Immunity during pregnancy: lymphocyte subpopulations and mitogen responsiveness.

Many hypotheses have been proposed to explain the alteration of maternal immune status that allows the fetus to escape rejection. Published data using monoclonal antibodies have stated that there are small variable reductions in circulating T-lymphocytes and little or no change in helper-to-suppressor ratios. Specific decreased levels of helper T-cells have been claimed by other workers. Our laboratory has previously reported alterations in tritiated thymidine uptake (3H-TdR) and HLA antibodies during pregnancy. The present study evaluates total T-cells, lymphocyte T-cell subsets, helper-to-suppressor ratios of T-cells, B-cells, and lymphocyte blast transformation (LBT) throughout pregnancy. These lymphocyte measurements were compared to hormonal changes occurring during pregnancy to determine whether or not hormonal levels have a significant correlation on the maternal immune response during gestation. Data from 100 women revealed no significant alteration of total T-cells or T-cell subsets during pregnancy or after parturition, as measured by monoclonal antibodies. Helper-to-suppressor ratios were within normal limits. B-cells showed a significant decrease (P less than 0.001) during the postpartum period. There was decreased lymphocyte responsiveness to mitogenic stimulation by phytohemagglutinin-P (PHA-P), concanavalin-A (CON-A), and pokeweed mitogen (PWM) in the first, second and third trimesters (P less than 0.01). The mechanisms of fetal protection from maternal immune recognition remain obscure.