According to established view, the underlying factor in allergic immune response is imbalance between the number and function of allergen-specific CD4+ lymphocytes. In atopic allergy, type 2 helper T lymphocytes (Th2 cells) are elevated in relation to regulatory T lymphocytes (Treg cells). In healthy subjects the ratio is vice versa. It seems that tolerance generated through allergen-specific Treg cells is the immunologic reaction mode towards allergens in healthy non-atopic subjects. It has been found that hyposensitization will correct the imbalance of Th2 and Treg cells and restore the normal immune response to allergens.