Anaphylaxis: An overwhelming allergic reaction

Swift action is needed to short-circuit potentially deadly symptoms.

Sarah Lyman had no reason to worry when her husband John left the house for a jog after lunch: he looked his usual healthy self. Twenty minutes later, she got word that he had collapsed by the side of the road — fighting for breath. At the hospital, she learned that the cause was anaphylaxis (also called anaphylactic shock or allergic shock), likely brought on by the lobster salad they'd eaten for lunch. Fortunately, John was treated in time and survived. That he was allergic to shellfish was news to him.

Anaphylaxis is a severe and sometimes life-threatening reaction that can develop within an hour — and sometimes within minutes or even seconds — after exposure to an allergen, a substance to which an individual's immune system has become sensitized. Many allergens can touch off anaphylaxis, including foods, medications, and insect stings (see "Anaphylaxis triggers"). In John Lyman's case, his postprandial jog likely played a role: anaphylaxis is occasionally triggered by aerobic activity like jogging — especially after ingesting allergenic foods or medications. Sometimes, the cause is unknown.

Anaphylaxis occurs when allergen-sensitized cells in blood and other tissues release large amounts of histamine and other inflammation-causing chemicals. While most allergic reactions involve only one physiological system (the upper respiratory tract or the skin, for example), anaphylaxis is a cascading response involving multiple systems. (See "Signs and symptoms of anaphylaxis in affected body systems," below.)

Symptoms are variable but can include flushing, itching, nasal congestion, wheezing, difficulty breathing, and swelling of the throat and tongue, sometimes accompanied by nausea, vomiting, and diarrhea. Blood pressure may drop precipitously, causing faintness. An immediate injection of epinephrine (adrenaline) can stop the cascade, which otherwise may result in fainting, shock, and even death.

Anaphylaxis is probably more common than once thought. It's treated in many different places — hospitals, emergency rooms, and clinicians' offices, as well as non-medical settings — and health authorities don't keep track of cases, so there's no single source of data on it. Also, because the symptoms are so variable, anaphylaxis may be confused with something else, such as an asthma attack, a panic attack, even an intestinal infection or food poisoning. And people with mild symptoms may not seek medical help. A 2006 study by Harvard Medical School researchers found evidence that anaphylaxis is vastly underreported as the cause of serious allergic reactions treated in emergency rooms — a problem, because proper diagnosis is the first step in preventing another anaphylactic reaction.

In 2008, a panel convened by the American College of Allergy, Asthma, and Immunology reviewed data from a number of sources and concluded that anaphylaxis affects 1% to 2% of the population, and the frequency is increasing (Current Opinion in Allergy and Clinical Immunology, August 2008). The study also found that risk is higher in women than in men. (According to the panel, laboratory findings suggest that the female hormone progesterone may boost the body's response to an allergen.) Fatal anaphylaxis is thought to be rare, but underreporting could be a problem here, too, because the reaction may be overlooked as the cause of death in people who have asthma, lung disease, or cardiovascular disease.

Anyone who's had anaphylaxis is at risk for further such reactions in the future, so it's important to identify and avoid the triggers and to prepare for any accidental exposure. That means having self-injectable epinephrine on hand at all times and knowing how to use it.

Anatomy of allergy and anaphylaxis

Allergies typically develop for two reasons: first, genetic predisposition (your risk of developing allergies is 50% if one parent has allergies, 70% if both parents are allergy sufferers); and second, environmental factors, especially in early childhood. According to the "hygiene hypothesis," the immune system in people who aren't exposed to a wide variety of germs early in life is more likely to incorrectly develop an allergic immune reaction to harmless foreign antigens.

Having an allergy means that your immune system reacts to an allergen as a threat and mounts a defense against it each time it comes in contact with it. Your first contact with the allergen may produce no obvious symptoms, but it stimulates the production of large amounts of an antibody protein called immunoglobulin E, or IgE. In allergy-prone people, IgE is produced in response to generally harmless substances, such as a food or medication. IgE locks onto immune cells (mast cells in tissue and basophils in blood) to prepare for the next encounter with the allergen — a process known as sensitization. Now, whenever you're subsequently exposed to the allergen, IgE signals the mast cells and basophils to disgorge inflammation-causing chemicals called mediators. The symptoms depend on the mediator and the tissue in which it's released — for example, the mediator histamine can cause blood vessels to dilate and airways in the lungs to narrow.

