Universal blood donor type: Is there such a thing?

People who need to get donor red blood cells right away, called an emergency transfusion, most often get blood group type O blood. This type of blood has the lowest risk of causing serious reactions when there isn't time to do routine testing. 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 typed as:

• Type A.
• Type B.
• Type AB.
• Type O.

Blood also is grouped by Rh factor. If your blood has the Rh factor, you're Rh positive. If your blood lacks the Rh factor, you're Rh negative.

It's best if people receive donated blood that works with their type and Rh factor. Even if the types match, small samples from the person getting the blood and the donated blood are mixed to make sure that they're OK together. This process is called crossmatching.

Anyone might get type O red blood cells in an emergency. This is most likely to happen for people who might die without the blood or if there is a shortage of the matching blood type.