Allergic Reaction

Skip to the navigation

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 all over, swelling, fainting, belly pain, nausea, vomiting, 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.

Current as of: February 20, 2015

Author: Healthwise Staff

Medical Review: William H. Blahd, Jr., MD, FACEP - Emergency Medicine & H. Michael O'Connor, MD - Emergency Medicine