Ankylosing spondylitis (AS) has features of an autoimmune disease, but it doesn’t fit neatly into that category. Scientists have debated this classification for years, and the current consensus is that AS sits somewhere between autoimmune and autoinflammatory, with evidence supporting both mechanisms working together to drive the disease.
Why the Classification Is Complicated
In a classic autoimmune disease like rheumatoid arthritis or lupus, the adaptive immune system (the part that learns to recognize specific threats) mistakenly attacks the body’s own tissues, often producing measurable antibodies against them. In a purely autoinflammatory disease, the innate immune system (the body’s first-response defense) misfires on its own, causing inflammation without those targeted antibodies.
AS borrows from both sides. The innate immune system plays a dominant role, with several types of first-responder immune cells showing abnormal activity at the sites where the disease strikes, particularly the spine and the joints connecting the spine to the pelvis. That pattern looks autoinflammatory. But researchers have also found specific autoantibodies in AS patients, and there is clear evidence that certain immune cells called T cells undergo activation and clonal expansion, meaning they multiply to target something specific. That pattern looks autoimmune.
A 2021 review in Nature Reviews Rheumatology examined both lines of evidence and concluded that rather than choosing one label, the disease is best understood as a condition where autoinflammatory and autoimmune pathways overlap. For practical purposes, AS is grouped with immune-mediated inflammatory diseases. Most doctors and patient resources still refer to it as autoimmune, which is a reasonable shorthand even if it’s not the full picture.
The Role of HLA-B27
The strongest genetic link in AS is a gene called HLA-B27. Roughly 85% of AS patients carry it, compared to about 6 to 8% of the general population. This gene codes for a protein on the surface of cells whose normal job is to present small fragments of viruses or bacteria to the immune system so it can mount a defense. The leading theory is that in people with HLA-B27, this protein may also present fragments of the body’s own tissues, essentially tricking the immune system into attacking healthy cells. This “arthritogenic peptide hypothesis” has been studied for over 50 years and remains the most widely supported explanation for why the gene increases disease risk.
Carrying HLA-B27 does not guarantee you’ll develop AS. Most people with the gene never do. Other genes inside and outside the same region of the genome contribute additional risk, and environmental triggers, particularly changes in the gut, appear to be necessary for the disease to actually take hold.
The Gut-Joint Connection
One of the most striking findings in AS research is that up to 50% of patients have subclinical gut inflammation, meaning their intestines show signs of immune activity even without digestive symptoms. About 5 to 7% develop full inflammatory bowel disease. This overlap is not a coincidence. The intestinal inflammation correlates with worse spinal inflammation and can drive progression from early to established disease.
Animal studies have made the connection even clearer. Mice engineered to carry the HLA-B27 gene do not develop arthritis when raised in a germ-free environment. But once gut bacteria are introduced, the disease appears rapidly. This means microbial triggers are not just associated with AS; they appear to be necessary for it to develop in genetically susceptible individuals.
Patients with AS also tend to have an imbalanced gut microbiome, with fewer beneficial bacteria and more pro-inflammatory species. These shifts can increase intestinal permeability, sometimes called “leaky gut,” allowing bacterial products to cross into the bloodstream and activate immune pathways that target distant tissues like the spine and entheses (the points where tendons and ligaments attach to bone).
How Inflammation Drives Spinal Damage
The immune pathway most central to AS is a signaling chain involving two inflammatory molecules: IL-23 and IL-17. IL-23 activates a subset of immune cells that produce IL-17, which in turn stimulates a cascade of inflammation. IL-17 prompts fibroblasts, macrophages, and other cells to release additional inflammatory signals, creating a self-reinforcing loop. In animal models, simply overproducing IL-23 is enough to cause a disease that closely resembles human AS, with inflammation at the entheses of the spine.
What makes AS distinct from many other inflammatory conditions is that this cycle of inflammation doesn’t just erode bone. It also triggers new bone formation. Over time, the vertebrae can develop bony growths that bridge the gaps between them, gradually fusing the spine and reducing mobility. This process, called ankylosis, is the hallmark of advanced disease and the reason early treatment matters.
What AS Feels Like
The hallmark symptom is inflammatory back pain, which behaves differently from the mechanical back pain most people experience from lifting something heavy or sitting too long. Inflammatory back pain typically starts before age 40, comes on gradually rather than after an injury, and lasts at least three months. The most telling features: it improves with movement and exercise but does not improve with rest. Pain at night is common, often waking you in the early morning hours and easing once you get up and start moving. Prolonged morning stiffness, sometimes lasting 30 minutes or more, is another red flag.
Mechanical back pain works in the opposite direction. It tends to worsen with activity and improve with rest. It can start at any age and often has a clear triggering event.
About 80% of AS patients first notice symptoms before age 30, and the condition affects men roughly twice as often as women. Only about 5% of patients develop symptoms after age 45.
Symptoms Beyond the Spine
Because AS is driven by systemic immune dysfunction, it can affect organs well beyond the joints. The most common extra-articular symptom is acute anterior uveitis, an inflammatory eye condition that causes sudden eye pain, redness, light sensitivity, and blurred vision. Between 20 and 30% of AS patients will experience at least one episode. Having a family history of spondyloarthritis significantly increases the risk, with one study finding the odds more than seven times higher in those with affected relatives.
Psoriasis occurs in roughly 5 to 10% of AS patients, and inflammatory bowel disease in about 5 to 11%, depending on the population studied. These overlapping conditions reinforce the idea that AS is part of a broader family of immune-mediated diseases rather than a condition limited to the spine.
How Treatment Targets the Immune System
The fact that AS responds to drugs designed to suppress specific immune pathways is itself strong evidence that immune dysfunction drives the disease. First-line biologic treatments for AS are TNF inhibitors, which block tumor necrosis factor, one of the key inflammatory molecules produced in the IL-23/IL-17 cascade. These drugs can significantly reduce pain, stiffness, and inflammation in many patients.
For people who don’t respond well to TNF inhibitors, IL-17 inhibitors offer an alternative. These drugs directly block the inflammatory molecule most central to AS pathology. A newer class, JAK inhibitors, works by interrupting the signaling inside immune cells that allows them to respond to inflammatory signals in the first place. All of these treatments aim to break the cycle of immune-driven inflammation before it causes permanent structural damage to the spine.
Exercise remains a cornerstone of AS management alongside medication. Regular movement helps maintain spinal flexibility, reduces stiffness, and can slow the progression of fusion. Physical therapy programs tailored to AS focus on posture, stretching, and strengthening the muscles that support the spine.