T helper 1 and T helper 2 cells regulate cellular function. Specific Th1 cytokines promote Th1 proliferation and differentiation, but these cytokines act to inhibit the differentiation and effector function of Th2 cells and cytokines. For example, IFN-g selectively inhibits Th2 proliferation, whereas IL-10 (a Th2 cytokine) inhibits Th1 cytokine production. Likewise, throughout the entire body, Th1 and Th2 cells interact to suppress each other. Pregnancy is a special condition in which there is systemic “immune tolerance” partly mediated through a shifting of the Th1/Th2 balance. During pregnancy, the Th1 system is suppressed while the Th2 system is enhanced. This facilitates fetal survival because Th1 cytokines can be harmful to the placenta, whereas IL-10 is protective against fetal loss.
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