Karl Landsteiner won the 1930 Nobel Prize in Physiology or Medicine for discovering human blood groups.
In 1900, Landsteiner showed that human blood could be divided into distinct groups based on reactions between red blood cells and blood serum. His work established the ABO blood group system, identifying groups A, B, and C, with group C later called O. A fourth group, AB, was identified in 1901 by his colleagues Alfred von Decastello and Adriano Sturli.
The discovery transformed blood transfusion practice. Before blood groups were understood, transfusions could cause severe or fatal reactions because antibodies in a recipient's plasma might attack a donor's red blood cells. Matching compatible blood became possible after Landsteiner's findings.
Landsteiner's Nobel recognition concerned the discovery of blood groups, not the later Rh blood group system. He and Alexander S. Wiener identified the Rh factor in 1940, but that work was not the achievement cited for his 1930 Nobel Prize.