Immunoglobulin Classes Carrying Homocytotropic and Passive Hemagglutinating Antibody Activities in the Marsupial Setonix brachyurus (the Quokka)

On the basis of their physicochemical and antigenic properties, and by analogy with guinea pig immunoglobulins, two antibody classes in the serum of the marsupial Setonix brachyurus have been classified as IgG1 and IgG2 isotypes. The analogy between these marsupial immunoglobulins and those of eutherian (placental) species was extended by an investigation of their biological activities. The electrophoretically slow antibody (IgG2) fixed hemolytic complement, precipitated soluble antigen and predominated early in the response mounted when quokkas were immunized with antigen emulsified in Freund’s complete adjuvant (FCA). The more anodic antibody (IgG1) in this marsupial did not fix complement, was inefficient in precipitating antigen and was the predominant antibody synthesized by quokkas immunized with antigen adsorbed to alumina gel. The IgG1 isotype in this marsupial appears to possess both passive hemagglutinating (HA) and homocytotropic antibody (HCA) activities. However, the HCA and IgG1 HA activities did not develop in parallel during the course of the immune response, thus suggesting that only a functional subpopulation of the IgG1 antibodies possess homocytotropic activity.