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An allergic reaction is an overreaction of the immune system to a substance called an allergen. Allergens include chemicals, foods, medicines, mould, plants, and pollen.
Symptoms of an allergic reaction can range from mild and annoying to severe and life-threatening.
Allergic reactions may not occur the first time a person is exposed to an allergen. A person may become more sensitive to the allergen with each exposure.
Current as of: June 27, 2018
Author: Healthwise Staff
Medical Review:William H. Blahd, Jr., MD, FACEP - Emergency Medicine & Adam Husney, MD - Family Medicine & Martin J. Gabica, MD - Family Medicine & H. Michael O'Connor, MD, MMEd, FRCPC - Emergency Medicine
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