Anaphylaxis is an unusual or exaggerated allergic reaction, usually to drugs or to insect bites or stings.

The patient with an anaphylactic reaction may quickly develop symptoms of shock.

In this discussion, “anaphylaxis” and “anaphylactic shock” are used interchangeably. The most common drug that brings on anaphylactic shock is penicillin, although any drug is a potential source of anaphylactic shock.

“Anaphylaxis” is derived from “ana,” meaning without, and “phylaxis,” meaning protection. Hence, the casualty is without protection from the poison or drug. In anaphylaxis, the release of histamine is profound, causing the bronchial tree to constrict and go into spasm.