Mucosal defense mechanism and secretory IgA system.

globulin in external body fluids of mammals. The mucosal surface is exposed to a myriad of antigenic substances, such as microbes and food ; the sIgA provides an immunological barrier to foreign matter by preventing both absorption of these antigens by the mucosal epithelium and penetration into the body, and interferes in the attachment of microbes and their toxins to the epithelium (29, 30) . On the other hand, intestine and respiratory tracts are richly populated lymphoid tissues, and contact between antigens on the mucosal surface and the mucosal lymphoid tissues initiates a diverse series of immunologic events. These include the production of immunoglobulins of IgA class that are secreted into extracellular body fluids and, in striking contrast, induce hyporesponsiveness of certain nonmucosal immunologic reactions (25, 27), whereas the humoral immune responses in the nonmucosal sites are largely of the IgG class. The cell-mediated immune mechanism is also distinctive in mucosal and nonmucosal lymphoid tissues. It is of interest to notice that the study of germfree mice and rats, which lack indigenous microbial flora, reveals the scarcity of secondary lymphoid follicles of the lymphoid tissue and of plasma cells and lymphocytes in the lamina propria of villi (6, 21). It has recently been recognized that the immune system at the mucosal sites plays an important role in the host-environment interaction (2, 21). In this review I summarized the recent information about immune responses in the intestinal mucosa, with emphasis on the role of microbial environments.