Allergies are an overreaction of the immune system to substances that generally do not affect other individuals. These substances, or allergens, can cause sneezing, coughing, and itching. Allergic reactions range from merely bothersome to life-threatening. Some allergies are seasonal, like hay fever. Allergies have also been associated with chronic conditions like sinusitis and asthma.
Anyone may have or develop an allergy - from a baby born with an allergy to cow's milk, to a child who gets poison ivy, to a senior citizen who develops hives after taking a new medication.
Allergies can generally not be prevented but allergic reactions can be. Once a person knows they are allergic to a certain substance, they can avoid contact with the allergen. Strategies for doing this include being in an air-conditioned environment during peak hay-fever season, avoiding certain foods, and eliminating dust mites and animal dander from the home. They can also control the allergy by reducing or eliminating the symptoms. Strategies include taking medication to counteract reactions or minimize symptoms and being immunized with allergy injection therapy.
"Allergies" definition provided by Communication at CDC.