TBX21 and tetanus: These results are consistent with a model (Figure 5) in which the influenza and tetanus-specific memory CD4 T cell responses include differing proportions of at least two differentiation states, each capable of expressing multiple cytokine and transcription factor phenotypes – Th1 cells (Tbet+ TNFα+ IFNγ+/- IL-2+/-) and Thpp cells (Tbet- TNFα+ IFNγ- IL-2+/-).