Each person has two types of these molecules, in any combination. Since type O refers to a lack of the antigenic molecule, the resulting types are type A (AA or AO molecules), type B (BB or BO molecules), type AB, or type O. People of a given blood type form antibodies against other blood types, causing a hemolytic (blood breakdown) reaction. This is most relevant when a patient needs a blood transfusion or transplant. The blood types must be matched to avoid a reaction based on ABO incompatibility. For example, a patient with type A blood will react against type B or AB blood. Similarly, a patient with type B blood will react against type A or AB blood, and patients with type O blood will react against type A, type B or type AB blood. Type O blood does not cause an immune response, and therefore type O cells (universal donor) can be given to patients of any blood type.

Since antibodies are present in the blood plasma, or liquid portion of the blood, plasma transfusions as well as whole blood transfusions must be matched to avoid inducing this reaction.

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