THE BIOLOGICAL ROLE AND CLINICAL IMPLICATIONS OF IGA

The total body pool of human IgA (3,600-9,100 mg) is larger than that for all other isotypes combined. Most IgA resides in the small and large intestines in the form of polymeric or secretory IgA (sIgA). Serum IgA is primarily monomeric and comprises the second highest concentration of any immunoglobulin in the circulation. IgA is composed of two subclasses: IgA1 and IgA2. Secretory IgA consists of approximately equal amounts of IgA1 and IgA2. Serum IgA is predominantly IgA1 (90%). Secretory IgA functions to neutralize toxins, enzymes, and viruses; agglutinate pathogens; inhibit penetration of foreign antigen into the mucosa and attachment; activate the alternate complement pathway; and possibly opsonize pathogens. Secretory IgA is the first line of defense against most infectious agents, and the integrity of the sIgA system is important to the health of the individual. The biological role and clinical implications of IgA are discussed.