Complex chemical substances found on the surfaces of red blood cells are different for each blood group. The two most important blood group systems in transfusion work are the ABO and Rh systems.

Within the ABO system people can be one of four types – O, A, B or AB, whilst in the Rh system they can be either Rh positive or Rh negative. Each system is inherited independently of the other. Thus, there are eight main blood groups. They are, from the most common to the rarest: