Alloimmunisation

When to suspect this adverse reaction

Alloimmunisation results in the stimulation of alloantibodies to red cells, human leucocyte antigens (HLA) or human platelet antigens (HPA).
 
Alloimmunisation by red cells results in an acute or delayed haemolytic transfusion reaction or haemolytic disease of the newborn.

Usual causes

Approximately 1% of red cell transfusions are associated with alloantibody formation but may be much higher (up to 30%) in patients who are frequently transfused, such as those with sickle cell disease and thalassaemia.
 
HLA and HPA antibody formation may lead to platelet refractoriness where less than an expected rise in platelet count is seen after a platelet transfusion. This may occur in about 25-70% of patients receiving multiple platelet transfusions.

Post-transfusion purpura can also occur as a consequence of development of HPA and HLA antibodies.
 
Alloimmunisation against HLA antigen is also implicated in transplant rejection, febrile non-haemolytic reactions and TRALI.

Investigation

Obtain patient history of any previous transfusions, transplantations or pregnancies.
 
Perform an antibody screen on the patient’s plasma to detect clinically significant red cell, HLA or HPA antibodies.

What to do

Treatment depends on the type and severity of the transfusion reaction with most reactions being mild.
 
 Alloimmunisation can’t be completely prevented.
 
In Australia, all cellular blood components are leucodepleted, with most white cells being removed by filtration, reducing the risk of HLA sensitisation.
 
RhD immunoglobulin is also given to pregnant women who are RhD negative to prevent haemolytic disease of the newborn.
 
To reduce the risk of TRALI,  plasma for fresh frozen plasma and cryoprecipitate is only accepted from males, and apheresis platelets collected from males and nulliparous women only.

If a red cell alloantibody is identified, antigen-negative blood should be selected if further transfusion is needed.
 
With patients requiring long-term transfusion support, e.g. those with thalassaemia, giving phenotyped-matched red cells early in their treatment course may reduce the risk of further antibody development.

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