For emergency transfusions, blood group type O negative blood is the variety of blood that has the lowest risk of causing serious reactions for most people who receive it. Because of this, it's sometimes called the universal blood donor type.

Blood group types are based on proteins called antigens that are present on red blood cells. There are major antigens and minor antigens coating the red blood cells. Based on the major antigens, blood groups may be classified as one of these four types:

• Type A
• Type B
• Type AB
• Type O

Blood is also classified by rhesus (Rh) factor. If your blood has the Rh factor, you're Rh positive. If your blood lacks the Rh factor, 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, 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.