Allergic reactions are sensitivities to a specific substance, called an allergen, that makes contact with the skin, or is inhaled into the lungs, swallowed, or injected. Some allergic reactions are mild, while others can be severe and life-threatening. Allergic reactions often occur more frequently in people with a family history of allergies. Substances that don't bother some people (including certain medications) can trigger allergic reactions in others.

Reviewed: January 2012