HLA Typing (Overview)
Dr. med. Kaimo Hirv
The HLA system (human leukocyte antigen system) plays a major role in the T cell-mediated immune response. The proteins of the HLA system are highly polymorphic molecules of the cell membrane that bind peptides and present them to the T cells. HLA molecules are encoded by a large number of genes in the MHC region (major histocompatibility complex) on the short arm of chromosome 6. The genes of the MHC region are divided into two classes. Class I is made up of the loci HLA-A, -B, -C, while class II is encoded by the loci HLA-DP, -DQ, -DR. One reason for the clinical significance of HLA typing is tissue typing. During transplantation of an organ or stem cells, the MHC loci of the donor and the recipient have to match as accurately as possible to minimize the risk of transplant rejection and to prevent graft-versus-host disease.
Another reason is that more than 30 diseases have been associated with certain HLA alleles that may indicate predisposition to the disease. These diseases are essentially autoimmune diseases such as systemic lupus erythematodes (SLE), ankylosing spondylitis (Bechterew's disease), multiple sclerosis (MS) or insulin-dependent diabetes mellitus (IDDM) as well as rheumatic diseases. Twin studies showed that environmental factors play an important role regarding the manifestation of these diseases so that they occur sporadically in predisposed individuals from affected families. The occurrence of reactive arthritis (Reiter's syndrome) after intestinal infection with gram-negative bacteria (shigella, salmonella, yersinia) for example, has been associated with the HLA-B27 antigen in almost all cases; however, not all HLA-B27 carriers develop the Reiter's syndrome.