Anaphylaxis is defined as “a severe allergic reaction that is rapid in onset and can cause death.” In the most severe forms, it is referred to as “anaphylactic shock.”

It is caused by a particular form of hypersensitivity, commonly called “allergy,” to an antigenic substance (called an allergen). Common causes include insect stings, foods and medications. It typically presents with a variety of clinical manifestations including itching, angioedema (swelling) of the face and throat, and rapid drop in systemic blood pressure.

At the pathophysiologic level, anaphylaxis is a type I hypersensitivity reaction due to the release of mediators by certain types of white blood cells activated by immune and non-immune mechanisms. It is diagnosed based on the symptoms and signs that occur.

Exposure to the substance can occur by inhalation, ingestion, contact or inoculation of the allergen. The reaction of anaphylaxis proper occurs against an antigen with which the subject has already come into contact previously.


The typical symptoms of the anaphylactic reaction are related to the massive and generalized release of histamine, a substance that causes swelling (edema), constriction of the bronchioles (bronchospasm), heart failure, cardiovascular failure, collapse and – in the most severe cases – even death. The therapy is based on the rapid administration of adrenaline (epinephrine), which is why anaphylactic shock requires prompt medical help.

The mechanism and symptomatology of anaphylactic shock are caused by immunoglobulin E (IgE) and other anaphylatoxins that induce the release of large amounts of histamine and other substances.

Anaphylaxis typically presents with many different symptoms that emerge within minutes or hours after contact, with an average onset of 5 to 30 minutes if the exposure is intravenous and 2 hours for food. The most commonly affected areas are: skin (80-90%), respiratory tract (70%), gastrointestinal system (30-45%), heart and vessels (10-45%), central nervous system (10-15%). More than one system may also be involved at the same time.


Typical signs and symptoms include generalized hives, itching, redness or swelling of the lips. Patients who experience swelling or angioedema describe a burning sensation of the skin, or itching. Swelling of the tongue or throat occurs in up to about 20% of cases. Other possible manifestations include rhinorrhea and swelling of the conjunctiva. If edema involves the upper or lower airways, the skin may also turn bluish (cyanosis) due to lack of oxygen (hypoxemia).

Respiratory system

A number of respiratory signs and symptoms may occur in anaphylaxis, including shortness of breath, asthma attacks, and the presence on auscultation of hissing or stridor. The hissing is usually due to spasms of the bronchial muscles while the stridor is related to obstruction of the upper airways due to a major inflammatory state or, in severe cases, to angioedema. Hoarseness and pain following swallowing or coughing may also occur.

Heart and circulatory system

Coronary spasm may occur in affected patients, resulting in possible myocardial infarction, arrhythmias, or cardiac arrest. Patients who have a history of coronary artery disease have an increased risk that the anaphylactic episode may affect the heart. Coronary spasm is related to the presence of histamine released from cells in the heart. The onset of tachycardia is frequent due to lowering of blood pressure (hypotension); a Bezold-Jarisch reflex has been described in 10% of cases: this results in a slowing of the heart rate (bradycardia) with low blood pressure. The drop in blood pressure or shock (either distributive or cardiogenic) may cause lightheadedness or lead to loss of consciousness. Rarely, lowered blood pressure may be the only sign of anaphylaxis.

Other effects

Gastrointestinal manifestations may include abdominal cramping, diarrhea, and vomiting. Confusion, loss of bladder control, or cramp-like pelvic pain are also possible. Dilation of blood vessels in the brain may cause headaches. Some anaphylaxis sufferers have also described a feeling of anxiety varying to a sense of “imminent death” probably due to breathing difficulties.

Leave a Comment