Australian Red Cross Blood Service

The human leucocyte antigen (HLA) system arises from polymorphic cell surface molecules involved in the presentation of antigen to T cells.

HLA polymorphisms distinguish 'self' from 'non-self' in an immunologic sense, and may also be involved in the pathogenesis of certain 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.

HLA compatibility has an important influence on the outcome of solid organ and haematopoietic stem cell transplantation. Furthermore, HLA antigens present in blood cells are responsible for some mild to serious complications of blood transfusion.

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.

Although their main role is to present antigens to T cells, HLA molecules can also be recognised as foreign by the host T cells. This mechanism is known as allorecognition.

Because of allorecognition, HLA antigens are considered as one of the main barriers to the success of solid organ or bone marrow transplantation. They are also responsible for the strong alloimmunisation seen in patients following transplantation or blood transfusion.

HLA antibodies are implicated in some cases of refractoriness to platelet transfusions.