Host Defense Mechanisms in the Gastrointestinal Tract

Abstract

In this review of host defense mechanisms in the gastrointestinal tract, I have attempted to update the current conception of basic host defenses. Nonimmunologic defenses such as indigenous intestinal flora, peristalsis, secretions, gastric barrier, and antibacterial substances (lysozymes, bile salts, and natural antibodies) were discussed with respect to their contribution in preventing the penetration of microorganisms, enterotoxins, and other antigens across the mucosal surface. The local immunologic system, including SIgA, cell-mediated immunity and other immunoglobulins were discussed. It was emphasized that the immune response at epithelial surfaces was independent from the systemic immune response and related to the antigenic stimulus at the epithelium. Several conditions, including prematurity, malnutrition, and primary immunodeficiencies, were reviewed in order to illustrate that altered host defenses may predispose to pathologic uptake of antigens from the gut and the development of clinical disease. Finally, it was suggested that an approach to the management of these conditions requires the passive replacement of defense substances by an oral route in order to maintain protection within the gastrointestinal lumen. The use of human colostrum/milk as a substitute was suggested.