Abstract
Immunosuppressive effect of pregnant serum obtained in the second and third trimesters was demonstrated in the PHA-induced homologous lymphocyte stimulation system, where 3H-thymidine in-corporation into DNA was measured. Such an inhibitory factor was not present in the first trimester serum. The immunosuppressive principle detected in the second trimester serum was characterized by a potent inhibitory activity even at an increased concentration of PHA up to 75μg, whereas the inhibitory activity found in the third trimester serum at optimum PHA concentration (15μg) was not detectable when the PHA concentration was raised to 75μg. Thus, the second and third trimester sera must contain different immuno-suppressive principles.
