During gestation, the immune system is challenged with establishing tolerance to the partial allograft represented by the paternal contribution to the fetal genome. That fact that cell-mediated immunity is decreased while T-cell-dependent immunoglobin production remains intact or is increased is generally accepted. This latter phenomenon can be placed in teleological perspective in terms of providing passive immunity to the fetus. T-cell lineages that diverge based on function and cytokine production have been identified. Pregnancy is associated with a relative increase in Th2-associated immunity, characterized by increased production of the cytokines IL-4 and IL-10. These changes occur concomitantly with increased immunoglobulin production and decreased Th1 immunity. IL-2 and interferon secretion characterize Th1 immunity, facilitating allograft rejection. These changes in Th1 and Th2 have implications for the clinical course of a several autoimmune diseases that may complicate pregnancy and the postpartum period.