Causes
The cause of AS is not completely understood, but it's believed to be at least partly related to genetics. AS is more common in people with a family history of the condition. Scientists believe that AS is "triggered" by something in the environment, such as an infection, for people whose genes put them at risk of AS. The immune system responds to this trigger, producing chemicals that cause inflammation in the spine and other joints of the body. There is no evidence, however, that an infection causes the disease.
It is also known that people with a molecule called HLA B27 on the surface of their cells are at higher risk of developing AS. HLA B27 can be passed down from parent to child. Although it increases the risk of AS, not everyone with HLA B27 will get AS.