An allergic reaction is an overreaction of the immune system to a substance called an allergen. Allergens include chemicals, foods, medicines, mold, plants, and pollen.
Symptoms of an allergic reaction can range from mild and annoying to severe and life-threatening.
Allergic reactions do not occur the first time a person is exposed to an allergen. A person may become more sensitive to the allergen with each exposure.
WebMD Medical Reference from Healthwise