Bachrach Studios · Public domain

Landsteiner identified the ABO blood groups in 1901, the discovery that made safe blood transfusion possible by explaining why mixing certain people's blood caused fatal clumping. In 1937, with Alexander Wiener, he went on to identify the Rhesus factor, closing another gap that had been costing lives in transfusion and childbirth; he had also helped discover the polio virus decades earlier. He won the 1930 Nobel Prize in Physiology or Medicine and is still described as the father of transfusion medicine.