ABO blood group system

The ABO blood group system classifies human blood by the presence of the A and B antigens on red blood cells; the Austrian physician Karl Landsteiner discovered it at the University of Vienna, dividing blood into three groups in 1901 after finding that some people's serum clumped others' red cells. His students found a fourth group, later named AB, the following year. It is the most important blood group system for transfusion, since a mismatch can cause a potentially fatal reaction. Landsteiner won the Nobel Prize for it in 1930.