A universal donor is someone who can donate blood to anyone else, with a few rare exceptions. People with the O- blood type have traditionally been considered universal blood cell donors. Conversely, a universal recipient can safely take blood from anyone, again with a few exceptions. People with AB+ blood have been considered universal recipients.
Historically, 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 on the blood cells. 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 on the B blood cells, rejecting the transfusion and triggering a transfusion reaction. O blood, however, can be given to someone with an A blood type, because there are no antigens in the donor blood to react with the recipient's body.
Things get a bit more complicated than that, as the "+" and "-" symbols people 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, there are a plethora of blood types: A-, A+, B-, B+, AB-, AB+, O-, and O+. This complicates matters, because the presence of the Rh factor can cause a transfusion reaction in someone with a negative blood type. This makes it unsafe, for example, for B+ blood to be transfused into a B- recipient.
In emergency situations, patients may be given O- blood; however, this is not ideal. The best blood for a person to receive is an exact match for both type and Rhesus factor. This is because of the presence of antibodies and other antigens in the blood, which can cause dangerous reactions. Doctors perform a test called crossmatching to determine if the donor blood is compatible with the recipient.
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 cells 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.
In addition to universal blood cell donors, there are people who are universal blood plasma donors. Antibodies are found in blood plasma, and are the opposite of the blood type. Someone with type A blood has B antibodies in his or her plasma. Type AB blood plasma has no antibodies, and therefore can safely be donated to anyone.
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.