Anaphylaxis is a severe allergic reaction that may occur when you come into contact with something you are allergic to (called an allergen). Symptoms of the reaction usually start within minutes to an hour or two after contact with the allergen. In rare cases, symptoms may occur up to 4 hours later. Examples of substances that can cause some people to have a severe allergic reaction are penicillin and bee stings.

If you have a severe allergy your blood pressure may drop suddenly. Because there is less blood flow, less oxygen reaches your brain and other vital organs. Since these organs do not function properly with a lack of oxygen, your body goes into shock. In addition, your body responds to the allergen by releasing chemicals that cause swelling of the skin, a red rash, and severe itching.

Complications of anaphylaxis can include brain damage, kidney failure, and death.

Anaphylaxis can be caused by an allergic reaction to foods, insects, allergy shots, medicines, vaccines, pets, smoke, or other chemicals.