There is a growing evidence supporting the role of regulatory T cells in controlling the development of asthma and allergic disease in a variety of models, although it is not clear whether the subsets of regulatory T cells are the most important. A revised hygienehypothesis proposes that limited exposure to infectious pathogens during infancy, particularly mycobacteria and parasites, may prevent the establishment of both Th1 and T regulatory repertoire, explaining in part the increased prevalence of allergies in developed countries. Regulatory T cells can control or regulate all effectors mechanisms activated during allergy and Th2 responses through the production of IL-10/TGF-β and/or cell to cell contact. IL-10 is a potent suppressor of total and allergen-specific IgE, whereas it induces an antibody isotype switch towards IgG4. TGF-β also decreases IgE production and induces immunoglobulin isotype switch towards IgA. TGF-β contributes to the generation of both Th17 and Treg cells. TGF-β directs the peripheral conversion of effector T cells into Fox p3 Treg cells, where as in the presence of IL-6, TGF-β promotes the generation of Th17 from naive T cells.