Causes of Ankylosing Spondylitis
The precise cause of ankylosing spondylitis (AS) isn't known, but the medical community does have some ideas about what could cause it. They think it may be hereditary, and AS possibly has a connection to certain bacteria.
Heredity: Family Connection
For most patients with ankylosing spondylitis, another family member has or had AS as well.
HLA-B27 is an antigen—a protein that helps the body to make antibodies to fight infection—and in 1973, medical researchers found a link between ankylosing spondylitis and this genetic marker. However, a link doesn't imply a direct connection; not everyone who has the HLA-B27 antigen will necessarily have AS. Also, not everyone with AS has the HLA-B27 marker—but about 90% of AS patients do have the marker.
Researchers aren't sure exactly how HLA-B27 relates to the development of AS, but they believe that it does help trigger the disease. There are 2 potential theories.
- The HLA-B27 marker changes the way certain bacteria interact with the body, allowing inflammation to develop that can lead to new bones—where your body shouldn't have bones (eg, connecting your vertebrae).
- The HLA-B27 antigen impacts the way other proteins work in the body, changing the body's immune system.
As mentioned above, some researchers believe that particular kinds of bacteria attack the body and cause inflammation around the ligaments. HLA-B27 potentially has something to do with this, but more research is needed to confirm the connection.
That inflammation—and especially chronic inflammation like in AS—can cause the body to release chemicals that can harm surrounding tissues, such as the cartilage. The damaged tissue is replaced by scar tissue, which can cause more pain since it doesn't protect the joint as well as cartilage does. The scar tissue can then harden and form new bone as a result of ankylosing spondylitis' effect on the body. This process of new bone formation is called ossification, and in patients with AS, bone develops where it shouldn't, limiting your spine's mobility and flexibility.