At one time, type O negative blood was considered the universal blood donor type. This implied that anyone — regardless of blood type — could receive type O negative blood without risking a transfusion reaction. However, we now know even type O negative blood may have antibodies that cause serious reactions during a transfusion.
There are four types of blood. They are classified as:
- Type A
- Type B
- Type AB
- Type O
Blood is also classified by rhesus (Rh) factor, which refers to a specific red blood cell antigen in the blood. If your blood has the antigen, you're Rh positive. If your blood lacks the antigen, you're Rh negative.
Ideally, blood transfusions are done with donated blood that's an exact match for type and Rh factor. Even then, small samples of the recipient's and donor's blood are mixed to check compatibility in a process known as crossmatching. In an emergency, however, type O negative red blood cells may be given to anyone — especially if the situation is life-threatening or the matching blood type is in short supply.