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.

Are all allergic reactions serious?

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.