An anaphylactic reaction usually comes on fast, and it involves at least two different body systems (the skin and the lungs, for example). Under certain circumstances, a drop in blood pressure alone may be a tip off that you're having an anaphylactic reaction. Your susceptibility to anaphylaxis is increased if you have a history of allergies, a previous episode of anaphylaxis, or asthma (even in a mild form) in addition to a food allergy. Asthma that isn't well controlled raises the risk of death from anaphylaxis, as does cardiovascular disease.

Certain cardiovascular medications (alpha-adrenergic blockers and beta blockers) can lower the effectiveness of epinephrine, the key treatment for anaphylaxis; so if you're taking one of those medications and have allergies, consult your clinician.

What to do

If you have a history of anaphylaxis, you should carry injectable epinephrine with you at all times. It comes in an autoinjector device (EpiPen, Twinject), available by prescription. One study suggests that epinephrine isn't as widely prescribed as it should be; if you haven't received a prescription, speak to your clinician. Another problem is that people are not always instructed in how to use the device. When the need arises, you will be under great stress, so you must know what to do in advance. It's best to carry two devices, in case one malfunctions.

Inject epinephrine at the first sign of intensifying allergic symptoms, especially lightheadedness, trouble breathing, or tightness in the throat. Other drugs used to treat allergies, such as antihistamines and asthma inhalers, can help with some symptoms (hives, for example) but not the most dangerous ones. Epinephrine is the only drug that affects all the physical changes that occur with anaphylaxis. It prevents or reverses airflow obstruction and protects against cardiovascular collapse (the sudden loss of blood flow due to cardiac or vascular factors), which are the chief causes of death from anaphylaxis.

Here's how to use an epinephrine autoinjector device:

• If possible, lie down before using your epinephrine injector (but don't delay if lying down isn't an option). Grasp the device firmly around its center with your writing hand, making a fist.

• With the other hand, remove the safety cap. (Leaving the cap on is a common mistake.)

• Rest the needle end of the device on your outer thigh, and push it in hard until it clicks. (The needle is designed to go through clothes, so don't waste time adjusting them.)

• Leave the device in place for a count of 10; remove it and check to see that the container has emptied. Massage the injected area for 10 seconds.

• Call 911, or ask someone to make the call. Put the injector back into its case, needle end first, and take it to the hospital for disposal.

• Be prepared to use your second EpiPen (or second Twinject dose) if you don't get relief within 20 minutes, or if there's a delay in getting to an emergency room and symptoms recur.

• If you're feeling weak or dizzy, lie down with your legs elevated. Do not try to sit up; it may prevent blood from reaching the heart and brain.

Even if the injection relieves your symptoms, you should get to the emergency room as soon as possible. Up to 20% of people with anaphylaxis have a biphasic pattern — that is, symptoms return (usually within eight hours) after the original reaction has seemingly ended. Depending on the severity of your reaction, you may need treatment with oxygen, a breathing tube, intravenous fluids, and various medications.

If you're susceptible to anaphylaxis, it's important to take preventive measures. See an allergist for a full evaluation. She or he can perform tests to help identify the allergen that triggered your reaction and advise you on how to avoid problems in the future. Make sure your epinephrine prescription is up to date and keep it with you at all times. Don't let it freeze nor repeatedly expose it to extreme heat.

If you've had an anaphylactic reaction or any systemic response to an insect sting, the main preventive treatment is immunotherapy — allergy shots, which are 97% effective at preventing a severe or life-threatening reaction from future stings. Researchers are working on immunotherapies for some food allergies, but their work is still considered experimental.

If you've ever had an anaphylactic reaction, wear a medical identification bracelet or necklace with a list of known allergies as well as the names and phone numbers of emergency contacts. (You can buy a medical bracelet through the nonprofit MedicAlert Foundation, 888-633-4298, www.medicalert.org.) Make sure family and close friends know about your allergies and when and how to administer your epinephrine in case you can't do it yourself.