Allergy is an immunological disorder, which is driven by uncontrolled allergen-activated T cell subsets, leading to immediate type hypersensitivity against otherwise harmless environmental allergens. These allergens are tolerated by healthy individuals as well as by patients, who successfully underwent allergen-specific immunotherapy (SIT). The successful SIT is characterized by the induction of T cell unresponsiveness against the given allergen. Regulatory T cells (Tregs), which are installed or enhanced by SIT and govern the activity of potentially pro-allergic effector T cells, mediate this unresponsiveness. The current article reviews the mechanisms underlying the balance of these cell populations along with suppressive mechanisms of SIT, which may serve as future drug targets.