Karl Landsteiner was an Austrian-born American pathologist awarded the 1930 Nobel Prize in Physiology or Medicine for discovering the major blood groups and developing the ABO system of blood typing. He showed that certain blood groups are incompatible with others, and that clots form if such groups are mixed. His findings explained why some blood transfusions in the past had been beneficial, whereas others had been fatal. In 1940 he helped to identify the rhesus factor.