The four primary human blood types--A, B, AB, and O--arise from differences in molecules called antigens that dot the surface of red blood cells and provoke responses from the body's immune system. Individuals with the type A antigens make antibodies to type B antigen, which causes their bodies to attack and reject transfused type B blood as foreign. The same grim scenario unfolds for type B individuals who get type A blood. Blood from type O individuals lacks both antigens and can be safely transfused into people with all four blood types. Type O individuals can receive only type O blood, however, which means that during blood shortages, they may want for blood that’s been donated to patients with other blood types.
