Spondylitis is caused by a combination of genetic susceptibility, immune system dysfunction, and environmental triggers that together drive chronic inflammation in the spine and pelvis. The single strongest risk factor is a gene called HLA-B27, carried by roughly 90% of people with ankylosing spondylitis but fewer than 8% of the general population. Having the gene alone isn’t enough to cause the disease, though. Most people with HLA-B27 never develop spondylitis, which points to a more complex chain of causes.
The Role of HLA-B27 and Other Genes
HLA-B27 is a gene that codes for a protein on the surface of your cells. This protein’s normal job is to present fragments of bacteria and viruses to your immune system so it can mount a defense. In people with spondylitis, something goes wrong in that process. The leading theories suggest the protein either misfolds inside cells (triggering an internal stress response that promotes inflammation), or it presents pieces of the body’s own tissue in a way that confuses the immune system into attacking healthy joints.
HLA-B27 accounts for only about 20 to 30% of the overall genetic risk. Researchers have identified over 100 other gene variants that each contribute a small amount of susceptibility, many of them involved in how the immune system regulates inflammation. This is why spondylitis often clusters in families: if you have a first-degree relative with the condition, your risk is significantly higher than someone without that family history, even if you don’t carry HLA-B27 yourself.
How the Immune System Drives Spinal Inflammation
At the molecular level, spondylitis is driven by an overproduction of specific inflammatory signaling molecules. Two of the most important are TNF-alpha and IL-17, both of which are found at elevated levels in affected joints. TNF-alpha amplifies the inflammatory response and recruits more immune cells to the site. IL-17 sustains that inflammation and contributes to tissue damage over time. A third molecule, IL-23, acts as an upstream trigger, activating immune cells that then release IL-17 and TNF-alpha in a self-reinforcing cycle.
What makes spondylitis distinct from other forms of arthritis is where this inflammation concentrates: the entheses. These are the points where tendons and ligaments attach to bone, and the spine has hundreds of them. Inflammation at these sites, called enthesitis, is the hallmark of the disease and the starting point for the structural damage that follows.
From Inflammation to Bone Fusion
The progression from inflammation to a fused spine follows a specific sequence. Repeated inflammation at the entheses first erodes small areas of bone near the attachment points. The body then attempts to repair that damage, but the repair process overshoots. A signaling molecule called IL-22 drives excessive new bone formation at these sites, creating bony growths called syndesmophytes that bridge the gaps between vertebrae. Over years, these bridges can connect enough vertebrae to significantly restrict spinal movement, a condition sometimes called “bamboo spine.”
This process also explains why certain parts of the body are affected more than others. The sacroiliac joints at the base of the spine and the vertebral connections in the lower back experience the most biomechanical stress, which is why they’re almost always the first sites involved. Physical strain on the entheses appears to prime these locations for the inflammatory cascade, particularly in people who are already genetically susceptible. Research has shown that repeated biomechanical stress, combined with the IL-17, IL-22, and IL-23 inflammatory pathway, helps explain the specific pattern of joint involvement.
The Gut Connection
One of the more surprising causes of spondylitis involves the gut. Up to 60% of people with ankylosing spondylitis show signs of microscopic intestinal inflammation, even when they have no digestive symptoms. A systematic meta-analysis found that people with spondylitis have measurably different gut bacteria compared to healthy individuals: lower overall microbial diversity, reduced levels of beneficial Bifidobacterium, and a shift toward higher levels of Bacteroidetes.
The connection appears to work in both directions. The HLA-B27 gene itself may contribute to disrupting the normal balance of gut bacteria, which then increases intestinal permeability. When the gut barrier becomes “leaky,” bacterial fragments can enter the bloodstream and reach the joints, potentially triggering or sustaining the inflammatory response. This gut-joint axis helps explain why spondylitis and inflammatory bowel disease so frequently overlap, and why treatments that calm gut inflammation sometimes improve joint symptoms as well.
Environmental and Lifestyle Triggers
Genetics and immune dysfunction set the stage, but environmental factors can push the disease into activity or accelerate its progression. Smoking is the best-studied trigger. Smokers with spondylitis have significantly higher levels of systemic inflammation, worse spinal mobility (including reduced chest expansion and cervical rotation), and faster progression of structural damage on imaging. Even exposure to fewer than 10 pack-years of cigarette smoke increases the likelihood of hip joint involvement, one of the more disabling complications of the disease.
Other environmental contributors include long-term exposure to air pollution, heavy metals, and a high-fat diet, all of which promote oxidative stress and may amplify the inflammatory process. The role of bacterial infections is less clear. Most studies have not found a direct link between common infections and spondylitis onset, with one exception: chronic periodontitis (a bacterial infection of the gums) showed an association in one large case-control study. There’s also a long-standing hypothesis that a bacterium called Klebsiella pneumoniae may interact with HLA-B27 to trigger the immune response, though this remains unproven.
Who Gets Spondylitis
Spondylitis typically appears in late adolescence or early adulthood, with most people first experiencing symptoms in their 20s or 30s. The average age at clinical assessment is around 38 years, though the delay between first symptoms and diagnosis often spans 7 to 10 years. Men are affected roughly twice as often as women in radiographic forms of the disease, where visible joint damage shows up on X-rays. In non-radiographic forms, where inflammation is present but hasn’t yet caused structural changes visible on standard X-rays, the gender gap narrows considerably.
This distinction matters because spondylitis exists on a spectrum. Radiographic axial spondyloarthritis (the classic form, ankylosing spondylitis) is diagnosed when X-rays show clear damage to the sacroiliac joints. Non-radiographic axial spondyloarthritis involves the same symptoms and inflammatory process but without visible X-ray changes, sometimes detectable only on MRI. Some people in the non-radiographic category eventually progress to the radiographic form, while others never do.
Conditions That Share the Same Roots
Spondylitis rarely exists in isolation. The same immune pathways that inflame the spine can target other organs, creating a cluster of related conditions. The most common overlap is with anterior uveitis, a painful inflammation of the eye that affects up to 40% of spondylitis patients at some point. Psoriasis (an inflammatory skin condition) and inflammatory bowel disease (including Crohn’s disease and ulcerative colitis) also share the TNF-alpha and IL-17 pathways and frequently co-occur with spondylitis.
These overlapping conditions are now understood as part of a broader spectrum of immune-mediated inflammatory diseases rather than separate, unrelated problems. If you develop one, your risk of developing the others is elevated. This shared biology is why treatments that target TNF-alpha or IL-17 can improve symptoms across multiple conditions simultaneously, and why doctors managing spondylitis routinely screen for eye, skin, and gut involvement.