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.
- Allergens can affect different tissues in the body, such as the airways, eyes, gastrointestinal tract, nose, lungs, and skin.
- Some allergic reactions, such as hives or itching around an insect bite or where a plant or chemical touched the skin, affect only one area of the body.
- Other allergic reactions may affect the whole body, causing itching, swelling, or difficulty breathing.
- A severe allergic reaction (called anaphylaxis) can lead to shock and even death.
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.
Last Revised: July 3, 2013
Author: Healthwise Staff
Medical Review: William H. Blahd, Jr., MD, FACEP - Emergency Medicine & H. Michael O'Connor, MD - Emergency Medicine
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