A sensitivity to a specific substance, called an allergen , that is contacted through the skin, inhaled into the lungs, swallowed, or injected. Allergic reactions are fairly common. They occur more often in people with a family history of allergies .
Most reactions happen soon after contact with an allergen .
Most of these reactions are mild and can be treated at home. While first-time exposure may only produce a mild reaction, repeated exposures may lead to more serious reactions. Allergic reactions vary. They can be mild or serious. They can be confined to a small area of the body or may affect the entire body.