Blood Types: Systems, Testing, and Transfusion

Definition and inheritance of blood types

Blood type (blood group) is a classification based on antigens on the surface of red blood cells (RBCs) and the corresponding antibodies in the blood. These antigens can be proteins, carbohydrates, glycoproteins, or glycolipids, and multiple antigen variants (alleles) together define a blood group system. Blood types are inherited from both parents, and—except for rare circumstances—an individual’s blood group remains the same throughout life. The ABO and Rh systems are the most important for transfusion compatibility. In ABO, people are classified as A, B, AB, or O depending on which A and/or B antigens are present on RBCs; the matching antibodies in serum help determine safe transfusion. In the Rh system, the presence or absence of the RhD (D) antigen determines the “+” or “−” status. Although most blood types are stable, they can change after events such as bone marrow transplantation, where new blood-forming cells replace the recipient’s original cells. Blood type inheritance also has clinical implications beyond transfusion. For example, RhD incompatibility can lead to hemolytic disease of the newborn (HDN) when an RhD-negative mother becomes sensitized to RhD-positive fetal blood. Additionally, certain blood group antigens are associated with susceptibility or resistance to infections (e.g., Duffy antigen and malaria), reflecting evolutionary selection pressures in different populations.

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Human blood group systems recognized by ISBT

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