Mechanisms of allergen-specific immunotherapy and the role of regulatory T cells in allergic diseases. An allergen is taken up by regional dendritic cells leading to the induction of regulatory T cells. These cells suppress allergic responses directly and indirectly by the following mechanisms. 1. Suppression of mast cells, basophils and eosinophils. 2. Suppression of effector T cells. 3. Suppression of inflammatory cell migration to tissues and tissue inflammation. 4. Suppression of mucus production. 5. Suppression of inflammatory dendritic cells and induction of tolerogenic dendritic cells. 6. Suppression of allergen-specific IgE and induction of IgG4 from B cells.