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
- 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 do not occur the first time a person is
exposed to an allergen. A person may become more sensitive to the allergen with