Identification of an immunosuppressive factor in pregnancy serum

Complement-inactivated allogeneic serum was found to contain a factor capable of suppressing BCG-induced inhibition of leukocyte migration in vitro. Although serum from late pregnancy was the most potent in this context, the ability to antagonize the leukocyte response appears to be an intrinsic property of human serum. The inhibitory factor was purified by salf fractionation and gel chromatography and the effect found to be associated with alpha-globulins of high lipid content which are probably alpha-lipoproteins. Estrogens, progesterone, or corticosteroids, associated with the lipoprotein, did not appear to be the cause of the observed suppression. The factor appears to block the immune response at the level of the lymphocyte and its nonspecific immunosuppressive properties could contribute to acceptance of the fetus by the mother.