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What Is Anaphylaxis?

Anaphylaxis is a sudden, severe allergic reaction that occurs when the body is exposed to something that it became sensitized to during a previous exposure. It usually (but not always) happens a few minutes to two hours after exposure.

The life-threatening reaction causes a sudden release of chemicals from cells in the blood and the body's tissues. The most common of these chemicals are called histamines. They cause blood vessels to dilate and blood pressure to lower. The first sign is usually hives and swelling, especially around the face and throat. These chemicals also act on the lungs, where they cause airways to constrict, making breathing very difficult.

In some cases, anaphylaxis may be mild, causing only hives and itching. But it can be deadly. Blood pressure can drop severely, leading to loss of consciousness. Swelling of the throat and airways can cause difficulty breathing, speaking, and swallowing. Anaphylaxis is a medical emergency.

What Causes Anaphylaxis?

Common allergies that may lead to anaphylactic shock are:

  • Foods
    • Peanuts and tree nuts are responsible for about 90% of fatal anaphylaxis
    • Shellfish, especially shrimp and lobster
    • Dairy products
    • Eggs
  • Stings from wasps, bees, or ants. As many as one in twenty Americans are sensitive to insect venoms. These allergic reactions are on the rise, because of the northward spread of fire ants and Africanized bees in the U.S.
  • Latex
  • Medications

The most severe cases may be fatal just 10 minutes after exposure. Sometimes symptoms may disappear and then return later.

If administered in time, an injection of epinephrine (adrenaline) will stop the anaphylaxis. People with known allergies usually are prescribed an EpiPen® to carry and administer in case of an allergic reaction.

Anaphylaxis can happen fast, so signs of an allergic reaction should be taken seriously. Do not wait to see how serious the reaction may become: call 911 right away if you or your child experiences any one of the following:

  • Swelling, itching and redness, especially around the face and neck
  • Worsening hives
  • Gasping for air, trouble breathing, wheezing
  • Difficulty swallowing

For more information about allergies and allergic reactions, visit the American Academy of Allergies, Asthma & Immunology website.


If you have questions about anaphylaxis, please call a public health nurse at 617-665-3800.


Food Allergies

Insect Bites

How to administer an EpiPen®



Cambridge Public Health Department
119 Windsor Street
Cambridge, MA 02139
617-665-3826 (main)
617-665-3800 (main)
617-665-3888 (fax)
617-643-0744 (TTY)

Hours: Monday-Friday:
8:30 a.m.–5 p.m.
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Claude-Alix Jacob
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Cambirdge Health Alliance

Cambridge City
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