In recent years, however, it has been observed that IL-21 is induced in the periphery and mucosal tissues of humans and mice after infection with some parasites (Stumhofer et al., 2013; Carpio et al., 2015; Perez-Mazliah et al., 2015; Cai et al., 2016; Ryg-Cornejo et al., 2016; Moretto et al., 2017; Inoue et al., 2018; Wikenheiser et al., 2018) (Table 1). Here, IL21 is linked to infection.