Humans have over fifty different blood antigens—the antigens of the A-B-O (or ABO) group are the most common. This classification system is used by most hospitals and blood donation groups to distinguish blood types for compatibility for blood transfusions. The antigens are known as antigens A and B—both responsible for producing the blood types A, B, AB, and O (also called the universal donor blood type).

Organ and tissue transplants—including blood transfusions—can only be safe if certain markers (antigens) of the donor and recipient are the same or very similar; if not, the recipient’s body (the immune system) will reject the blood or tissue as though they were disease-causing organisms by producing antibodies. For example, people with type A have antibodies in their blood against type B; those with type B have antibodies against type A. People with type AB have no anti-A or anti-B antibodies, but those who have type O have both anti-A and anti-B antibodies. People with AB blood are called universal recipients, as they can receive any of the blood types. And usually the most sought-after blood types for donation come from people with type O blood: called universal donors, their blood can be given to people with any of the other blood types.