Which blood-group system did Karl Landsteiner discover in 1900?

Karl Landsteiner discovered the ABO blood group system in 1900.

Working in Vienna, Landsteiner found that human blood samples could react differently when red blood cells and serum from different people were mixed. These observations led to the recognition of the A, B, and C groups; group C was later renamed O, and AB was identified soon afterward by his colleagues.

The ABO system is based mainly on whether red blood cells carry A antigen, B antigen, both antigens, or neither. Matching this system is essential for safe red-cell transfusion because incompatible antibodies can cause dangerous agglutination and destruction of transfused cells.

Landsteiner’s discovery is sometimes mixed up with the Rh system, which was identified decades later. The ABO system is not the only blood-group classification, but it remains the most important for routine transfusion compatibility. Landsteiner received the 1930 Nobel Prize in Physiology or Medicine for his work on blood groups.