HLA

The human leukocyte antigen system (HLA) is the name of the major histocompatibility complex (MHC) in humans. The superlocus contains a large number of genes related to immune system function in humans. This group of genes reside on chromosome 6, and encode cell-surface antigen-presenting proteins and many other genes. The HLA genes are the human versions of the MHC genes that are found in most vertebrates (and thus are the most studied of the MHC genes). The proteins encoded by certain genes are also known as antigens, as a result of their historic discovery as factors in organ transplantations. The major HLA antigens are essential elements for immune function. Different classes have different functions:
HLA class I antigens (A, B & C) present peptides from inside the cell (including viral peptides if present). These peptides are produced from digested proteins that are broken down in the proteasomes. The peptides are generally small polymers, about 9 amino acids in length. Foreign antigens attract killer T-cells (also called CD8 positive- or cytotoxic T-cells) that destroy cells.
HLA class II antigens (DP,DM, DOA,DOB,DQ, & DR) present antigens from outside of the cell to T-lymphocytes. These particular antigens stimulate T-helper cells to multiply, and these T-helper cells then stimulate antibody-producing B-cells to produce Antibodies to that specific antigen. Self-antigens are suppressed by suppressor T-cells.
HLA class III antigens encode components of the complement system.
HLA have other roles. They are important in disease defense. They may be the cause of organ transplant rejections. They may protect against or fail to protect (if down regulated by an infection) cancers. They may mediate autoimmune disease (examples: type I diabetes, coeliac disease). Also, in reproduction, HLA may be related to the individual smell of people and may be involved in mate selection.
Aside from the genes encoding the 6 major antigens, there are a large number of other genes, many involved in immune function, located on the HLA complex. Diversity of HLA in human population is one aspect of disease defense, and, as a result, the chance of two unrelated individuals having identical HLA molecules on all loci is very low. Historically, HLA genes were identified as a result of the ability to successfully transplant organs between HLA similar individuals.

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