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Medical Content Created by the Faculty of the
Harvard Medical School

Anaphylaxis

What Is It?

Anaphylaxis is a severe, sometimes life-threatening, allergic reaction that occurs within minutes of exposure an allergy-causing substance (allergen). Anaphylaxis also is called anaphylactic shock.

In an allergic reaction, the body's immune system responds to the presence of an allergen by releasing histamine and other body chemicals. These chemicals cause the symptoms of allergies, which are usually mild but annoying, such as the runny nose of hay fever (allergic rhinitis) or the itchy rash of poison ivy. However, in some cases, the symptoms can be much worse and involve the entire body. Anaphylaxis is the most severe allergic reaction. In anaphylaxis, these immune chemicals cause serious skin symptoms, such as hives and swelling, as well as severe breathing problems, such as swelling in the throat, narrowing of the lower airways and wheezing). The chemicals also cause blood vessels to widen dramatically, which leads to a rapid, severe drop in blood pressure (shock). Anaphylaxis is a life-threatening medical emergency.

Although the specific allergen that triggers anaphylaxis may be different for each patient, it often can be traced to one of the following sources:

• Foods — Especially eggs, seafood, nuts, grains, milk and peanuts

• Drugs — Especially an antibiotic from the penicillin or cephalosporin group

• Insect stings — From bees, yellow jackets, paper wasps, hornets or fire ants

• Injected anesthetics — Procaine, lidocaine

• Dyes — Used in diagnostic X-rays and scans

• Industrial chemicals — Latex and rubber products used by health care workers

• Allergy shots (immunotherapy)