What is the ABO blood group system?

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)