A child developing in the womb carries foreign antigens from its father as well as immunologically compatible self antigens from its mother, and might be expected to trigger a graft rejection. But the uterus is an "immunologically privileged" site where immune responses are subdued. One source of protection appears to be a substance produced by the child, perhaps in response to antibodies from the mother. The substance promotes the development of special white blood cells in the uterus, and these cells release a factor that blocks the actions of IL-2. Another substance, produced by the uterus, helps disguise antigens on the fetal surface of the placenta, shielding them from the mother's immune defenses.