Furthermore, circulating IL-21, which plays an important role in activating TFH cells, was also reported to be higher in AAV or GPA patients than in healthy controls [17], and we previously demonstrated that serum IL-21-positive AAV patients exhibited a higher cross-sectional Birmingham vasculitis activity score (BVAS) than serum IL-21-negative AAV patients [18]. Here, IL21 is linked to vasculitis.