HLA-B27

Human Leukocyte Antigen B*27 (subtypes B*2701-2724) is a class I surface antigen encoded by the B locus in the major histocompatibility complex (MHC) on chromosome 6 and presents microbial antigens to T-cells. HLA-B27 strongly associated with a certain set of autoimmune diseases referred to as the "seronegative spondyloarthropathies". In the general population, about 8% Caucasian, 4% African, 2-9% Chinese, and 0.1-0.5% Japanese have the HLA-B27 antigen. In Northern Scandinavia (Lapland), 24% of people are HLA-B27 positive while 1.8% have ankylosing spondylitis (AS).

Mystery
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 nearly all 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 (although rarely) people who are HLA-B27 negative develop it? The literature is inconclusive, though several theories have been suggested and research continues.

Associated pathology
In addition to its connection with AS, HLA-B27 is implicated in Reiter's syndrome, certain eye disorders such as acute anterior uveitis and iritis, psoriatic arthritis and Crohn's disease. Reiter's syndrome then again, is statistically associated with AS.