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A classification system for human blood that identifies four major blood types based on the presence or absence of two antigens, A and B, on red blood cells. The four blood types (A, B, AB, and O, in which O designates blood that lacks both antigens) are important in determining the compatibility of blood for transfusion.
ABO SystemThe major alloantigen system in humans, which is based on 3 carbohydrate antigens expressed on red cells, encoded on chromosome 9q34.
Significance ABO blood groups determine to whom one can donate blood from whom one can accept blood. Type A or AB blood causes an immune reaction in type Bs. Type B and AB blood cause an immune reaction in type As. Conversely, type O blood has no A or B antigens—type Os are universal donors. Since AB blood already produces both antigens, type ABs can accept any of the other blood types without suffering an immune reaction, and are thus known as universal recipients, as they form antibodies to neither A nor B antigens.