Vgamma9/Vdelta2 T cells are a minor subset of T cells in human blood and differ from all other lymphocytes by their specific responsiveness to (E)-4-hydroxy-3-methyl-but-2-enyl pyrophosphate (HMB-PP), a metabolite produced by a large range of microbial pathogens. Vgamma9/Vdelta2 T cells can be skewed towards distinct effector functions, in analogy to, and beyond, the emerging plasticity of CD4(+) T cells. As such, depending on the microenvironment, Vgamma9/Vdelta2 T cells can assume features reminiscent of Th1, Th2, Th17 and Treg cells as well as professional APCs. We here demonstrate that Vgamma9/Vdelta2 T cells express markers associated with follicular B helper T (T(FH) ) cells when stimulated with HMB-PP in the presence of IL-21. HMB-PP induces upregulation of IL-21R on Vgamma9/Vdelta2 T cells. In return, IL-21 plays a co-stimulatory role in the expression of the B-cell-attracting chemokine CXCL13, the CXCL13 receptor CXCR5 and the inducible co-stimulator by activated Vgamma9/Vdelta2 T cells, and enhances their potential to support antibody production by B cells. The interaction between HMB-PP-responsive Vgamma9/Vdelta2 T cells, IL-21-producing T(FH) cells and B cells in secondary lymphoid tissues is likely to impact on the generation of high affinity, class-switched antibodies in microbial infections.