central lymphoid tissues in that they are central to the prenatal development of the immune system but do not participate in the immune response in the adult. The remaining lymphoid organs are actively involved in the immune response and constitute the peripheral lymphoid tissue.
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The detailed microanatomic structure of the principal lymphoid organs has been described in Chapter 4: The Immune Response. The intimate juxtaposition of macrophages—which phagocytose and process antigen—and lymphocytes provides for an optimal immune response.
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It is important to recognize that this microanatomic arrangement is subject to rapid modification in the presence of an active immune response. The histologic appearances of lymphoid tissue are largely dependent upon the degree of antigenic stimulation.
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Note: Reactive follicles (foci of B cell proliferation) only appear following exposure to antigen. Likewise, immunoblasts are only present in the face of recent antigenic stimulation, while plasma cells indicate activity of some weeks' duration.
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The cellularity of the lymph node (ie, the number of immune cells present) and the overall size of the node also depend ...