IL21 and allergic disease: Additionally, TFH cells predominantly produce IL-2, IL-4 and IL-21 in B cell follicles and closely regulate antibody class-switching in severe inflammatory and allergic diseases, including AD, asthma, and COVID-19-induced airway inflammation (Yao et al., 2018; Crotty, 2019; Gong et al., 2019; Papillion et al., 2019; Yao et al., 2019; Zhou et al., 2019; Gong et al., 2020).