Studies on the maternal immune response to placental antigens: absence of a blocking factor from the blood of abortion-prone women.

The leucocyte migration-inhibition assay was employed to assess the reactivity of cells from postpartum women to placental antigen preparations. Leucocytes from normal primigravidae responded to their own placental antigens and displayed a limited degree of cross-reactivity with allogeneic preparations. Autologous plasma suppressed this antigen recognition and the factor responsible was shown to be an IgG antibody. Leucocytes from five women with spontaneous abortions also responded to autologous placental preparations but no immunosuppressive factor could be detected in their blood. It is suggested that target antigens for maternal lymphocytes are present on the placenta, a structure critical to the continuance of pregnancy, and that the presence of the blocking antibody is necessary for fetal well-being.