Germinal centers require CD4⁺ follicular helper T cells (T_FH cells), whose hallmark is expression of the transcriptional repressor Bcl-6, the chemokine receptor CXCR5 and interleukin 21 (IL-21). To track the development and fate of T_FH cells, we generated an IL-21 reporter mouse by introducing sequence encoding green fluorescent protein (GFP) into the Il21 locus; these mice had expression of IL-21–GFP in CD4⁺CXCR5⁺PD-1⁺ T_FH cells. IL-21–GFP⁺ T_FH cells were multifunctional helper cells that coexpressed several cytokines, including interferon-γ (IFN-γ), IL-2 and IL-4. T_FH cells proliferated and gave rise to transferrable memory cells with plasticity, which differentiated after recall into conventional effector helper T cells and T_FH cells. Thus, we demonstrated that T_FH cells were not terminally differentiated but instead retained the flexibility to be recruited into other helper T cell subsets and nonlymphoid tissues.