An allergen is a molecule that stimulates an allergic response in a sensitive individual. Allergens are almost always proteins; however, not all proteins are allergens. A protein that acts as an allergen displays two fundamental properties: induction of an IgE immune response on first exposure and induction of a clinical response to the same or similar protein on subsequent exposures. Some examples of allergens include dust mites, peanuts, pollen, and pet dander; however, a comprehensive list of allergens is impossible to assemble, as the list of possible substances is extensive (i.e., food, air particles, drugs, animal products, and insect stings). Sensitivities vary greatly from one person to another; and the response to allergens also varies in severity from allergic rhinitis and hives to asthma and ...