Whose discovery identified blood groups for safe transfusions?

Austrian scientist Karl Landsteiner discovered that human blood is not all the same. In 1901 he identified the A, B, and O blood groups, showing why some transfusions succeed and others fail. Later, the AB group was also recognized, completing the ABO system. This discovery dramatically reduced the danger of blood transfusions and became essential to modern medicine. Landsteiner received the Nobel Prize in Physiology or Medicine in 1930.