Objective To test the level of follicular helper T cells (Tfh) and interleukin (IL)-21,CXCL13 in the peripheral blood of patients with ankylosing spondylitis (AS),and to analyze the relationship between Tfh and clinic features and explore the possible immunological pathogenesis of AS.Methods The Tfh cells were obtained from patients and normal controls and detected by flow cytometry.While the levels of IL-21,CXCL13 in patients and normal controls were measured by enzyme-linked immunosorbent assay (ELISA) tests.Data analysis were performed by Student's t-test,Rank-sum test,Spearman's correlation test.Results The expression of CD4+CXCR5 qCOS + cells (Tfh) (mean rank 33.71) and IL-21 [(299±27) ng/L],CXCL13 [(5.8±1.0) μg/L] in the peripheral blood of AS was significantly higher than normal controls [mean rank 23.54,(176±26) ng/L,(4.2±0.8) μg/L] (Z=-2.258,t=17.221,t=6.464,all P＜0.05).It was similar in AS with peripheral joint involvement compared with AS of non-peripheral joint involvement,and there was no difference between AS patients with positive HLA-B27 and those without HLA-B27.Mean -while,no correlation was found between the expression of Tfh,IL-21,CXCL13 and level of ESR,CRP,BASDAI.And there was no significant correlation between the expression of Tfh and IL-21,CXCL13 (P＞0.05).Conclusion The expression of Tfh and the levels of IL-21,CXCL13 are increased significantly,but are not closely relatedto disease activity.These results indicate that the abnormality of Tfh may play an important role in the pathogenesis of AS.