The major mucosal immunoglobulin, IgA, consist of two basic units joined by a J chain. The addition of a secretion molecule prevents its digestion by enzymes present in mucosal and intestinal secretions. Thus, IgA2 is the major IgA molecule in secretions and is quite effective in neutralizing antigens that enter via these mucosal routes. IgA1, the main IgA molecule in serum,is, however,susceptible to inactivations by serum proteases and is thus less active for defense. Its functions is unclear at present.