Medicine: 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....

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