Fixation of C′ and C′la by Rabbit γG- and γM-Antibodies with Particulate and Soluble Antigens

Abstract

Summary C′ and C′1a fixation by γG- and γM-antibodies was studied with A substance and type A human erythrocytes as antigens and specifically purified rabbit antibodies. In soluble antigen systems, the maximal C′ fixation by γG-antibody was obtained at the equivalence antigen to antibody ratio, whereas the maximal fixation by γM-antibody was obtained in excess antibody. At the optimal antigen to antibody ratio for C′ fixation, the C′-fixing activity of the γG- and γM-antibodies was comparable on a molar basis. Essentially no C′ fixation was observed by soluble antigen-γM-antibody complexes formed in excess antigen. Comparisons of C′1a fixation by γG- and γM-antibodies in both soluble and cell antigen systems indicated that γM-antibody is more effective than γG-antibody on the cell surface, while the effect was reversed when the soluble antigen was used. It was confirmed that a single γM-antibody molecule on the cell surface established one C′1a-fixing site. The γM-antibody complexes with insoluble A substance had much higher C′ and C′1a-fixing activities than those combined with soluble A substance. The different efficiency of C′1a fixation by γM-antibody, depending on the nature of antigen, was ascribed to the number and distribution of antigenic determinants on the surface of antigen involved. The results strongly suggested that a combination of γM-antibody molecules with antigen through multiple combining sites is essential for the induction of C′-fixing activity by γM-antibody.