Non-determinant specificity of feedback immunosuppression by IgG antibodies injected after the antigen.

The determinant specificity of the IgG-mediated suppression of the humoral immune response in mice was studied. One hour before or 2.5, 6, 12, or 24 h after the injection of sheep erythrocytes (SRBC) or SRBC-TNP, CBA/Ca mice received SRBC-specific monoclonal IgG antibodies. The antibodies did not cross-react with TNP or goat erythrocytes, the latter an antigen which shows 30% cross-reactivity with SRBC. Five days later the determinant-specific plaque-forming cell response against SRBC and the non-determinant-specific response against goat erythrocytes and TNP were determined. Regardless of whether the antibodies were injected before or after the antigen, they suppressed not only the response to the antigenic determinant they bound to, but also the response to other epitopes on the same antigen. This shows that Fc parts of the IgG molecules play a crucial part in suppression of the in vivo antibody response even when, as in a natural situation, the antigen is presented to the immune system before the antibody.