A universal donor is someone who can donate blood to anyone else, with a few rare exceptions. The blood type of universal donors is O-. Conversely, a universal recipient can safely take blood from anyone, again with a few rare exceptions. People with AB+ blood are considered universal recipients.

Universal donors are determined on the basis of the ABO blood typing system. Under this system, people can be divided into four blood types: A, B, AB, and O. Blood type is determined by the antigens present in the blood. In the case of people with A blood, A antigens are present. B blood types have B antigens, AB blood types have both, and O types have no antigens. Some people refer to the O group as the “null” or 0 group, referencing the fact that no antigens are present.

If someone with an A blood type is given blood from a B donor, the recipient's blood will react with the antigens in the B blood, rejecting the transfusion and triggering a transfusion reaction. O blood, however, can be safely given to someone with an A blood type, because there are no antigens in the donor blood to react with the recipient's body.

However, things get a bit more complicated than that, as the “+” and “-” symbols you are probably used to seeing after blood types would suggest. The ABO blood typing system can be further classified with the use of the Rhesus blood group system. Blood types under this system are determined by testing for A and B antigens, and looking for something called the Rhesus or Rh factor. If the Rh factor is present, the blood is “positive,” and if it is not, the blood is “negative.”

When the two systems are combined, you get a plethora of blood types: A-, A+, B-, B+, AB-, AB+, O-, and O+. As you might imagine, this complicates matters, because the presence of the Rh factor can cause a transfusion reaction in someone with a negative blood type, making it unsafe, for example, for B+ blood to be transfused into a B- recipient. When all eight of these blood types are considered as a group, only one blood type, O-, is totally nonreactive, making someone with O negative blood a universal donor. People with AB+ blood can take blood from anyone because their blood will not react with the Rh factor or the A and B antigens.

Many blood banks like to stock as much blood from universal donors as they can. In an emergency situation where blood transfusions are urgently needed, O negative blood can be safely given to most patients. As a result, being identified as a universal donor can make someone very popular with a local blood bank.

There are a few cases in which blood from a universal donor can be dangerous. Some rare blood types fall outside the ABO system, and these blood types can react with O negative blood. For people with these blood types, it is a good idea to carry a medical information card clearly indicating this, as otherwise blood from a universal donor may be transfused under the assumption that it will be safe.