Severe Allergic Reaction (Anaphylactic Shock)

Reviewed on 7/8/2022

What Is Anaphylactic Shock (Severe Allergic Reaction)?

Anaphylaxis, a severe allergic reaction, can cause difficulty breathing and potentially lead to death.
Anaphylaxis, a severe allergic reaction, can cause difficulty breathing and potentially lead to death.
  • Anaphylaxis is a severe allergic reaction that occurs rapidly and causes a life-threatening response involving the whole body. This reaction can lead to difficulty breathing and shock ultimately leading to death.
  • Most frequently, severe allergic reactions occur due to reactions to food, insect stings, or medications (immunologic causes), but they can also occur due to non-immunologic causes.
  • For an anaphylactic reaction to occur, you must have been exposed in the past to the substance that causes the reaction, called the antigen. This is called "sensitization."
  • Another bee sting may produce a sudden, severe allergic reaction known as anaphylaxis or anaphylactic shock.
  • These reactions usually occur within seconds to minutes of exposure. Occasionally, they are delayed.
  • You may develop sensitivity and anaphylaxis to a substance that you have been exposed to many times in the past without a reaction, and often people don't recall the previous exposure.

What Causes Anaphylactic Shock (Severe Allergic Reaction)?

An anaphylactic reaction occurs when the body's immune system overreacts to an antigen, which it recognizes as an "invader" or foreign substance.

  • The body's white blood cells produce substances called antibodies as a reaction to that antigen. The antibodies circulate in the bloodstream and attach themselves to certain cells in the body.
    • In an allergic reaction, the antibody is called immunoglobulin E, or IgE.
    • When the antibodies come in contact with the antigen, they signal other cells to produce certain chemicals called "mediators." Histamine is an example of a mediator.
    • The effects of these mediators on organs and tissues of the body cause the symptoms of the reaction.
  • Triggers of anaphylaxis include many substances. Only a trace amount of the trigger may be needed to cause a severe reaction. Triggers of allergic reactions, including anaphylaxis, may include:
    • Prescription and over-the-counter medications (see Drug allergy)
    • Venom of stinging insects such as yellow jackets, bumble bees, honey bees, wasps, fire ants
    • Foods, especially high-protein foods - most commonly, shellfish, fish, nuts, fruit, wheat, milk, eggs, soy products (see Food allergy)
    • Food additives, such as sulfites
    • Transfusion of blood or blood products
    • Numerous other substances such as latex (natural rubber)
    • Dyes and contrast materials used during radiologic procedures or tests
  • Sometimes the trigger of the reaction is obvious -- a bee sting, or a new prescription drug. Often, however, the trigger is unknown.
  • People with asthma, eczema, or hay fever are slightly more likely to have an anaphylactic reaction than people who do not have these conditions.

What Are the Symptoms of Anaphylactic Shock (Severe Allergic Reaction)?

The symptoms of anaphylaxis can vary. In some people, the reaction begins very slowly, but in most the symptoms appear rapidly and abruptly.

The most severe and life-threatening symptoms are difficulty breathing and loss of consciousness.

  • Difficulty breathing is due to swelling and/or spasm in the airways (which can include swelling of the tongue or the large and small parts of the airways). In very rare cases, breathing can stop altogether.
  • Loss of consciousness is due to dangerously low blood pressure, which is called "shock."
  • In the most serious cases, the heart can stop pumping altogether.
  • These events can lead to death from anaphylaxis.
  • While some symptoms are life threatening, others are merely uncomfortable.

Generally, a reaction must involve at least two different body systems, such as skin and heart, to be considered anaphylaxis.

Skin: Most anaphylactic reactions involve the skin.

  • Hives (urticaria, welts, or wheals [raised bumps]): Hives can cause severe itching
  • Generalized erythema (redness)
  • Swelling in the face, eyelids, lips, tongue, throat, hands, and feet
  • Angioedema

Breathing: Swelling of the surrounding tissues narrows the airways.

  • Difficulty breathing, wheezing, chest tightness
  • Coughing, hoarseness
  • Nasal congestion, sneezing

Cardiovascular: Blood pressure may drop to dangerously low levels.

  • Rapid or irregular heartbeat
  • Dizziness, faintness
  • Loss of consciousness, collapse


When Should I Call a Doctor for an Allergic Reaction?

Act quickly if someone experiences the symptoms of an anaphylactic reaction. True anaphylaxis is a medical emergency and requires immediate treatment in an emergency department of a hospital, where the person can be watched closely and life-saving treatment can be given.

  • It is impossible to predict how severe the allergic reaction will be. Any person who shows symptoms of anaphylaxis must be transported to a hospital emergency department.
  • If swelling develops rapidly, particularly involving the mouth or throat, and you have trouble breathing or feel dizzy, light-headed, or faint, call 911 for ambulance transport to the hospital.

How Do Medical Professionals Diagnose a Severe Allergic Reaction?

Anaphylactic reactions are diagnosed solely on the basis of signs and symptoms.

  • No specific tests are helpful.
  • Your health care provider may order tests to rule out other conditions.

Can I Take Benadryl for an Allergic Reaction?

Do not attempt to treat severe reactions or to "wait it out" at home. Go immediately to the nearest emergency department or call an ambulance.

While waiting for the ambulance, try to stay calm.

