Sir Peter Medawar, Nobel Laueate and the "father of transplantation biology."

1953: Peter Medawar of the University of London finds that animals exposed to foreign tissues while they’re young—still embryos—don’t reject them. MacFarlane Burnet of Melbourne University postulates that this is because the immune cells that patrol the bloodstream in search of foreign invaders somehow learn very early on to accept whatever tissues are there as part of the body, and only attack things that show up later. In 1960, Medawar and Burnet win a Nobel Prize for their discoveries.