Anaphylaxis is an acute multi-system severe type I hypersensitivity reaction. The term comes from the Greek words ἀνά ''ana'' (against) and φύλαξις ''phylaxis'' (protection).
Due in part to the variety of definitions, between 1% and 15% of the population of the United States can be considered "at risk" for having an anaphylactic reaction if they are exposed to one or more allergens. Of those people who actually experience anaphylaxis, up to 1% may die as a result.
Anaphylaxis results in approximately 1,500 deaths per year in the U.S. In England, mortality rates for anaphylaxis have been reported as up to 0.05 per 100,000 population, or around 10-20 a year.
Anaphylactic reactions requiring hospital treatment appear to be increasing, with authorities in England reporting a threefold increase between 1994 and 2004.
Based on the pathophysiology, anaphylaxis can be divided into "true anaphylaxis" and "pseudo-anaphylaxis" or "anaphylactoid reaction."
The symptoms, treatment, and risk of death are the same; however, "true" anaphylaxis is caused by degranulation of mast cells or basophils mediated by immunoglobulin E (IgE), and pseudo-anaphylaxis occurs without IgE mediation.
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