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Anaphylactic reactions (anaphylaxis) are sudden, widespread, potentially severe and life-threatening allergic reactions.

• These reactions begin with a feeling of uneasiness, followed by tingling sensations and dizziness.
• People then rapidly develop severe symptoms, including generalized itching and hives, wheezing and difficulty breathing, fainting, or a combination of these and other allergy symptoms.
• These reactions can quickly become life threatening.
• Avoiding the trigger is the best approach.
• Affected people should always carry antihistamines and a self-injecting syringe of epinephrine.
• Anaphylactic reactions require emergency treatment.

Anaphylactic reactions are most commonly caused by the following:

• Drugs (such as penicillin)
• Insect stings
• Certain foods (particularly eggs, seafood, and nuts)
• Allergy injections (allergen immunotherapy)
• Latex

But they can be caused by any allergen. Like other allergic reactions, an anaphylactic reaction does not usually occur after the first exposure to an allergen but may occur after a subsequent exposure. However, many people do not recall a first exposure. Any allergen that causes an anaphylactic reaction in a person is likely to cause that reaction with subsequent exposures, unless measures are taken to prevent it.

Anaphylactoid Versus Anaphylactic Anaphylactoid reactions resemble anaphylactic reactions. However, anaphylactoid reactions, unlike anaphylactic reactions, may occur after the first exposure to a substance. For example, anaphylactoid reactions may occur after the first injection of certain drugs, such as polymyxin, pentamidineSome Trade Names NEBUPENT , opioids, or the radiopaque dyes sometimes used with x-ray procedures. Also, anaphylactoid reactions are not allergic reactions because immunoglobulin E (IgE), the class of antibodies involved in allergic reactions, does not cause them. Rather, the reaction is caused directly by the substance. AspirinSome Trade Names BAYER and other nonsteroidal anti-inflammatory drugs (NSAIDs) can cause anaphylactoid reactions in some people, particularly those with year-round allergic rhinitis and nasal polyps. Dyes that can be seen on x-ray (radiopaque dyes) are a common cause. Other triggers include blood transfusions and exercise. If possible, doctors avoid using radiopaque dyes in people who have anaphylactoid reactions to such dyes. However, some disorders cannot be diagnosed without dyes. In such cases, doctors use dyes that are less likely to cause reactions. In addition, drugs that block anaphylactoid reactions, such as prednisone, diphenhydramineSome Trade Names BENADRYL , or ephedrine, are usually given before the dye is injected.

Symptoms and Diagnosis

Anaphylactic reactions begin within 1 to 15 minutes of exposure to the allergen. Rarely, reactions begin after 1 hour. Symptoms vary, but people usually have the same symptoms each time.

The heart beats quickly. People may feel uneasy and become agitated. Blood pressure may fall, causing fainting. Other symptoms include tingling (pins-and-needles) sensations, dizziness, itchy and flushed skin, throbbing in the ears, coughing, a runny nose, sneezing, hives, and swelling of tissue under the skin (angioedema). Breathing may become difficult and wheezing may occur because the windpipe (upper airway) constricts or becomes swollen. People may have nausea, vomiting, abdominal cramps, and diarrhea.

An anaphylactic reaction may progress so rapidly that it leads to collapse, cessation of breathing, seizures, and loss of consciousness within 1 to 2 minutes. The reaction may be fatal unless emergency treatment is given immediately.

The diagnosis is based on symptoms. Because symptoms can quickly become life threatening, no tests are done.

Prevention and Treatment

Avoiding the allergen is the best prevention. People who are allergic to certain unavoidable allergens (such as insect stings) may benefit from long-term allergen immunotherapy (see Anaphylactoid Versus Anaphylactic).

People who have these reactions should always carry a self-injecting syringe of epinephrine and antihistamine tablets for prompt treatment. If they encounter a trigger (for example, if they are stung by an insect) or if they start to develop symptoms, they should immediately inject themselves and take the antihistamines. Usually, this treatment stops the reaction. Nonetheless, after a severe allergic reaction and immediately after injecting themselves, such people should go to the hospital emergency department, where they can be closely monitored and treatment can be adjusted as needed. People should also wear a Medic Alert bracelet with their allergies listed.

In emergencies, doctors give epinephrine by injection under the skin, into a muscle, or into a vein. If breathing is severely impaired, a breathing tube may be inserted into the windpipe (trachea) through the person's mouth or nose (intubation) or through a small incision in the skin over the trachea. If blood pressure is very low, fluids are given intravenously, sometimes with drugs that cause blood vessels to narrow (vasoconstrictors). Antihistamines (such as diphenhydramineSome Trade Names
BENADRYL
) and histamine-2 (H₂) blockers (such as cimetidineSome Trade Names
TAGAMET
) are given intravenously until symptoms disappear. Beta-agonists that are inhaled (such as albuterolSome Trade Names
PROVENTIL-HFA VENTOLIN HFA
) are given to widen the airways and help with breathing.

Last full review/revision September 2008 by Peter J. Delves, PhD