Inhibition of PHA-induced lymphocyte stimulation by factors in maternal serum during late pregnancy.

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 incorporation 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 75 micrograms, whereas the inhibitory activity found in the third trimester serum at optimum PHA concentration (15 micrograms) was not detectable when the PHA concentration was raised to 75 micrograms. Thus, the second and third trimester sera must contain different immunosuppressive principles.