All Forms of Human IgA Antibodies Bound to Antigen Interfere with Complement (C3) Fixation Induced by IgG or by Antigen Alone

Polyclonal human secretory IgAI and lgA2 antibodies to a bacterial protein antigen Streptococcus mutans Agl/II, and polyclonal human serum IgA1 and IgA2 antibodies to staphylococcal a‐toxin, were found to interfere with antigen‐mediated C3b fixation. In fluid phase, immune complexes of antigen and IgA failed to fix C3b, whereas antigen‐IgG complexes did fix C3b. Partial removal of glycan chains with Streptococcus mitis SK96 glycosidases diminished the capacity of IgA antibodies to interfere with antigen‐mediated C3b fixation by the alternative complement pathway. The authors conclude that native serum or secretory IgA antibodies suppress C3b fixation, and that the glycan chains play a significant role in maintaining this property.