IgA (Fc = α) is found in exocrine gland secretions(milk, saliva, tears), where it protects mucous membranes(e.g., in the respiratory tract). It is present in the serum as amonomer at a concentration of 1 to 2 mg/mL, but humans secreteabout 1 g of the dimer per day in the mucosal fluids.Secretory IgA consists of two IgG-like units linked togetherat the Fc regions by a peptide known as the secretory fragmentand a J fragment. The secretory fragment is actually partof the membrane receptor for IgA. The IgA molecule on themucosal side of the membrane binds antigen, then binds to thereceptor. By a process of transcytosis, the IgA–antigen complexis moved from the mucosa to the bloodstream, whereIgG and IgM can react. Because it is distributed on the mucosa,IgA has an anatomically specific distribution, unlike theother antibodies. IgA is the mediator of oral polio vaccination(the mucosal reaction gives way to systemic protection).
