IL21 and infection: Cytokines utilizing the STAT3 signaling pathway including IL-6, IL-21, and IL-27 have been implicated in driving Tfh differentiation (Nurieva et al., 2008; Batten et al., 2010; Eto et al., 2011; Ma et al., 2012; Choi et al., 2013a; Harker et al., 2013; Ray et al., 2014), but due to their partially compensatory pathways, separating the requirements for individual signals throughout infection has proved challenging.