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.
Ideally the donor’s blood types should always be an exact match to the recipient’s blood type. Universal donors should only be used in the case of medical emergency where there is an unavailability or shortage of the patient’s blood type or when a blood transfusion needs to be performed immediately, not allowing the time to cross type the blood compatibility. Because many instances where a blood transfusion is needed is in an emergency situation, type O negative blood is often in short supply which further increases the need for type O negative blood donors to generously donate their blood to hospitals and blood banks.
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.
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