Primary and secondary IgG are equally efficient immunosuppressors in relation to antigen binding capacity

Secondary, hyperimmune IgG antibodies can suppress the humoral immune response against the relevant antigen. Whether IgG antibodies derived from a primary antigen response also have this capacity is not clear, although the role of primary IgG is of great interest in a physiological situation. In this study we compared the in vivo immunosuppressive potential of primary and secondary IgG anti-SRBC (sheep erythrocytes) on the primary anti-SRBC PFC response in CBA/Ca mice. Both primary and secondary IgG antibodies are potent immunosuppressors causing more than 99% specific suppression. Preparations of primary and secondary IgG antibodies which, measured by an ELISA method, were shown to bind to SRBC to the same extent, also had very similar immunosuppressive potency. This emphasizes the strong correlation between the antigen binding and the immunosuppressive capacities of IgG antibodies.