Blood groups

Blood groups arise because of the expression of antigens (agglutinogens) on the extracellular surface of red blood cell (RBC) membranes. In some cases these antigens are found only on RBCs; however, in many cases they are also found in other tissue types. A transfusion reaction occurs when antibodies (agglutinins) in a recipient’s plasma interact with the RBC agglutinogens, leading to ‘clumping’ (agglutination) of the donor RBCs and haemolysis. The first blood group to be described in detail was the ABO system, which gives rise to A, B, AB and O blood groups. The ABO system is currently the only system in which individuals naturally possess antibodies to non-self. Currently, there are 29 recognized blood group systems, including the Rhesus system, which is responsible for haemolytic disease of the newborn. An understanding of blood group systems has implications beyond transfusion medicine into areas such as transplantation, autoimmunity and population biology.