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Ankylosing spondylitis tends to run in families with close relatives suffering from either ankylosing spondylitis itself or one of the other seronegative spondarthritides such as Reiter's disease, psoriatic arthritis or enteropathic arthritis.
All these conditions are strongly associated with the major histocompatibility antigen HLA B27.
It has been postulated that a genitourinary or bowel infection may trigger the development of the disease in genetically predisposed individuals. Klebsiella bacteria carry antigens which mimic B27.