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What is ankylosing spondylitis, in plain terms?

The short answer

Ankylosing spondylitis is a form of arthritis where the immune system inflames the joints where the spine meets the pelvis, and sometimes the spine itself, the eyes, the gut and the skin. Over years, inflammation can be followed by new bone that stiffens the spine, which is where the name comes from. Most people who have it carry a gene called HLA-B27, though most people with the gene never get AS, so other triggers, including the gut, are involved. Researchers now use the broader term axial spondyloarthritis, or axSpA, for the whole spectrum, with AS as its X-ray-visible end.

5 papers behind this answer, best first

Clicking through to a study? How to read a paper is the ten-minute version.

Rutledge BJ · 2026
Review

Why Do Some Therapies Work While Others Fail? Mechanisms of Response and Emerging Treatments in axial Spondyloarthritis: 2026 SPARTAN Annual Meeting Proceedings.

Current rheumatology reports

This 2026 review explains why some drugs work in axial spondyloarthritis and others do not: HLA-B27 can present fragments that expand a specific population of T cells making TNF and IL-17, while IL-23 matters less once inflammation is established, which fits the failure of IL-23 blockers. It names the next targets, including an interferon signature seen in people who fail TNF and IL-17 drugs and a macrophage factor called MIF.

IL-23 / IL-17 AxisHLA-B27 GeneticsMacrophage M1 Polarisation
Lobiuc · 2025
Review

Gut microbiota and ankylosing spondylitis: current insights and future challenges

Microbial Cell

Comprehensive review of how gut bacterial imbalance disrupts the IL-23/IL-17 inflammatory pathway and weakens the intestinal barrier in AS. Identifies the lack of standardised methodology and large cohorts as the bottleneck for translating this into treatments.

Gut–Immune AxisIL-23 / IL-17 Axis
Zheng M · 2026
Editorial

Ten millennia of purifying selection on HLA-B27 reveals an ancient epidemic-scale burden of spondyloarthritis in West Eurasia.

Autoimmunity reviews

Ancient DNA from nearly 16,000 people shows HLA-B27 was carried by about half of West Eurasians 8,500 years ago and has been steadily selected against ever since, down to about 4 percent today. The gene likely protected against infections in the past, and the disease it predisposes to was once common enough to shape human genetics.

HLA-B27 Genetics
Stankey · 2024
Study

A disease-associated gene desert directs macrophage inflammation through ETS2

Nature

Identified a single 'switch' gene called ETS2, sitting on chromosome 21, that turns immune cells inflammatory across AS, IBD, and several other related diseases. Because no current drug targets ETS2, this points to a treatment direction beyond what's already available with TNF and IL-17 blockers.

ETS2 / Functional GenomicsMacrophage M1 PolarisationIL-23 / IL-17 Axis
Risch I · 2026
Study

Refinement of the Pathogenic T Cell Receptor Motif Suggests Type 17 CD8+ T Cells Initiate HLA-B*27-associated Autoimmunity.

bioRxiv : the preprint server for biology

A specific family of killer T cells that recognise HLA-B27 was found in people with axial spondyloarthritis, uveitis and psoriatic arthritis but not healthy controls, and these cells appear to switch into an IL-17-producing mode in the gut. It is a direct line from the HLA-B27 gene, through the gut, to the inflammation that IL-17 drugs block, in one set of cells.

HLA-B27 GeneticsGut–Immune AxisIL-23 / IL-17 Axis

Plain-language guides on this from patient organisations

Reviewed sources written for patients. They cover the “what do I do” part that research papers can’t.

Want the mechanism? These threads go deeper

HLA-B27 GeneticsGut–Immune Axis