Immunologic structure and function of the gastrointestinal tract.

Host defenses within the gastrointestinal tract exclude bacteria and other intraluminal substances, which if released into the systemic circulation, would be toxic to the body. This is accomplished via complex interactions between these external pathogens and local immune responses and nonimmunologic processes. In addition to the mechanical and chemical barriers of the nonimmunologic defense system within the gastrointestinal tract, there is an effective immunologic barrier composed of aggregated and nonaggregated lymphoid cells. Gut-associated lymphoid tissue protects the intestinal mucosa from invading pathogens by intricate pathways of antigen processing. Gut-associated lymphoid tissue also transfers protection to other secretory sites within the body through the common mucosal immune system. The integrity of both the immunologic and nonimmunologic barriers may be affected by any number of pathologic insults as well as by nutritional influences. This article reviews the structural and functional characteristics of this complex and critically important host defense system. Specific nutrient requirements of the immunologic processes are discussed.