Anaphylactic reaction

Anaphylactic reaction


Anaphylaxis is a life-threatening type of allergic reaction.

Alternative Names

Anaphylactic reaction; Anaphylactic shock; Shock - anaphylactic


Anaphylaxis is an severe, whole-body allergic reaction. After an initial exposure to a substance like bee sting toxin, the person's immune system becomes sensitized to that allergen. On a subsequent exposure, an allergic reaction occurs. This reaction is sudden, severe, and involves the whole body.

Tissues in different parts of the body release histamine and other substances. This causes constriction of the airways, resulting in wheezing, difficulty breathing, and gastrointestinal symptoms such as abdominal pain, cramps, vomiting, and diarrhea.

Histamine causes the blood vessels to dilate (which lowers blood pressure) and fluid to leak from the bloodstream into the tissues (which lowers the blood volume). These effects result in shock. Fluid can leak into the alveoli (air sacs) of the lungs, causing pulmonary edema.

Hives and angioedema (hives on the lips, eyelids, throat, and/or tongue) often occur. Angioedema may be severe enough to block the airway. Prolonged anaphylaxis can cause heart arrhythmias.

Some drugs (polymyxin, morphine, x-ray dye, and others) may cause an anaphylactoid reaction (anaphylactic-like reaction) on the first exposure. This is usually due to a toxic reaction, rather than the immune system mechanism that occurs with "true" anaphylaxis. The symptoms, risk for complications without treatment, and treatment are the same, however, for both types of reactions.

Anaphylaxis can occur in response to any allergen. Common causes include insect bites/stings, horse serum (used in some vaccines), food allergies, and drug allergies

Anaphylaxis occurs infrequently. However, it is life-threatening and can occur at any time. Risks include prior history of any type of allergic reaction.


Symptoms develop rapidly, often within seconds or minutes. They may include the following:

  • Difficulty breathing
  • Wheezing
  • Abnormal (high-pitched) breathing sounds
  • Confusion
  • Slurred speech
  • Rapid or weak pulse
  • Blueness of the skin (cyanosis), including the lips or nail beds
  • Fainting, light-headedness, dizziness
  • Hives and generalized itching
  • Anxiety
  • Sensation of feeling the heart beat (palpitations)
  • Nausea, vomiting
  • Diarrhea
  • Abdominal pain or cramping
  • Skin redness
  • Nasal congestion
  • Cough

Exams and Tests

Examination of the skin may show hives and swelling of the eyes or face. The skin may be blue from lack of oxygen or may be pale from shock. Angioedema in the throat may be severe enough to block the airway.

Listening to the lungs with a stethoscope may reveal wheezing or indicate fluid (pulmonary edema). The pulse is rapid, and blood pressure may be low. Weakness, pale skin, heart arrhythmias, mental confusion, and other signs may indicate shock.

Testing for the specific allergen that caused anaphylaxis (if the cause is not obvious) is postponed until after treatment.


Anaphylaxis is an emergency condition requiring immediate professional medical attention. Assessment of the ABC's (airway, breathing, and circulation from Basic Life Support) should be done in all suspected anaphylactic reactions.

CPR should be initiated if needed. People with known severe allergic reactions may carry an Epi-Pen or other allergy kit, and should be assisted if necessary. Emergency interventions by paramedics or physicians may include placing a tube through the nose or mouth into the airway (endotracheal intubation) or emergency surgery to place a tube directly into the trachea (tracheostomy or cricothyrotomy).

Epinephrine should be given by injection without delay. This opens the airways and raises the blood pressure by constricting blood vessels.

Treatment for shock includes intravenous fluids and medications that support the actions of the heart and circulatory system.

Antihistamines, such as diphenhydramine; and corticosteroids, such as prednisone may be given to further reduce symptoms (after lifesaving measures and epinephrine are administered).

Outlook (Prognosis)

Anaphylaxis is a severe disorder which has a poor prognosis without prompt treatment. Symptoms, however, usually resolve with appropriate therapy, underscoring the importance of action.

Possible Complications

  • Shock
  • Cardiac arrest (no effective heartbeat)
  • Respiratory arrest (absence of breathing)
  • Airway obstruction

When to Contact a Medical Professional

Go to the emergency room or call the local emergency number (such as 911) if severe symptoms of anaphylaxis develop.


Avoid known allergens. Any person experiencing an allergic reaction should be monitored, although monitoring may be done at home in mild cases.

Occasionally, people who have a history of drug allergies may safely be given the offending medication after pretreatment with corticosteroids (prednisone) and antihistamines (diphenhydramine).

People who have a history of allergy to insect bites/stings should be instructed to carry (and use) an emergency kit consisting of injectable epinephrine and chewable antihistamine. They should also wear a Medic-Alert or similar bracelet/necklace stating their allergy.

The diagnosis and management of anaphylaxis: an updated practice parameter. J Allergy Clin Immunol. 2005 Mar;115(3 Suppl):S483-523.

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