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How was the AB blood group discovered after Karl Landsteiner's original work?

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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 AB blood group was not discovered by Karl Landsteiner in his initial 1901 study; rather, it was identified by a different research team one year later, in 1902.12

In his original work, Landsteiner identified three blood groups—which he labeled A, B, and C (later renamed O)—based on the immunological reactions that caused red blood cells to clump, or agglutinate, when mixed. The subsequent identification of the fourth group, AB, revealed that individuals in this group possess both A and B antigens on their red blood cells, causing them to react with blood from either the A or B groups. This discovery completed the ABO blood group system, which remains fundamental to the safety of modern blood transfusions.12