A component in extracts of Group A streptococci suppresses antibody formation in mice against heterologous erythrocyte and protein antigens. Large doses are not toxic and repeated injection does not change its effectiveness. It is most effective when injected 1 or 2 days before antigen and it is not suppressive when given after antigen. The active factor occurs as a large polydisperse complex and activity can be increased 10- to 25-fold by filtration through Sepharose 2B. Both direct (γM) and indirect (γG) antibody-forming cells are suppressed in primary and secondary responses. Injection before a primary response does not reduce memory cell development. It increases rather than depresses the "background" antibody-forming cells to sheep erythrocytes, and is equally effective if injected intraperitoneally or intravenously. Ribonuclease increases activity while deoxyribonuclease has no effect. Proteases destroy immunosuppressive action.

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