Men who receive transplanted kidneys from women are more likely to lose or reject them than they would organs from other men. Physician Martin Zeier of the University of Heidelberg in Germany uncovered this pattern by scouring a giant database called the Collaborative Transplant Study, which collects records of organ donation from patients in 49 countries. After analyzing records of nearly 167,000 kidney, heart, and liver transplants, he found that men had a 22 percent higher risk of rejecting or losing a donated kidney from a woman than one from a man within one year of transplant. Men were also 13 percent more likely to lose a woman's heart. No parallel effect was seen in women who received kidney or heart transplants from men.
This pattern hints at a fundamental, still-unknown difference between the immune systems of men and women. The presence or absence of estrogen, or a gender-related difference in the distribution of chemical receptors on the surface of cells, might influence immune reactions that can lead to organ acceptance or rejection. Nonetheless, Zeier would never advise a man to turn down a female kidney, especially since rejection can often be reversed with the proper drug regimen. "The organ shortage is so heavy in Western countries that it would screw up our allocation system. And there are other factors—the age and health of the donor, for example—that are much more important than the gender difference," he says.