ABSTRACT
The mechanisms responsible for the maternal immune tolerance of the allogeneic fetus are not fully known. Several mechanisms are thought to play a role in fetal- maternal tolerance; these mechanisms may involve pregnancy-associated cytokines; hormones; environmental factors, such as semen exposure, infection, stress and the placenta. Levels of Treg cells are elevated systemically during normal human and mouse pregnancy, and levels of Treg cells have already increased by the late follicular phase of the menstrual cycle. The decidua is populated with different subsets of leukocytes during pregnancy, and the levels of these subsets vary during pregnancy. The memorizing function of the adaptive immune system is also seen in pregnancy. It is, for example, known that the duration of the sexual relationship is inversely related to immune-associated complications of pregnancy such as preeclampsia and fetal growth restriction. The effects of immune imbalances have been reviewed extensively.
