Human leucocyte antigens

Human leucocyte antigens (HLA) are polymorphisms in cell surface molecules that distinguish 'self' from 'non-self'. They may also be involved in pathogenesis of certain autosystem autoimmune and infectious diseases.

HLA falls into two classes:

  • HLA class I antigens (A, B and C) are expressed on the majority of tissues and cells including T and B lymphocytes, granulocytes and platelets.

  • HLA class II antigens (DR, DQ, DPA and DPB) are constitutively expressed on B lymphocytes, monocytes and dendritic cells but can also be detected on activated T lymphocytes and activated granulocytes. It is not clear whether they are also present on activated platelets.

Although their main role is to present antigens to T cells, HLA antigens can also be recognised as foreign by the host T cells. This mechanism is known as allorecognition. People exposed to non-self antigens through pregnancy, transfusion or transplantation, may become alloimmunised and develop antibodies directed against these HLA antigens.

A number of techniques to detect HLA antibodies have been described, which include complement-dependent lymphocytotoxicity test (LCT), enzyme-linked immunosorbent assay (ELISA) and flow cytometry.

HLA matched platelets

Human leucocyte antigen (HLA) matched platelets may be required in patients who are at risk of developing, or have developed antibodies to HLA antigens. HLA antibodies are implicated in approximately 20% of cases of platelet refractoriness. Platelet refractoriness is the failure to achieve satisfactory responses to platelet transfusions from random donors.

HLA matched platelets can be supplied at the direction of the Australian Red Blood Service Transfusion Medicine Specialist for the following clinical indications:

  • congenital platelet function disorders such as Bernard-Soulier syndrome, Glanzmanns thrombasthenia or other congenital platelet disorders where development of HLA alloantibodies may make future platelet support very difficult;
  • patients who are to undergo stem cell transplantation using stem cells from a donor who is not a full HLA match and where development of HLA antibodies could result in an adverse transfusion outcome;
  • patients who are refractory to random platelet transfusions due to the presence of HLA alloimmunisation

References

  1. AABB Technical Manual.18th ed. USA, 2014