REGULATION OF THE IMMUNE RESPONSE BY IgM ANTIBODY: A PARADOXICAL SUPPRESSION OF THE IN VITRO PRIMARY IMMUNE RESPONSE TO SHEEP ERYTHROCYTES BY PASSIVE IgM.

Summary Using a purified preparation of mouse anti‐sheep erythrocyte IgM, a marked suppression of the in vitro primary response of mouse spleen cells to sheep red cells was demonstrated. This effect was shown to be specific in that a primary response to the dinitrophenyl determinant was unaffected. The response to donkey erythrocytes, however, was suppressed, suggesting cross‐reactivity of the anti‐sheep red cell antibody, although this was not demonstrable by serologic techniques. The same preparations of IgM were observed in in vivo experiments to have the marked enhancing effect on the immune response to sheep red cells reported by otber workers, suggesting that the net enhancement seen in vivo must be due to factors not operating in vitro . A study was also made of early appearing mouse IgG antibody against sheep red cells, which was demonstrated to be suppressive both in vitro and in vivo . A common mechanism of antibody‐induced specific immune suppression as detected at the cellular level, involving blocking of a proportion of antigenic determinants, is proposed for both IgG and IgM classes of antibody. The paradoxical enhancing activity of IgM on the in vivo sheep red cell response is proposed to reflect merely an increase in the dose of antigen thereby caused to localize in the spleen. This increase in antigen dose must counteract the suppressive action of IgM.