Diffuse lymphatic tissue and lymphatic nodules guard the body against pathogenic substances and are the site of the initial immune response
The alimentary canal, respiratory passages, and genitourinary tract are guarded by accumulations of lymphatic tissue that are not enclosed by a capsule. Lymphocytes and other free cells of this tissue are found in the lamina propria (subepithelial tissue) of these tracts. This form of lymphatic tissue is called diffuse lymphatic tissue or mucus-associated lymphatic tissue (MALT) because of its association with mucous membranes (Fig. 13.13). These cells are strategically located to intercept antigens and initiate an immune response. Following contact with antigen, they travel to regional lymph nodes, where they undergo proliferation and differentiation. Progeny of these cells then return to the lamina propria as effector B and T lymphocytes.
The importance of diffuse lymphatic tissue in protecting the body from antigens is indicated by
• The regular presence of large numbers of plasma cells, especially in the lamina propria of the gastrointestinal tract, a morphologic indication of local antibody secretion
• The presence of large numbers of eosinophils, also frequently observed in the lamina propria of the intestinal and respiratory tracts, an indication of chronic inflammation and hypersensitivity reactions
Lymphatic nodules are discrete concentrations of lymphocytes contained in a meshwork of reticular cells
In addition to diffuse lymphatic tissue, localized concentrations of lymphocytes are commonly found in the walls of the alimentary canal, respiratory passages, and genitourinary tract. These concentrations, called lymphatic nodules or lymphatic follicles, are sharply defined but not encapsulated (Fig. 13.14). A lymphatic nodule consisting chiefly of small lymphocytes is called a primary nodule. However, most nodules are secondary nodules and have distinctive features that include
A germinal center located in the central region of the nodule (Fig. 13.15), which in histologic sections appears lightly stained. The germinal center develops when a lymphocyte that has recognized an antigen returns to a primary nodule and undergoes proliferation. The lighter staining is due to the large lymphocytes (lymphoblasts and plasmablasts) that it contains. These lymphocytes have large amounts of dispersed euchromatin in their nuclei rather than the dense heterochromatin of small lymphocytes. The germinal center is a morphologic indication of lymphatic tissue response to antigen. The presence of a germinal center represents a cascade of events that includes proliferation of lymphocytes, differentiation of plasma cells, and antibody production. Mitotic figures are frequently observed in the germinal center, reflecting the proliferation of new lymphocytes at this site. The number of macrophages in the germinal center often increases dramatically following a period of intense response to an antigen.
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