how many different blood groups are there?
<answer> 4 <context> The ABO Blood Group System. There are four major blood groups determined by the presence or absence of two antigens – A and B – on the surface of red blood cells: 1  Group A – has only the A antigen on red cells (and B antibody in the plasma). 2  Group B – has only the B antigen on red cells (and A antibody in the plasma).