Ankylosing Spondylitis
An HLA-B27-associated spondyloarthropathy in which inflammation begins at tendon and ligament entheses rather than the synovium, and the body's attempt to heal that inflammation lays down new bone that fuses the spine.
First principles
Ankylosing spondylitis is a disease of the enthesis, not the synovium
Unlike rheumatoid arthritis, where the primary target is the synovial membrane, ankylosing spondylitis is driven by inflammation at the enthesis (the site where tendons and ligaments insert into bone), particularly around the sacroiliac joints and spine, in people carrying the HLA-B27 allele. This distinction matters clinically: enthesitis produces pain at insertion points (Achilles tendon, plantar fascia, costochondral junctions) rather than the boggy synovial swelling of rheumatoid arthritis, and it is why ankylosing spondylitis is classified as a seronegative spondyloarthropathy: rheumatoid factor and anti-CCP are typically negative because there is no synovial autoantibody-driven process.
Educational content pending clinical review. Not medical advice.