Recently the Th1/Th2 concept has been revised and Th17 cells have been implicated in allergy. Despite clear correlative evidence, the cellular and molecular basis for the connection between increased IL-17A and IgE in allergy has not been elucidated. Here we show using flow cytometry that allergic patients have higher numbers of IL-17A+ cells compared to nonallergic donors. The selective removal of IL-17A+ cells from peripheral blood mononuclear cells of allergic donors after an IL-17A secretion assay reduces IgE levels, whereas re-addition of recombinant IL-17A restores it, as measured by ELISA, showing their important functional implication for IgE production. In addition, IL-17A directly promotes the differentiation of IgE-secreting cells and IgE production upon anti-CD40/IL-4 costimulation, as shown by enzyme-linked immunospot technique and ELISA. IL-17A triggers rapid degradation of IκBα and subsequent translocation of NF-κB into the B-cell nucleus, followed by transcription of epsilon germ-line, activation-induced cytidine deaminase, and IFN regulatory factor 4, as analyzed by flow cytometry, western blot, and quantitative real-time RT-PCR, respectively. Our study shows that IL-17A+ cells promote IgE production and that IL-17A exerts its pro-allergic effect directly at the level of B cells. Therefore, IL-17A might be a target for the treatment of IgE-dependent diseases, including atopic dermatitis.