Allergens are special types of (otherwise harmless) environmental antigens that elicit allergic reactions in sensitized individuals. One group of allergens includes small environmental or therapeutical chemicals that become allergenic after binding to self-proteins (contact or drug allergy). Other allergens are inhaled, dietary or insect-sting proteins that induce, mainly in predisposed individuals, an immune response characterized by immunoglobulin E production (atopic allergy).