What Causes Ankylosing Spondylitis?

Ankylosing spondylitis (AS) is a type of inflammatory disease that mostly affects the joints and bones in the spine and back. AS is also an autoimmune condition, meaning that the immune system attacks healthy tissues in the body by mistake.

The exact cause of AS is currently unknown, but research remains ongoing. However, experts theorize that a combination of genetic and environmental factors may increase your risk of developing the condition. 

woman sitting on couch with back pain

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Theories of Cause

Although the cause of AS is not fully understood, researchers have developed some theories that might play a role in the development of the disease. 

HLA-B27 Gene 

Experts have learned that most people who live with AS have the HLA-B27 gene—a protein located on the surface of white blood cells. Normally, human leukocyte antigens (HLAs) help your immune system tell the difference between healthy and harmful cells (e.g., viruses or bacteria).

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

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

Microbial Infections 

Some research suggests that certain types of infections can increase the risk of AS. 

One study found that exposure to Candida albicans (yeast) might trigger AS. Other researchers theorize that viruses, such as human papillomavirus (HPV), might affect your immune system by increasing the number of immune cells that cause inflammation in the body. Another theory is that some bacteria, like Klebsiella pneumoniae, can cause infections (e.g., pneumonia) which may lead to inflammation in the joints and trigger AS symptoms. 

The link between infections and AS largely remains a theory, however. More research is still needed to confirm the connection between infections and AS. 

Is Ankylosing Spondylitis Hereditary? 

People with an immediate family history (e.g., parent or sibling) of AS are more likely to develop the disease. Researchers have discovered more than 60 genes that are associated with an increased risk of AS.

Approximately 90% of people with AS have inherited the HLA-B27 gene. However, inheriting the HLA-B27 gene does not guarantee that you will develop AS. In fact, only 1-2% of people with the HLA-B27 gene will get AS. But, if you have both the HLA-B27 gene and a parent who lives AS, your chances of developing the condition increase to about 20%.

Some research also suggests that genetic variations in ERAP1, EL1A, and IL2R—genes that influence immune system functioning—might also raise your risk of developing AS.

Who Gets Ankylosing Spondylitis? 

Anyone can develop ankylosing spondylitis, but some people are at a higher risk than others. These factors include:

  • Age: Symptoms often begin in early adulthood, while most people with AS receive their diagnosis before turning 45. But, children and older adults can also have AS.
  • Sex: Previously, researchers believed that AS affects more men than women. But, new studies show that the men and women experience the condition at similar rates. However, in people under the age of 24, women have higher rates of AS. Men have slightly higher rates of AS among older adults.
  • Family history: If you have a family member with AS, you are more likely to develop the condition yourself. 
  • Ethnicity: AS is more common in white people than other racial groups because HLA-B27 is more common in people with European ancestry. 

Risk Factors 

In addition to genetic factors, environmental and lifestyle factors can also contribute to your risk of AS. These factors include: 

  • Having inflammatory bowel disease (IBD): People with a type of inflammatory bowel disease, such as Crohn's disease or ulcerative colitis, may increase their risk of other inflammatory conditions like AS.
  • Smoking cigarettes: Although experts aren’t sure whether smoking is linked to the development of AS, there is evidence to suggest that smoking can worsen symptoms of AS and progress your disease faster. Some studies also show that smoking can cause inflammation, leading to an earlier onset of symptoms and an overall lower quality of life.

A Quick Review 

Ankylosing spondylitis is an inflammatory disease and chronic autoimmune condition that causes pain and stiffness in the spine and back. As your condition progresses, inflammation can affect other joints and organs in the body. 

Researchers believe that a combination of genetic and environmental factors can raise your risk of developing AS. If you have a family history of AS or begin to notice symptoms, it’s a good idea to talk to your healthcare provider. Getting an early diagnosis can help get you started on treatment sooner—which can reduce pain and improve your quality of life. 

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11 Sources
Health.com uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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