HLA-B27

Human Leukocyte Antigen (HLA) B27 (subtypes B*2701-2759) is a class I surface antigen encoded by the B locus in the major histocompatibility complex (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 collectively as "spondyloarthritis".

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.

Serotype
Note: Serotyping is currently obsolete, as gene sequencing and protein structure determinations are currently used to detect various alleles and subtypes of HLA-B27. Deletion of this chart is suggested. Additionally, the new allele frequency database is obsolete as well.

Spondylarthropathies
The relationship between HLA-B27 and many diseases has not yet been fully elucidated. Though it is associated with a wide range of pathology, 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 fraction of people with HLA-B27 ever develop AS. This raises two important questions: why don't all HLA-B27 positive people develop AS, and why do some people who are HLA-B27 negative develop it? 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.,

Note: These are gene frequencies which are roughly half of the phenotype frequencies.

Associated pathology
In addition to its association with ankylosing spondylitis, HLA-B27 is implicated in other types of seronegative spondyloarthropathy as well, such as reactive arthritis (Reiter's Syndrome), certain eye disorders such as acute anterior uveitis and iritis, psoriatic arthritis and ulcerative colitis associated spondyloarthritis. The shared association with HLA-B27 leads to increased clustering of these diseases.