Who discovered the human ABO blood group system?

Karl Landsteiner discovered the human ABO blood group system.

In 1900 and 1901, Austrian immunologist Karl Landsteiner showed that human blood could react differently when samples from different people were mixed. He identified the patterns that led to the original A, B, and C group classification; group C was later renamed group O. His work explained why some blood transfusions caused dangerous clumping while others were compatible.

The ABO system is based mainly on whether red blood cells carry A antigen, B antigen, both antigens, or neither. The corresponding blood plasma contains antibodies that can react against unfamiliar A or B antigens. This is why ABO matching remains central to safe transfusion practice.

Landsteiner received the 1930 Nobel Prize in Physiology or Medicine for his discoveries of human blood groups. The Rh blood group system, discovered later, is separate from ABO, although both are routinely considered during transfusions.