The ABO blood group system consists of four major blood groups. According to the American Red Cross, these groups are based on the presence or absence of A and B on the surface of red blood cells.

The American Red Cross breaks down the groups as:

Group A: having only an A antigen on red cells (also has a B antibody in the plasma)

Group B: having only a B antigen on red cells (also has an A antibody in the plasma)

Group AB: having both A and B antigens on red cells (also has neither A nor B antibodies in the plasma)

Group O: having neither A nor B antigens on red cells (also has both A and B antibodies in the plasma)