The gastrointestinal mucosal immune system.

The mucosal immune system, consisting of lymphoid tissues associated with the lacrimal, salivary, gastrointestinal, respiratory and urogenital tracts and lactating breasts, quantitatively contains the majority of the lymphoid tissue of the body. There are a number of important features of the gastrointestinal mucosal immune system: the mucosal immune system contains specialized structures, such as the Peyer's patches, where immune responses are thought to be initiated; there is a pattern of relatively specific recirculation of lymphoid cells to the mucosa, known as mucosal homing; subsets of lymphoid cells, particularly IgA B cells and memory T cells, predominate at mucosal surfaces; and the predominant mucosal immunoglobulin, secretory IgA, is particularly well adapted to host defense at mucosal surfaces. These elements of the gastrointestinal mucosal immune system function together to generate an immune response which on the one hand protects the host from harmful pathogens, but on the other hand is tolerant of the ubiquitous dietary antigens and normal microbial flora.