anaphylactic shock Anaphylaxis is an acute systemic ( multi-system) and severe allergic reaction. The term comes from the Greek words ana (against) and phylaxis (protection).[1] Anaphylaxis occurs when a person is exposed to a trigger substance, called an allergen, to which they have already become sensitized. Minute amounts of allergens may cause a life-threatening anaphylactic reaction. Anaphylaxis may occur after ingestion, inhalation (though this is rare), skin contact, or injection of an allergen.[2] The most severe type of anaphylaxis—anaphylactic shock—will usually 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 may suffer from anaphylactic reactions, 0.002% of whom may experience fatal results.[3] Most common presentation is sudden cardiovascular collapse (88% of reported cases of severe anaphylaxis).[citation needed] Researchers typically distinguish between "true anaphylaxis" and "pseudo-anaphylaxis." 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 only important for researchers who are studying mechanisms of allergic reactions, and it may frustrate patients who feel they are being told that a life-threatening allergic reaction wasn't "real."