Objective To investigate the correlations of the serum level of interleukin-21 (IL-21) with anti-BP 180 antibodies,peripheral eosinophil level,bullous pemphigoid disease area index (BPDAI)in patients with bullous pemphigoid (BP).Methods A total of 31 patients with bullous pemphigoid and 31 healthy controls were enrolled into this study.Blood samples were collected from all the subjects,and enzyme-linked immunosorbent assay (ELISA) was performed to detect the serum level of IL-21 and serum titer of anti-BP180 antibodies.BPDAI was assessed among 13 patients,and the correlation of the IL-21 level with anti-BP180 antibody titers,eosinophil count and BPDAI were analyzed.Results No significant difference in the serum level of IL-21 was observed between the patients (M [P25-P75]:255.28 [156.19-361.59] ng/L) and healthy controls (150.47 [72.4-268.73] ng/L,P =0.14).There was no correlation between the serum level of IL-21 and serum titer of anti-BP180 antibodies (82.98 [26.82-132.92] U/ml,rs =0.18,P =0.332).Among the 27 BP patients who underwent blood cell count,no correlation was found between the serum levels of IL-21 and whole-blood eosinophil counts (rs =0.104,P =0.606).Among the 13 BP patients who underwent assessment of BPDAI,there was no correlation between the serum levels of IL-21 and BPDAI scores (rs =0.233,P =0.464).Conclusion The IL-21 level is uncorrelated with anti-BP 180 antibody level,peripheral eosinophil level or BPDAI in patients with BP.