Ankylosing spondylitis (AS) is a disease in which inflammation of the sacroiliac joints and spinal attachment points is the main symptom. There is a strong association with HLA-B27. Certain microorganisms (e.g. Klebsiella) have common antigens with the susceptible person’s own tissue and can trigger an abnormal immune response. It is a chronic inflammatory disease characterized by fibrosis and ossification of the sacroiliac joints, the large joints of the extremities, the intervertebral disc fibrous rings and their adjacent connective tissue, and joint ankylosis. Ankylosing spondylitis is a type of seronegative spondyloarthropathy. The cause of the disease is not clear. It can cause ankylosis and fibrosis of the hip, knee, and spine, as well as varying degrees of eye, lung, and muscle tissue lesions, and is an autoimmune disease.