Blood Types Discovered
Karl Landsteiner discovered the ABO blood group system, making safe transfusions possible
June 14, 1901
When Blood Transfusions Were Deadly
Blood transfusions had been attempted since the 1600s, but the results were unpredictable and often fatal. Patients receiving blood from another person would sometimes recover, but others would develop severe reactions and die. For more than two centuries, no one understood why some transfusions worked and others killed. The answer came in 1901 from an Austrian physician named Karl Landsteiner, who was working at the Vienna Pathological Institute. Landsteiner noticed that when he mixed blood samples from different people, the red blood cells sometimes clumped together. This clumping, he realized, was the key to understanding transfusion reactions.
Landsteiner's System
Through careful experiments, Landsteiner identified three blood types in 1901, which he labeled A, B, and C. His colleagues later renamed C to O. A fourth type, AB, was discovered by his students in 1902, completing the ABO system. The type of blood a person has depends on which antigens, or marker proteins, are present on the surface of their red blood cells. If a person receives blood with antigens their immune system does not recognize, it attacks the foreign cells, causing a dangerous reaction. Matching blood types before a transfusion prevents this. Landsteiner also went on to discover the Rh factor in 1937, giving us the positive and negative designations still used today.
The Impact on Medicine and Surgery
Karl Landsteiner received the Nobel Prize in Physiology or Medicine in 1930 for his discovery. Blood typing made safe transfusions possible, which transformed surgery and emergency medicine. Soldiers wounded in World War I and World War II benefited enormously from blood matching and blood banking technology. Today, blood banks store millions of units of blood that are typed and screened before transfusion. Understanding blood types also became essential for organ transplantation and treating conditions like hemolytic disease of the newborn, where a mother's immune system attacks a baby's blood cells. Landsteiner's 1901 discovery remains one of the most consequential in the history of medicine.