The fetus represents a foreign entity to the maternal immune system, yet this “natural”
allograft is not normally rejected, thereby indicating the existence of active tolerance
mechanisms that prevent rejection. Different mechanisms have been proposed to account
for tolerance at the fetomaternal interface (FMI). Molecules involved in apoptosis of maternal
leukocytes and negative signals delivered to activated T cells by regulatory costimulatory
pathways, which can also act as natural inhibitors for effector T-cell expansion, have been …