Immunology of pregnancy. Implications for the mother

Pregnancy is a unique event in which a genetically and immunologically foreign fetus usually survives to full term without apparent rejection by the mother's immune system. Over the past decade, more information has been gathered to provide insight into the complex immunological mechanisms that allow the fetus to grow and survive in most cases. Whereas the maternal-fetal interface was once felt to be an immunologically privileged site with complete separation between the fetus and the mother, it is now known that recognition of the foreign fetus does occur. However, despite this immunological recognition, several mechanisms have been discovered which may explain why the mother does not reject the foreign fetus. These mechanisms include fetal factors such as trophoblast cell properties and altered MHC Class I expression as well as local maternal factors such as specialized uterine natural killer cells and a shifting of the T-helper cell cytokine profile from a type 1 to a type II array. Other novel immunomodulators are found to be expressed in the local uterine environment to aid in fetal survival. Furthermore, the persistence of fetal cells in the maternal circulation long after pregnancy is over (termed chronic microchimerism) and may have implications for autoimmune diseases. This review presents investigations and developments relevant to an understanding as to why the fetus is not rejected by the maternal immune system.