An allergen is an otherwise harmless substance that causes an allergic reaction. Allergic rhinitis, or hay fever, is an allergic response to specific allergens. Pollen is the most common allergen in seasonal allergic rhinitis. These are allergy symptoms that occur with the change of seasons. Nearly 8 percent of adults in the United States experience allergic rhinitis of some kind, according to the American Academy of Allergy, Asthma & Immunology (AAAAI). Between 10 and 30 percent of the worldwide population may also have allergic rhinitis.