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Who discovered the A, B, and O blood groups in 1901?

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Close-up illustration of multiple human red blood cells with their characteristic disc shape, floating against a red background. — blood group The major component of human blood is the red blood cell.

The A, B, and O blood groups were discovered in 1901 by the Austrian immunologist and pathologist Karl Landsteiner.123

Landsteiner identified these groups by observing that the mixing of blood from different individuals could result in the clumping, or agglutination, of red blood cells. He determined that this reaction was caused by an immunological response involving antigens on the surface of the red blood cells and antibodies in the serum. Initially, he labeled these antigens A, B, and C (the latter was later renamed O). A fourth blood type, AB, was identified by another research team the following year. This discovery was fundamental to the development of safe blood transfusion practices and earned Landsteiner the 1930 Nobel Prize for Physiology or Medicine.13