  • If you can identify the cause of the reaction, prevent further exposure.
  • Take an antihistamine (one to two tablets or capsules of diphenhydramine [Benadryl]) if you can swallow without difficulty.
  • If you are wheezing or having difficulty breathing, use an inhaled bronchodilator such as albuterol (Proventil) if one is available. These inhaled medications dilate the airway.
  • If you are feeling light-headed or faint, lie down and raise your legs higher than your head to help blood flow to your brain.
  • If you have been given an epinephrine kit (EpiPen), inject yourself as you have been instructed or have someone else perform the injection. The kit provides a premeasured dose of epinephrine, a prescription drug that rapidly reverses the most serious symptoms of anaphylaxis (see Follow-up).
  • Bystanders should administer CPR to a person who becomes unconscious and stops breathing or does not have a pulse.

If at all possible, you or your companions should be prepared to tell medical personnel what medications you take and your allergy history.


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What Is the Treatment for a Severe Allergic Reaction?

The first priority in the emergency department is to protect the airway (breathing) and maintain adequate blood pressure.

The emergency team will make sure that your airway is open and that you are getting adequate oxygen.

  • Oxygen may be given through tubes into the nose or by face mask.
  • In severe respiratory distress, mechanical ventilation may be required. In this situation, a tube is placed via the mouth into the air passages to keep the airway open. The tube is connected to a ventilator (providing oxygen directly into the lung).
  • In rare cases when the swelling prevents placing an airway through the mouth, a surgery is performed to open an airway (tracheostomy).

If blood pressure is dangerously low, medication to increase blood pressure will be given.

  • An intravenous (IV) catheter will be inserted.
  • This is used to give saline solution to help boost blood pressure.
  • The IV line may also be used to give medication to combat the reaction, as well as medications to stabilize blood pressure.

You may need to be admitted to the hospital for further monitoring and treatment.

What Medications Treat Severe Allergic Reactions?

  • Epinephrine: Given in severe allergic reactions, epinephrine is extremely effective and fast-acting; it acts by constricting blood vessels, which increases blood pressure, and widening the airway. Epinephrine is given by injection into the muscle, through an IV line, or by injection under the skin.
  • H1-receptor blockers/antihistamines: Usually diphenhydramine (Benadryl); these drugs do not stop the reaction but relieve some of the symptoms. They may be given by IV, by injection in the muscle, or by mouth
  • Inhaled beta-agonists (albuterol): Used to treat bronchospasm (spasms in the lung) and dilate the airways; inhaled
  • H2-receptor blockers: Usually ranitidine (Zantac); given by IV or by mouth
  • Corticosteroids (prednisone, Solu-Medrol): These drugs help decrease the severity and recurrence of symptoms; may be given orally, injected in muscle, or by IV line
  • If low blood pressure does not improve, additional medications, such as dopamine, may be given.

What Is the Follow-up Care for Severe Allergic Reaction?

You will usually be observed for at least six hours after the beginning of the reaction. Occasionally, a reaction will seem to get better and then recur, and even worsen, in a few hours. Sometimes the severity of the reaction will require admission to the hospital.

Upon leaving the hospital emergency department, you should immediately obtain the medication prescribed for you. You should carry these at all times to prevent another reaction or lessen its severity.

  • The epinephrine autoinjector (known as EpiPen) should be kept with you at all times in case you are exposed to the antigen that caused the first reaction.
  • The autoinjector contains a premeasured dose of epinephrine in an easy-to-use syringe. As soon as an exposure occurs, you immediately inject the epinephrine into your thigh muscle. This is extremely effective and fast-acting.
  • Anyone who has experienced an anaphylactic reaction should carry one of these autoinjectors after consulting with your physician.
  • Medical attention is always required right away, even if you have treated yourself with epinephrine.
  • A follow-up appointment with your primary care doctor, and possibly an immunologist, should be made.

How Do I Prevent a Severe Allergic Reaction?

Strictly avoid contact with the substance (allergen) that was the trigger.

  • If the trigger is a food, you must learn to read food labels carefully. When ordering foods at restaurants or eating in friends' homes, ask about ingredients. Be aware of ingredients that may contain triggers. Avoid eating foods if you can't confirm their ingredients. If your reactions are severe, contact the manufacturer to assure that the triggering food was not processed in the same area as a food to which you are allergic.
  • If the trigger is a drug, inform all health care providers of the reaction. Be prepared to report what happened when you had the reaction. Wear a tag (necklace or bracelet) that identifies the allergy. Make sure all your medical records are updated to include this allergy.
  • Insect stings are more difficult to avoid. Wear long-sleeved clothing outdoors. Avoid bright colors and perfumes that attract stinging insects. Use caution with sweetened beverages outdoors, such as uncovered soft drinks.

People who are likely to be re-exposed to (or are unable to avoid) an allergen that has caused them a severe anaphylactic reaction in the past should see an allergist for desensitization. Skin testing may be required to help identify the allergen.

What Is the Prognosis for a Severe Allergic Reaction?

With appropriate and timely treatment, you can expect full recovery. With severe anaphylaxis, although rare, people may die from low blood pressure (shock) or respiratory and cardiac arrest.

Where Can People Find More Information About Severe Allergic Reaction?

American Academy of Allergy, Asthma and Immunology

American College of Allergy, Asthma and Immunology

Food Allergy and Anaphylaxis Network

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Anaphylactic Shock Symptom

Low Blood Pressure

For low blood pressure to be a problem there needs to be a symptom associated with that low number. Readings below 120/80 may be normal depending upon the clinical situation. Many people have systolic blood pressures below 100, but some people develop symptoms with pressures that low. Symptoms of low blood pressure occur because one or more of the body's organs is not getting enough blood supply.

Reviewed on 7/8/2022
Jameson, J. Larry, et al. Harrison's Principles of Internal Medicine, 20th Ed. New York: McGraw-Hill Education, 2018.