The ability of passively administered antibody to suppress the immune response against homologous antigenic determinants while concomitantly enhancing the response against other unrelated determinants of the same antigen molecule has been established in two distinct antigen-antibody systems: (a) guinea pig γ2-immunoglobulin + passive anti-F(ab')2 antibody, where suppression of anti-F(ab')2 antibody synthesis is accompanied by enhancement of the anti-Fc response; and (b) human secretory IgA + passive anti-serum IgA antibody, where suppression of antibody production against the α and L chains accompanies augmentation of the response to the secretory component.

The mechanisms of the suppressive and enhancing effects are probably unrelated for the following reasons: (a) Enhancement of the response to certain determinants may be obtained without discernible suppression of the response to the homologous determinants; and (b) the F(ab')2 fragments of passive antibody can mediate immune suppression but were not observed to enhance the response against the unrelated determinants of the same antigen molecule. Also, the timing for achieving maximum suppression or enhancement of antibody formation is not the same; enhancement was obtained only at a later time.

Both the enhancement and suppressive effects were obtained with the purified γG fraction of antisera. This finding rules out an exclusive role of γM antibody in the enhancement phenomenon.

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