Pregnancy in the human presents a paradox for the mother's immune system as the mechanisms which are essential to protect her from infection have the potential to destroy her antigenically foreign fetus. The maternal decidua is comprised principally of immune cells and it is into this tissue that the fetal trophoblast must invade to establish the placenta. The major factor which appears to prevent the rejection of the trophoblast is its expression of HLA-G, a nonpolymorphic transplantation antigen. Both local and systemic nonspecific suppressor mechanisms have been described which may down-regulate maternal immune responses without significantly impairing the ability to fight infections, but there is little evidence to suggest that specific blocking factors (antibodies and suppressor cells) play an essential role. The placental barrier restricts the traffic of cytotoxic cells to the fetus, and cytotoxic antibodies are removed by the placenta before they reach the fetal circulation. Thus a combination of immune adaptations ensures the success of the pregnancy.