Pregnancy: Why mother's immune system does not reject developing fetus as foreign tissue

Researchers at NYU School of Medicine have made an important discovery that partially answers the long-standing question of why a mother's immune system does not reject a developing fetus as foreign tissue.The researchers discovered that embryo implantation sets off a process that ultimately turns off a key pathway required for the immune system to attack foreign bodies. As a result, immune cells are never recruited to the site of implantation and therefore cannot harm the developing fetus.