Background
Knowledge of red cell blood group antigens and the clinical significance of red cell alloantibodies has provided the basis for safe and effective red cell transfusions. This knowledge has also provided the basis for understanding the aetiology of haemolytic disease of the newborn, and provides an essential contribution to the clinical management of this condition.
Blood group antigens have also provided geneticists and forensic scientists with useful genetic markers.
However the biological significance and evolutionary background of these blood group systems are largely unknown.
Antigens
Antigens are substances that can cause the production of antibodies by the body's immune system. Antigens are usually protein and can be found on the surface of bacteria, viruses, pollen and more importantly for transfusion on the surface of red cells, white cells and platelets.
The presence or not of these antigens on red cells allows laboratories to identify the blood group of individuals. Most people are aware of the blood groups ABO and Rh (e.g. A Positive, O Negative etc.) however there are nearly 300 red cell antigens now recognised, representing 29 blood group ‘systems'.
The response by the immune system to some of these antigens can range from clinically benign to massive and complicated immune reactions possibly resulting in death.
The avoidance of incompatibility in transfusion practice relies on laboratories performing thorough testing of blood groups and identifying the presence of any red cell antibodies. This is basically the group and type (or group and antibody screen) that forms part of the crossmatch procedure.
Red cell antigens are not exclusively expressed on red cells, and may be found on other blood cells and/or tissues.
Antibodies
Antibodies are produced by the body's immune system in response to antigenic stimulus. These antibodies (also called immunoglobulins) are made of gammaglobulins of which there are five main types; IgG, IgM, IgA, IgD and IgE. Each type performs a different role.
Almost all antibodies to red cells are IgG or IgM, and a minority have an IgA component. Antibodies which are reactive at 37°C are potentially capable of mediating destruction or sequestration of transfused allogeneic red cells. IgG antibodies are capable of crossing the placenta and may cause haemolytic disease of the newborn.
Of critical clinical importance are naturally occurring anti-A and anti-B that are reactive at 37ºC. Most other naturally occurring antibodies are not reactive at this temperature, and as a consequence are usually of little clinical significance. Naturally occurring antibodies are usually IgM and are found in individuals who have never been transfused with red cells or have not been pregnant with a foetus carrying the relevant red cell antigen.
They are believed to be produced in response to exposure to substances found within the environment or diet, which have a similar structure to the corresponding red cell antigen.
Immune red cell antibodies are predominantly IgG and these atypical antibodies are formed as a result of exposure to foreign red cell antigens during transfusion or pregnancy. The incidence of an immune red cell alloantibody is determined by the frequency of the antigen in the population and by its immunogenicity. The main purpose of pre-transfusion testing is to ensure ABO compatibility and detection of pre-existing atypical red cell antibodies.