Th2 cells predominate in most patients with asthma and differentiate from uncommitted precursor T cells under the influence of IL-4. Th2 cells orchestrate allergic inflammation through the release of the Th2 cytokines IL-4, IL-5, IL-9, and IL-13. Th1 cells differentiate under the influence of IL-12 and IL-27 and suppress Th2 cells through the release of IFN-γ. Th17 cells differentiate under the influence of IL-6 and IL-23. Tregs normally suppress other Th cells through the release of TGF-β and IL-10 and may have impaired function in asthma. Each Th cell type is regulated by a specific transcription factor: T-bet for Th1 cells, GATA3 for Th2 cells, retinoic acid orphan receptor-γt (RORγt) for Th17 cells, and FOXP3 for Tregs.