ALL FORMS OF HUMAN-IGA ANTIBODIES BOUND TO ANTIGEN INTERFERE WITH COMPLEMENT (C3) FIXATION INDUCED BY IGG OR BY ANTIGEN ALONE

Polyclonal human secretory IgA1 and IgA2 antibodies to a bacterial protein antigen Streptococcus mutans AgI/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.