IL21 and influenza: The ideal experiment would be a combination of in vivo and in vitro, but we only performed the IL-21R blockade assay in vitro. However, Abdullah et al. and Rimpei et al. have demonstrated that IL-21 secreted by Tfh cells help B cells differentiation through the IL-21R blockade assay in vitro after influenza vaccine, and our result was consistent with previous studies [6, 38].