1. Karl Landsteiner discovered blood types in 1901 by observing that blood from people of different types would clot when mixed together. He later classified them as A, B and O.
2. ABO isn’t the only blood grouping system, however. There are currently 33 systems recognized by the International Society of Blood Transfusion, with monikers like Lutheran, Duffy, Hh/Bombay and Ok.
3. Blood type refers to different molecules on the surface of red blood cells. A mismatch of these molecules between donor and recipient can trigger a fatal immune reaction after a blood transfusion, as the recipient’s body attacks the outsider blood.
