The three immunoglobulins, i.e., γG, γA and γM, were isolated from the sera of type O humans, who were immunized with A substance. γA-globulin was fractionated into polymer and monomer types. The effect of reduction with mercaptoethanol and alkylation by iodoacetate on antigen-binding activity of anti-A antibodies were studied. The results indicated that the stability of the activity to the treatment was different depending on the immunoglobulins with which the anti-A antibody was associated; the susceptibility of both polymer and monomer types γA-antibodies was intermediate between that of γG- and γM-antibodies. Upon reduction and alkylation, the polymer type γA-antibody was dissociated to subunits containing antigen-combining site(s). It was also found that a part of the antigen-combining activity remained after reduction and alkylation of γM-antibody. Evidence was presented that the activity was associated with subunits of the antibody. Reduction of γM-antibody followed by dialysis against saline resulted in reassociation of subunits to form fast-sedimenting molecules having agglutinating activity.

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