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.
Currently, there are 35 different blood group systems and over 300 blood group antigens located within these systems. Red cell antigens are not exclusively expressed on red cells and may be found on other blood cells and tissues.
Pre-transfusion testing is performed to identify the patient's blood group and to detect pre-existing atypical red cell antibodies. A group and type (or group and antibody screen) is performed to allow provision of compatible red cells. If a clinically significant antibody is detected, red cells that are negative for the relevant antigen must be crossmatched.
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.