Clues from the Placenta

Short of working with identical twins, surgeons have yet to discover any sure system for preventing the human body from rejecting transplanted organs. Yet nature turns a similar trick with ease--more than 100 million times a year. Every time a woman reaches the third month of pregnancy, Dr. James H. Nelson Jr. of New York's Downstate Medical Center told the American College of Obstetricians and Gynecologists, she is tolerating tissue that is not entirely her own.

The placenta, the spongelike pad of tissue on the wall of the womb to which the three-month fetus is attached by the umbilical cord, explained Dr. Nelson, "is, in essence, a homograft." This is because the placenta is derived partly from the embryo and therefore partly from the father. Yet the mother does not reject it through her immune mechanisms, as she would any tissue of comparable size received as a transplant. Why not?

The fact that women are somewhat more tolerant of skin grafts during pregnancy than at any other time gave Dr. Nelson and his associate, Dr. J. Edward Hall, a clue. Pregnancy, the doctors reasoned, must depress at least some of the body's elaborate immune mechanisms. From women who had to undergo surgery in mid or late pregnancy, they removed snippets of lymph-node tissue. A pathologist who studied these tissues under the microscope noted that the specimens contained very few cells of the type that triggers the immune reaction. Studies on women after childbirth showed that it takes at least four weeks after delivery for these cells to reappear in normal numbers.

The pregnant woman, the doctors concluded, is in a "hypoimmune state." The likeliest explanation is that she is secreting unusually large amounts of the gonadotropin group of sex hormones. If it can be proved that this is indeed the reason, then nature will have supplied, in the gonadotropins, a natural and essentially harmless substance for cutting down the severity of the rejection mechanism. And surgeons may have a practical medicine for making transplantation easier.

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