ONE OF the greatest mysteries yet to be unraveled in biology is the mechanism by which the
fetus, which is essentially an allograft, is able to survive the immunologic defenses of the
mother without being rejected. That a successful pregnancy so often is the outcome seems
even more remarkable, since it defies the basic tenets of the field of transplantation
immunology. Metchnikoff first noted in 1882 that starfish had an intrinsic ability to
differentiate self from nonself and, in their identification of that which was foreign, mounted a …