HLA-B27

Human leukocyte antigen (HLA) B27 (subtypes B*2701-2759) is a class I surface antigen encoded by the B locus in the (MHC) on chromosome 6 and presents antigenic peptides (derived from self and non-self antigens) to T cells. HLA-B27 is strongly associated with ankylosing spondylitis (AS), and other associated inflammatory diseases referred to as "spondyloarthropathies". Diseases associated with the HLA-B27 subtype can be remembered with the mnemonic PAIR, and include Psoriasis, Ankylosing spondylitis, Inflammatory bowel disease, and Reactive arthritis.

The prevalence of HLA-B27 varies markedly in the general population. For example, about 8% of Caucasians, 4% of North Africans, 2-9% of Chinese, and 0.1-0.5% of persons of Japanese descent possess this gene. In northern Scandinavia (Lapland), 24% of people are HLA-B27 positive, while 1.8% have associated ankylosing spondylitis.

A small group (<0.5%) of people infected with HIV are able to remain symptom-free for many years without medication. These long-term nonprogressors appear to be slightly more common among people who are HLA-B27 positive.

The relationship between HLA-B27 and many diseases has not yet been fully elucidated. Though it is associated with a wide range of pathology, particularly seronegative spondyloarthropathy, it does not appear to be the sole mediator in development of disease. For example, while 90% of people with ankylosing spondylitis (AS) are HLA-B27 positive, only a small fraction of people with HLA-B27 ever develop AS. People who are HLA-B27 positive are more likely to experience early onset AS than HLA-B27 negative individuals. There are additional genes being discovered that also predispose to AS and associated diseases. Additionally there are potential environmental factors (triggers) that may also play a role in susceptible individuals.

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