Karl Landsteiner (1868-1943), Austrian American immunologist discovered human blood existed in different groups, which he first identified as A, B, and O blood types in 1901. He completed his blood research work at Rockefeller Institute in the 1920s. Until his breakthroughs, many blood transfusion recipients died from immune reactions caused by incompatible blood types. Landsteiner received the 1930 Nobel Prize in Medicine.