Although the cause of ankylosing spondylitis is unknown, it is believed to involve a combination of genetic and environmental factors.
More than 90% of those affected have a specific human leukocyte antigen known as the HLA-B27 antigen. The underlying mechanism is believed to be autoimmune or autoinflammatory.
but only 2% of people with HLA-B27 develop clinical AS.
Reiter syndrome (after Hans Reiter, a German bacteriologist): a reactive arthritis (usually 1–3 weeks following certain bacterial infections) with involvement of at least one other non-joint area (urethritis, uveitis/conjunctivitis, skin lesions, mucosal ulcerations…). 75% are HLA-B27 positive