THE INTESTINAL IMMUNE-SYSTEM AND ITS RELATION TO DISEASE

The essential protective structure against the heavy enteric antigenic burden, the gut mucosa, prevents penetration of noxious agents, but a llows a minimal exchange of large molecules and particles between the gut lumen and the 'milieu interieur' of the body. M cells in the folli cle-associated epithelium of the gut, ideal gateways for the presentat ion of enteric antigens to the cells of the gut-associated lymphoid ti ssue (GALT), are also weak links in the mucosal barrier, and may provi de access for various microorganisms. The afferent limb of the GALT co nsists of distinct aggregates of lymphoid cells located in Peyer's pat ches, the vermiform appendix and the solitary lymphatic follicles, and of the mesenteric lymph nodes. The efferent limb subsumes the diffuse ly scattered mucosal leukocytes, mainly lymphocytes and plasma cells. Intraepithelial and mucosal T lymphocytes are instrumental in launchin g local immune responses, producing lymphokines, and in the specific l ysis of virally infected cells. Antigenic stimulation of the GALT resu lts in local secretion of antibodies, or in suppression of systemic im munologic responses to ingested antigens ('oral tolerance'). Poorly co ntrolled mucosal immune responses result in organ-specific diseases. E xtranodal lymphomas that mimic structures of the GALT may arise on a b ackground of inflammatory or immunologic (autoimmune) disorders.