Peter Medawar—eventually Sir Peter Medawar—was a zoologist from Oxford who had been called upon during World War II to help a plastic surgeon treat burn victims. Sir Medawar tried in vain to graft skin from donors onto the charred victims of shelling and bombing. The results were cruel. Even the failed grafts looked for several days or weeks like they were succeeding. That’s because skin doesn’t have as many blood vessels and as much blood flow as, say, kidneys or other internal organs. It would take time for the immune system cells, carried in the blood, to assess and reject the skin.
