Ankylosing spondylitis

Ankylosing spondylitis is a chronic inflammatory disease, meaning a long-lasting one, and it mostly affects the spine, the column of bones that runs down your back. What sets it apart is back pain that sticks around for a long time and spinal stiffness that keeps getting worse.

To see what's going on, you have to look at a very specific spot: the entheses, the points where tendons and ligaments, those tough bands of tissue, anchor into bone. When those spots become inflamed, it's called enthesitis, and that's the main lesion in this disease. Immune cells called T lymphocytes and macrophages move into the area, and cytokines are released. Cytokines are messenger chemicals these cells use to pass instructions to each other, and one of them is TNF-α. Those signals fuel the inflammation, and they also drive fibrosis, the buildup of tough, scar-like tissue, and ossification, the making of new bone.

Over time, that inflammation can make the spine stiff. Think of a bicycle chain: it bends because each link pivots on the next one, but if the links were welded together, the chain would turn into a solid bar. In severe cases, something similar happens. The vertebrae, the bones of the spine, can grow together, and the spine becomes rigid and hard to bend. In later stages, this fusion can be seen on an X-ray, and it has its own name: "bamboo spine."