Doctors don’t know exactly what causes ankylosing spondylitis (AS). This rare type of arthritis causes stiffness and inflammation, mainly along your spine. Both your genes and things in your environment are thought to play a role.
Most people who have ankylosing spondylitis have a gene that makes a protein or "genetic marker" called HLA-B27. This gene is involved in your body's immune response, which normally works to protect you against bacteria, viruses, and other harmful substances.
But most people who have HLA-B27 don’t get the disease. And people who don’t have this genetic marker can still get AS.
Some other gene variations have been linked to ankylosing spondylitis, too. Scientists don’t know how these gene differences affect your chances of getting the disease. But some of the genes they affect are important in your immune system.
If your genes already make you prone to AS, something else -- like an injury or a bacterial infection -- could trigger it to start. Scientists don't know exactly what these triggers may be.
One theory is that AS could start with a breakdown of the natural defenses in your intestines. This allows bacteria to pass into your bloodstream. That, in turn, may trigger changes in your immune response that lead to AS.
Anyone can get it. But your chances of having ankylosing spondylitis are higher if:
- Others in your family have the disease.
- You're 17-45 years old. AS usually, but not always, starts when you're a teen or young adult. This is earlier than other types of arthritis and rheumatic diseases.
- You’re a man. Men are at least twice as likely to get the disease as women.
- You have gastrointestinal infections regularly.
- You have another autoimmune disease, like psoriasis, Crohn’s disease, or ulcerative colitis.