Complement-Fixing Activity of Antibodies Associated with Different Classes of Immunoglobulins

Abstract

Abstract Antibodies belonging to different immunoglobulin classes and subclasses exhibit characteristic immunochemical properties. Among these properties, which serve to differentiate between these immunoglobulin classes, is the ability to fix C′. Using a C′1a-fixing system comprised of human anti-γA isoantibodies and type A red cells, both γG and γM antibodies have been shown to fix C′1a, whereas γA antibody in both monomeric and polymeric forms lacks this ability. It was also found that monospecific aggregates of γG and γM fixed C′, whereas aggregated γA failed to do so. Among the subclasses of γG globulins, evidence has been obtained that γG1, γG2, and γG3 globulins fix C′ and C′1a on aggregation but γG4 globulin does not. Although antibodies of both the γG and γM type have been shown to fix C′, whether the same mechanisms of fixation exist for both classes has not yet been resolved.