Macfarlane Burnet proposes in 1949, with Frank Fenner, that self/non-self immunological distinction develops during fetal life, theoretically predicting that exposure to foreign cells before immune-system maturation would produce tolerance rather than rejection — a reasoned hypothesis with no experimental demonstration of his own. Peter Medawar, Rupert Billingham, and Leslie Brent (UCL) confirm it experimentally in 1953, grafting tissue between genetically distinct mouse fetuses. The 1960 Nobel was jointly awarded to Burnet and Medawar, combining theoretical prediction and experimental confirmation — though, per Nobel archives, they were never jointly nominated by anyone.