The presence or absence of antigens on the surface of red cells allows laboratories to identify the blood group of individuals. ABO and Rh blood group systems are the most well-known and most significant in transfusion practice.
Nearly 300 red cell antigens are now recognised, representing 29 blood group systems. Red cell antigens are not exclusively expressed on red cells and may be found on other blood cells and tissues.
The avoidance of incompatibility relies on laboratories performing thorough testing of blood groups. Pretransfusion testing is performed to ensure ABO compatibility and to detect pre-existing atypical red cell antibodies. This is basically the group and type (or group and antibody screen), which forms part of the crossmatch procedure.
Immune red cell antibodies are predominantly IgG. These atypical antibodies are formed upon exposure to foreign red cell antigens during transfusion or pregnancy. IgG antibodies can cross the placenta and may cause haemolytic disease of the fetus and newborn.
The incidence of an immune red cell alloantibody is determined by the frequency of the antigen in the population and by its immunogenicity.
