Blood groups are complex chemical systems found on the surface of blood cells. The two main blood group systems in transfusion practice are the ABO system and the Rh(D) type.

Within the ABO system, people can be one of four groups - O, A, B or AB, while in the Rh(D) system they can be either Rh(D) positive or Rh(D) negative. As a result, there are eight main blood groups.