Leucocytes
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 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.
HLA class I antigens (A, B, C) are expressed on the majority of tissues and cells including T and B lymphocytes, granulocytes and platelets. The 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.
Throughout the years a number of techniques to detect HLA antibodies have been described. These include the complement-dependent lymphocytotoxicity test (LCT), enzyme-linked immunosorbent assay (ELISA) and flow cytometry.
Although the main role of the HLA molecules is to present antigens to T cells, the HLA molecules can themselves be recognised as foreign by the host T cells by a mechanism known as allorecognition.
Two pathways of allorecognition have been identified, direct and indirect. Because of this mechanism, HLA antigens are therefore one of the main barriers to the success of solid organ transplantation or bone marrow transplantation and are 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.