Anaphylaxis is an acute systemic (multi-system) and very severe Type I Hypersensitivity allergic reaction in humans and other mammals. The term comes from the Greek words ανα ana (against) and φύλαξις phylaxis (protection). Minute amounts of allergens may cause a life-threatening anaphylactic reaction. Anaphylaxis may occur after ingestion, skin contact, injection of an allergen or, in some cases, inhalation.

Anaphylactic shock, the most severe type of anaphylaxis, occurs when an allergic response triggers a quick release from mast cells of large quantities of immunological mediators (histamines, prostaglandins, leukotrienes) leading to systemic vasodilation (associated with a sudden drop in blood pressure) and edema of bronchial mucosa (resulting in bronchoconstriction and difficulty breathing). Anaphylactic shock can lead to death in a matter of minutes if left untreated.

An estimated 1.24% to 16.8% of the population of the United States is considered "at risk" for having an anaphylactic reaction if they are exposed to one or more allergens, especially penicillin and insect stings. Most of these people successfully avoid their allergens and will never experience anaphylaxis. Of those people who actually experience anaphylaxis, up to 1% may die as a result. Anaphylaxis results in approximately 18 deaths per year in the U.S. (compared to 2.4 million deaths from all causes each year in the U.S. ). The most common presentation includes sudden cardiovascular collapse (88% of reported cases of severe anaphylaxis).

Researchers typically distinguish between "true anaphylaxis" and "pseudo-anaphylaxis" or an "anaphylactoid reaction." The symptoms, treatment, and risk of death are identical, but "true" anaphylaxis is always caused directly by degranulation of mast cells or basophils that is mediated by immunoglobulin E (IgE), and pseudo-anaphylaxis occurs due to all other causes. The distinction is primarily made by those studying mechanisms of allergic reactions.

Causes

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

Hives and angioedema (swelling of 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. Some vaccinations are also known to cause "anaphylactoid" reactions. Antitoxins and antivenins may cause similar reactions.