The effect of plasma from multigravida women on the expression of cellular hypersensitivity to paternal antigens was investigated. It was found that autologous, but not homologous, maternal plasma specifically blocked MIF production by maternal lymphocytes using their husbands' cells as an antigenic stimulus. The blocking activity could be absorbed out by repeated incubation with paternal cells. Sephadex G-200 fractionation of maternal plasma showed the blocking activity to be in the IgG

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... n. The presence of specific blocking factor(s) in maternal plasma which prevent reactivity to paternal antigens may explain in part the success of the fetus as an allograft.