The most popularly known blood grouping is the ABO grouping. It is named ABO and not ABC, because "O" in it refers to having:
overdominance of this type on the genes for A and B types
one antibody only - either anti-A or anti-B on the RBCs
no antigens A and B on RBCs
other antigens besides A and B on RBCs
Option 1)
overdominance of this type on the genes for A and B types
Option 2)
one antibody only - either anti-A or anti-B on the RBCs
Option 3)
no antigens A and B on RBCs
Option 4)
other antigens besides A and B on RBCs