Immunoregulation of host-versus-graft responses in the uterus

The mammalian fetoplacental unit bears a variety of embryonal and paternally derived histocompatibiliy antigens against which the immune system of the mother can and does react(1-3). Nevertheless, there has been little evidence to suggest that these pregnancy-induced immune responses actually harm the fetus, and some investigators have suggested that immunization against paternal antigens may actually be helpful in ensuringfetal survival(2-4). In this review David Clark and his colleagues briefly outline data indicating that the so-called fetal allograft' is not always as successful as we have thought, and argue that a unique type of suppressor cell which accumulates at the implantation site in the decidua early in pregnancy may play an important role in preventing maternal refection of the embryo. They also summarize current information on the role of suppressor T cells and of fetal trophoblast cells in protection of the conceptus.