TBX21 and influenza: In contrast to the potential of Thpp cells to contribute to Tfh populations, Th1 cells are more likely to contribute to cytotoxicity during influenza infection [4], as higher expression levels of Granzyme B were detected in IFNγ+ cells than IFNγ- cells (Fig 2B) and Tbet is required for expression of cytotoxic functions [66].