The major human blood type system which depends on the presence or absence of two antigens A and B. Type O occurs when neither A nor B is present and AB when both are present. A and B are genetic factors that determine the presence of enzymes for the synthesis of certain glycoproteins mainly in the red cell membrane.

Synonyms: ABO Blood-Group Systems, Blood-Group System, ABO, ABO Factor, H Blood Group System, Blood Groups, H, Systems, ABO Blood-Group, ABO Blood-Group System, System, ABO Blood-Group, Blood Group, H, 0, Factors, ABO, Blood Group, ABH, Blood-Group Systems, ABO, Blood Groups, ABH, H Blood Group, Factor, ABO, ABO Factors, ABH Blood Group, ABH Blood Groups, ABO Blood Group System, Blood Group H Type 1 Antigen, H Blood Groups