Blood of the AB type has both A antigens and B antigens carried on red blood cells but lacks both A and B antibodies in its plasma. These characteristics make the AB blood type the most restricted with regard to red cell transfusions but a universal donor for plasma transfusions.
The AB blood type is one of four possible blood types in the ABO classification and is the least common. Depending on the ethnic group, it is found in between 2.2 and 7.1 percent of the population. To have this blood type, a person must inherit one gene for the A antigen from one parent and one gene for the B antigen from the other parent. This can occur if one parent is type A and the other is type B, if one parent is type A or type B and the other is type AB, or if both parents are type AB. Because a type O person does not carry the genes for either A or B antigens, a type O person cannot be the biological parent of a type AB child.
Blood typing of both the donor and the recipient is critical for a successful blood donation. Because people with A, B or O blood types have antibodies in their blood plasma for one or both of the antigens carried on the red blood cells of AB blood, a person with AB type blood can donate red cells or whole blood only to another person with AB type blood. On the other hand, the plasma of AB type blood does not have antibodies to either A or B antigens — if it did, it would react to its own red blood cells. Because plasma from the AB blood type has no antibodies against the antigens of other types, it can be given to anyone without causing an immune reaction.