What Causes Ankylosing Spondylitis?

Medically reviewed by David Ozeri, MD

The exact cause of ankylosing spondylitis (AS) is unknown, but genetic and environmental risk factors may play a role. Some risk factors include other autoimmune disorders, such as inflammatory bowel disease, and smoking.

AS is an autoimmune, inflammatory disease that mostly affects the joints and bones in the spine and back. There's ongoing research for the cause or causes of AS, but theories include specific genes and certain infections that may be responsible.

Related: Can You Get Arthritis in Your 20s?

Theories

The cause of AS is not fully understood. Researchers have developed some theories that might play a role in developing the disease.

HLA-B27 Gene

Research has shown that most people with AS have the HLA-B27 gene, or a protein located on the surface of white blood cells. Human leukocyte antigens (HLAs) normally help the immune system tell the difference between healthy and harmful cells like viruses or bacteria.

HLA-B27 is a protein related to immune system dysfunction. The immune system can mistakenly attack healthy cells in your body if you carry the HLA-B27 gene. This can make you more vulnerable to a number of autoimmune disorders, including AS.

It's important to note that it does not mean you will eventually develop AS if you have inherited the HLA-B27 gene. This gene just increases your risk of getting the condition.

Microbial Infections

Some evidence suggests that certain types of infections can increase the risk of AS. A review published in 2023 noted a link between exposure to Candida albicans (yeast) and AS. Other research has found that viruses, such as human papillomavirus (HPV), can increase the number of immune cells that cause inflammation in the body.

Some bacteria, like Klebsiella pneumoniae, may also cause infections that lead to inflammation in the joints and trigger AS symptoms. The link between infections and AS remains largely a theory. More research is needed to confirm the connection.

Is Ankylosing Spondylitis Hereditary?

People with an immediate family member (e.g., parent or sibling) with AS are more likely to develop the disease than others. Research has found more than 60 genes associated with an increased risk of AS.

About 90% of people with AS have inherited the HLA-B27 gene. Inheriting this gene does not always mean you will develop AS. Only 1% to 2% of people with the HLA-B27 gene will get AS. Having both the HLA-B27 gene and a parent who has AS raises your chances of also developing it to about 20%.

Some evidence suggests that genetic variations might increase your risk. The genes affected include ERAP1, EL1A, and IL2R, all of which influence immune system function.

Who Gets Ankylosing Spondylitis?

Anyone can develop AS, but some are at a higher risk than others:

• Age: Symptoms often begin in early adulthood, and most people with AS receive a diagnosis before turning 45. Children and older adults can also have AS.

• Ethnicity: AS is most prevalent in White people because HLA-B27 is more common in people with European ancestry.

• Sex: Researchers previously believed that AS affects more men than women, but men and women experience the condition at similar rates. Women have higher rates of AS among those younger than 24. Men have slightly higher rates of AS, in contrast, among older adults.

Risk Factors

Certain factors can also contribute to your risk of AS. These risk factors include inflammatory bowel disease (IBD), which is also an autoimmune disorder, and smoking.

Inflammatory Bowel Disease (IBD)

People with IBD, such as Crohn's disease or ulcerative colitis, may have an increased risk of other inflammatory conditions like AS. Crohn's disease affects the lining of the digestive tract, while ulcerative colitis affects the large intestine and rectum.

Smoking

Some evidence suggests that smoking can worsen AS symptoms and progress your disease faster than usual. Smoking can cause inflammation, which is higher in people with AS.

Related: What Causes Psoriatic Arthritis?

A Quick Review

AS is an autoimmune, inflammatory disease that causes pain and stiffness in the spine and back. Inflammation can affect other joints and organs in the body as the disease progresses.

A combination of genetic and environmental factors can raise your risk of AS. It's a good idea to talk to a healthcare provider if you have a family history of AS or notice symptoms. Getting an early diagnosis can help get you started on treatment sooner, which can reduce pain and improve your quality of life. 

For more Health.com news, make sure to sign up for our newsletter!

Read the original article on Health.com.