INTERACTION OF ANTIGENS AND ANTIBODIES AT MUCOSAL SURFACES

Infections often involve the mucosal surfaces of the body, which form a boundary with the outside world. This review focuses on immunoglobul in A (IgA) antibodies because IgA is the principal mucosal antibody cl ass. IgA is synthesized by local plasma cells and has a specific polym eric immunoglobulin receptor-mediated transport mechanism for entry in to the secretions. By serving as an external barrier capable of inhibi ting attachment of microbes to the luminal surface of the mucosal epit helial lining, IgA antibodies form the first line of immune defense. I n addition to this traditional mode of extracellular antibody function , recent evidence suggests that IgA antibodies can also function in a nontraditional fashion by neutralizing viruses intracellularly, if a v irus is infecting an epithelial cell through which specific IgA antibo dy is passing on its way to the secretions, IgA antibodies are also en visaged as providing an internal mucosal barrier beneath the mucosal l ining. Antigens intercepted by IgA antibodies here can potentially be ferried through the epithelium and thereby excreted. In addition to th e polymeric immunoglobulin receptor on mucosal epithelial cells, IgA a ntibodies can bind to receptors on a variety of leukocytes and have be en shown, in some experimental systems, to be capable of activating th e alternative complement pathway, making IgA antibodies potential part icipants in inflammatory reactions. Although the relationship of IgA a ntibodies to inflammation is not entirely clear, the bias presented is that IgA is basically noninflammatory, perhaps even anti-inflammatory . According to this view, the major role of the Pc portion of IgA anti bodies is to transport IgA across mucosal epithelial cells and not, as in the case of the other classes of antibody, to activate secondary p henomena of the kind that contribute to inflammation. Because of IgA's key role as an initial barrier to infection, much current research in mucosal immunology is directed toward developing new vectors and adju vants that can provide improved approaches to mucosal vaccines. Finall y, because of advances in monoclonal antibody technology, topical appl ication of antibodies to mucosal surfaces has significant potential fo r preventing and treating infections.