Australian Red Cross Blood Service

Blood group systems are necessary to identify suitable products for transfusion.

Their clinical importance also relates to the production of alloantibodies that destroy transfused blood cells or that cross the placenta and give rise to haemolytic disease in the fetus and newborn. This is dependent upon:

• The frequency with which alloantigens and alloantibodies occur, which may be different among ethnic groups.

• The functional characteristics of alloantibodies: thermal range, immunoglobulin class, titre, avidity and ability to fix complement.

• The presence of autoantibodies that are relevant to the diagnosis and management of autoimmune blood disorders and that may interfere with compatibility testing.

ABO blood group

ABO is the most important of all the blood group systems.

There are four different ABO blood groups, which is determined if an individual's red cells carry the A antigen, the B antigen, both A and B antigens, or neither antigens.

Normal healthy individuals, from early in childhood, make red cell antibodies against A or B antigens that are not expressed in their own cells.

These naturally occurring antibodies are mainly IgM immunoglobulins. They attack and rapidly destroy red cells carrying the corresponding antigen.

For example, anti-A reacts with red cells of Group A or AB, and anti-B against red cells of Group B or AB.

If ABO incompatible red cells are transfused, immediate red cell lysis can occur.

For example, if Group A red cells are transfused into a Group O recipient, the recipient’s anti-A antibodies bind to the transfused red cells.

An ABO incompatible transfusion reaction leads to overwhelming haemostatic and complement activation, resulting in shock and renal failure. Stopping tranfusion and immediate volume resuscitation is imperative.

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Rh blood group

The Rhesus or Rh system comprise of more than 50 antigens found on the red cell membrane protein. The Rh(D) antigen is the most potent and important antigen.

The presence of Rh(D) antigen on red cells confers Rh positivity; while persons who lack Rh(D) antigen are Rh-negative, which occurs in about 15% of individuals.

Exposure of Rh-negative individuals to even small amounts of Rh-positive cells, by either transfusion or pregnancy, can result in the production of anti-D alloantibody.

Alloantibodies directed against the D antigen are capable of causing severe haemolytic transfusion reactions and haemolytic disease of the fetus and newborn.

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