Whose Discovery Were the Blood Groups?

Karl Landsteiner noticed that mixing blood from different people sometimes caused red blood cells to clump together. In 1901 he identified three blood groups, later known as A, B and O, and his colleagues found the fourth group, AB, in 1902. This explained why many early transfusions had failed or proved fatal. Landsteiner received the Nobel Prize in Physiology or Medicine in 1930. In 1940 he and Alexander Wiener discovered the Rh factor, another important blood group system.