CURRENT CONCEPTS IN MUCOSAL IMMUNITY IV - HOW EPITHELIAL TRANSPORT OFIGA ANTIBODIES RELATES TO HOST-DEFENSE

The humoral arm of the mucosal immune system is principally composed o f locally synthesized polymeric IgA, whose Fc portion is adapted for b inding to the polymeric immunoglobulin receptor that is expressed on t he basolateral surface of mucosal epithelial cells, including enterocy tes. This receptor mediates the endocytosis and transcytosis of polyme ric IgA, which allows IgA to function in host defense at three anatomi c levels in relation to mucosal epithelium: IgA antibodies in the lami na propria can bind antigens and excrete them through the epithelium i nto the lumen; antiviral IgA antibodies in transit through epithelial cells can inhibit virus production by an intracellular action; and IgA antibodies secreted into the lumen can prevent antigens and microbes from adhering to and penetrating the epithelium. The ways in which IgA antibodies function in mucous membranes provide challenging investiga tive opportunities for cell physiologists and cell biologists.