Spondylitis is part of a group of conditions called spondyloarthritis, or SpA for short. All of these conditions are considered autoimmune diseases. They cause long-lasting inflammation, which can lead to symptoms like back pain and stiffness. Spondylitis is a general term for several types of inflammatory arthritis that mostly affect the neck and spine.

Spondyloarthritis is a long-term condition that usually gets worse over time. In some cases, it can cause serious problems, like bones in the spine fusing together or breaking more easily. Understanding spondylitis as an autoimmune disease helps explain its symptoms, guides treatment, and highlights the importance of getting an accurate diagnosis.

What Is an Autoimmune Disease?

When the immune system works properly, it responds to bacteria, viruses, and damaged cells by causing inflammation. This inflammation helps rid the body of infection and damaged cells. In autoimmune diseases, however, the immune system mistakenly attacks healthy tissue, which leads to tissue damage and ongoing inflammation.

There are more than 100 different autoimmune diseases, including multiple sclerosis, rheumatoid arthritis, inflammatory bowel disease, type 1 diabetes, systemic lupus erythematosus, and all types of spondyloarthropathy.

In many autoimmune diseases, the body makes special proteins called autoantibodies that attack healthy tissues by mistake. Doctors can sometimes use these autoantibodies to help diagnose the disease.

Why Is Spondylitis Classified as Autoimmune?

Researchers don’t completely understand how the immune system works in spondyloarthritic conditions. However, these conditions are thought to be related to abnormally functioning proteins called MHC-I proteins.

Spondylitis is one of several inflammatory diseases that are described as MHC-I-opathies, meaning they involve problems with certain immune system proteins called MHC‑I. MHC-I proteins, when functioning normally, allow the immune system to recognize and target viruses, bacteria, and cancer cells. Diseases in the SpA family feature abnormal MHC-I proteins that don’t work properly. This dysfunction is thought to cause an immune response that triggers spondyloarthritic conditions.

Research from Frontiers in Immunology has found evidence of autoantibodies among people living with SpA, which may lead to better treatments and earlier diagnoses.

How Genetics and Risk Factors Play a Role

Like many autoimmune conditions, the exact cause of spondylitis isn’t fully known, but doctors believe it develops when a mix of genes and outside factors sets off the immune system.

Certain inherited traits, like the HLA-B27 gene, are found more often in people with spondylitis. The HLA‑B27 gene codes for a molecule found on the surface of certain immune cells. Having HLA‑B27 may increase the chance of triggering an immune response. Other proteins, such as ERAP1 and the immune messengers interleukin-23 and interleukin-17, can also influence how the body’s immune defenses work. Having these markers doesn’t mean you will definitely get spondylitis, but it may raise the chances that your immune system will mistakenly attack healthy tissue — one of the hallmarks of autoimmune disease. At the same time, the absence of these markers does not exclude the disease.

Environmental triggers may add to this risk. Infections in the gut or urinary tract can sometimes spark reactive arthritis, a related form of spondylitis, especially in people with the HLA-B27 gene. Changes in gut bacteria, known as dysbiosis, may also lead to extra inflammation and play a role in these conditions. Together, these genetic and environmental factors help explain why spondylitis develops and why it’s considered an autoimmune disease.

How Is the Immune System Affected in Spondylitis?

While spondylitis can cause inflammation in many areas, it most often affects the lower back, causing a dull ache in that area or in the buttocks. Various types of SpA affect the joints differently, causing symptoms including joint pain, ankylosis (abnormal new bone growth at joints), and enthesitis (inflammation where ligaments and tendons connect to bone).

In addition to stiffness and pain in the spine and neck, common symptoms of spondylitis-related conditions include heel pain and swollen fingers. Depending on the type of SpA, other parts of the body, including the hips, eyes, bowels, tendons, and other joints, can also become involved. SpA can be associated with uveitis (eye inflammation), psoriasis, and inflammatory bowel diseases such as Crohn’s disease and ulcerative colitis.

Spondyloarthritic conditions are often classified based on where symptoms appear, how severe they are, what causes them, and whether there are any related conditions.

Is Spondylitis Autoimmune or Autoinflammatory?

The Spondylitis Association of America identifies conditions in the SpA family as both autoimmune and autoinflammatory. Many immune system dysfunctions don’t fit perfectly into either category, as they typically show some symptoms of one and some characteristics of the other.

For example, genetic links to spondyloarthritic conditions, like the HLA-B27 gene, support calling it an autoimmune disease, since these conditions often run in families. But some types of SpA, such as enteropathic arthritis and reactive arthritis, are linked to past infections and seem more like autoinflammatory diseases.

Why the Classification Matters

Labeling a condition as autoimmune helps doctors and researchers better understand how the disease works. It also shapes how we diagnose, study, and develop new treatments.

Treatments for autoimmune diseases have improved and now focus on targeted immunotherapy, rather than just treating symptoms. Today, people with ankylosing spondylitis may use biologic drugs that block certain proteins in the immune system. Biologic drugs are special medicines made from living cells that help control the body’s immune response. These include TNF inhibitors, which block a protein called tumor necrosis factor (TNF), and IL-17 inhibitors, which block a protein called interleukin-17.

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