1964: James Hardy of the University of Mississippi Medical Center attempts the first cardiac xenotransplant, using a heart from a chimp. The primate heart—too small to support the patient’s circulation—functions for only two hours. Two other transplants, using pig hearts, fail due to hyperacute rejection.

Outside the clinic, researchers studying animals are learning more about the causes of hyperacute rejection. They discover that human blood contains natural antibodies that can recognize cells from pigs, dogs, or other animals. When these antibodies encounter foreign tissue, they trigger a chain reaction that destroys the graft within hours.