How is pathogenesis of SpA?

Genetics (very tricky, first degree relatives increase risk, HLAB27 role in MHC I, microbes which can cross react with B27 Ab to bacteria this is known as molecular mimickery, role of gut >50% have ileal disease microscopic, breakdown of gut blood barrier can be inciting event of SpA) Inflammation (Most important is Enthesis, typical Sacroiliac joint, vertebral bodies, peripheral joint synovium, GI and eye, limited histopathology. Starts with early sacroilitis, then synovitis with myxoid bone marrow then destroyed bone replaced and endochondral ossification results in bone ankylosis)