A universal blood donor is a person who can safely donate blood to almost anyone. Historically, it was believed that universal donors, as they are also known, could donate blood safely to any individual. Later, research demonstrated that blood compatibility was actually more complicated and that there are rare situations where someone may experience a transfusion reaction upon being given blood from a universal blood donor, hence the “almost” caveat.
Blood compatibility is determined by the presence of antigens in the blood. Everyone's blood has a slightly different antigen composition. If blood from someone who has antigens is transferred into someone who does not have those antigens, the recipient's body will react to the transfusion and attempt to break down the donor blood. Ideally, when someone needs a blood transfusion, testing is performed and a donor with a close match is found. In a hurry, however, there may not be time to test the recipient or to find a match, in which case blood from a universal blood donor may be offered. This blood contains no common antigens and thus can safely be transfused into most people, even if their blood types are unknown.
The concept of the universal blood donor is based on two very commonly used blood typing systems, the ABO system and the Rhesus system. These systems can be used to classify blood on the basis of very common antigens. The ABO system includes four blood types, A, B, AB, and O, based on the presence of A and B antigens, with O blood containing no antigens. Each of these types can be divided into two subtypes on the basis of whether someone's blood contains another antigen known as the Rhesus factor. People with Rhesus factor are said to have positive blood, while people without have negative blood, resulting in blood types like AB- and O+. A person with O- blood is a universal blood donor.
There are a number of other blood typing systems based on a variety of other antigens that can be identified in the blood. These antigens are more rare, but they can complicate transfusions. Someone with O- blood might have a rare antigen that would cause a recipient to react with the donor's blood. Certain ethnic and racial groups are more likely to have unusual antigens and it is important to conduct thorough compatibility testing before transfusing blood to or from members of these groups to avoid adverse reactions.
People who know their blood types may want to note their blood types on their identification or carry a card in their wallets with basic information so that emergency responders can quickly provide safe transfusions if necessary. If someone has a rare antigen or has reacted poorly to transfusions from universal donors in the past because of an antigen incompatibility, this should also be noted.