The blocking by autologous serum of maternal cell-mediated immune reactions to placental antigen.

The reactivity of splenic lymphocytes to a placental antigen was assessed in pregnant and control rats by means of a macrophage migration inhibition test. The pregnant but not the control animals, showed increasing cell-mediated immunity, which persisted until the second week after parturition. Migration inhibition could be abolished when sensitised lymphocytes were first pulsed with serum from the pregnant animal in the presence of antigen. This blocking effect was obtained only with autologous but not with normal or heterologous pregnant rat serum, and did not occur when antigen was omitted from the pulse. The results indicate that pregnant animals have cell-mediated immunity against a placental antigen, but the serum of these animals has a central enhancing effect which is antigen-dependent.