Anaphylaxis is a rapid, severe allergic response that occurs when a person is exposed to an allergen, an allergy-causing substance, to which he or she has been previously sensitized.  It is brought on when the allergen enters the bloodstream, causing the release of chemicals throughout the body that try to protect it from the foreign substance.

Anaphylaxis has existed for many centuries.  The first report, in 2640 B.C., described the sudden death of the Egyptian pharaoh, Menes, after a wasp sting.  However, it was not until the early 1900s that the medical field began to understand the causes of sudden allergic emergency.