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The ABO blood group system is the most important of the human blood group systems. It is based on the presence or absence of A and B antigens on the surface of red blood cells. Individuals can have one of four main blood types: A, B, AB, or O. Type A blood has A antigens, type B has B antigens, type AB has both A and B antigens, and type O has neither A nor B antigens.

This classification is determined by inherited genes. The presence of antibodies in the plasma is also crucial. Individuals with type A blood have anti-B antibodies, those with type B blood have anti-A antibodies, type AB blood has neither anti-A nor anti-B antibodies, and type O blood has both anti-A and anti-B antibodies.

The ABO system is vital for blood transfusions. It is essential to transfuse red blood cells that are compatible with the recipient's antibodies. For example, a person with type A blood cannot receive type B blood because their anti-B antibodies would attack the transfused red blood cells. Type O individuals are considered universal red blood cell donors because their red blood cells lack A and B antigens, making them less likely to be rejected by recipients of any ABO type. Conversely, type AB individuals are universal plasma donors because their plasma lacks anti-A and anti-B antibodies. ABO incompatibility can lead to serious, even fatal, hemolytic transfusion reactions.