Mucosal immunity and gastrointestinal antigen processing

The intestine is the largest lymphoid organ in the body by virtue of lympho cyte numbers and quantity of immunoglobulin produced. This is largely relat ed to the enormous antigen load to which these cells are exposed on a daily basis. However, despite this, the mucosa-associated lymphoid tissue appear s to be regulated by unique mechanisms, and this is reflected in specific p henomena (oral tolerance, controlled or physiologic inflammation) as well a s unusual lymphoid populations (intraepithelial lymphocytes) that respond t o alternative pathways of activation. This, coupled with the existence of n ovel antigen-presenting cells (intestinal epithelial cells) sets the scene for distinct immune responses. It is these distinct regulatory factors that support immunosuppression or tolerance rather than active immunity at a si te juxtaposed to the external environment. This review defines these novel interactions and suggests how alteration in normal function may result in a llergic or inflammatory responses. A clearer understanding of mucosal immun oregulation may lead to new therapeutic approaches for these diseases. (C) 2000 Lippincott Williams & Wilkins, Inc.