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Home > Fitness & Health > Health Library > Allergic reaction
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 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: October 7, 2019
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 - Emergency Medicine
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