ABSTRACT
Successful mammalian pregnancy is characterized by the introduction of new patterns of gene expression in the female reproductive organs. In the case of pregnancy in outbred species where the fetus is internalized and maternal and fetal tissues are in direct contact, one major change is in the local immune environment, which must be modified so as to prevent maternal rejection of the semiallogeneic fetus(es) while concurrently permitting immune system components to provide host defense and participate in the reproductive process. Decades of experimentation have shown that pregnancy is well protected by overlapping and redundant protective mechanisms supplied by both the mother and the fetus. An emerging concept is that the altered local environment, where adaptive immunity is dampened and the innate immune system prevails, is programmed mainly by the placenta and its products. A second major emerging concept is that placental products have dual or multiple roles, participating in placental organogenesis and function as well as providing immune privilege. These and other recent insights on placental functions may lead to earlier diagnosis of pregnancy problems and to new approaches for treating women with less than optimal fertility.
