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Type O blood is a classification within the ABO blood group system. Individuals with type O blood have neither the A nor the B antigen on their red blood cells. However, their serum contains antibodies against both A and B antigens.1

This characteristic makes type O blood the universal red blood cell donor, meaning it can be transfused into recipients of any ABO blood type (A, B, AB, or O) without causing the red blood cells to clump together (agglutinate). Conversely, individuals with type O blood can only receive transfusions of type O red blood cells because their serum contains antibodies that would attack type A or type B red blood cells. Type O is the most common blood type globally.234

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