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 large joints of the extremities, as well as the intervertebral disc rings and adjacent connective tissue, and joint ankylosis. What are the causes of ankylosing spondylitis? 1, genetic: genetic factors have an important role in the development of ankylosing spondylitis. The risk of ankylosing spondylitis in a group of relatives is reported to be 20 to 40 times higher than in the general population, and the prevalence of ankylosing spondylitis in first-degree relatives is 24.2%, 120 times higher than in the normal population. All of this suggests that HLA-B27 is an important factor in the development of ankylosing spondylitis. 2, autoimmune: ankylosing spondylitis itself is an autoimmune disease. It has been found that 60% of patients with ankylosing spondylitis have elevated serum complement, most cases have IgA-type wet-like factor, significantly higher levels of serum C4 and IgA, and circulating immune complexes (CIC) in the serum, but the antigenic nature is not determined. The above phenomena suggest the involvement of immune mechanisms in the pathogenesis of the disease. 3, Infection: Recent studies suggest that the incidence of ankylosing spondylitis may be associated with infection. Reports also confirm that the incidence of ulcerative colitis and restrictive enteritis is much higher in patients with ankylosing spondylitis than in the general population, so it is speculated that ankylosing spondylitis may be related to infection. 4, other: trauma, endocrine, metabolic disorders and metabolic reactions are also suspected as pathogenic factors. In conclusion, the cause of this disease is unknown, there is no one doctrine that can fully explain all the manifestations of ankylosing spondylitis, it is likely that the genetic factors based on environmental factors (including infection) and other aspects of the impact of the disease.