Clinical relevance of the HLA system in blood transfusion.

HLA alloimmunization induced by pregnancy, multiple transfusions or transplantation is responsible for some of the serious complications seen in patients receiving blood and blood products. These complications are primarily the result of antibody and antigen triggering an acute immunological reaction, which in some cases can be fatal e.g. TRALI. Some adverse reactions are triggered by HLA antibodies present in the patient whereas others are initiated by antibodies or HLA reactive cells present in the transfused product. The introduction of universal leucodepletion for the prevention of vCJD transmission has resulted in a significant reduction in these reactions by eliminating the main source of alloimmunization, but residual cellular components or platelets are still able to activate the immune system and induce the development of HLA reactive antibodies or T cells. However, the use of more sensitive and specific techniques to detect HLA antibodies and antigens has not only improved the investigation of transfusion reactions and their subsequent diagnosis, but it has also facilitated the implementation of a number of measures such as the use of HLA antibody negative products to further reduce their development.