anaphylactic shock

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. A sudden, severe allergic reaction characterized by a sharp drop in blood pressure, urticaria, and breathing difficulties that is caused by exposure to a foreign substance, such as a drug or bee venom, after a preliminary or sensitizing exposure. The reaction may be fatal if emergency treatment, including epinephrine injections, is not given immediately. Also called anaphylaxis.

Wiktionary

1. n. A severe and rapid systemic allergic reaction to an allergen, constricting the trachea and preventing breathing.

GNU Webster's 1913

1. n. (Med.) a severe form of physiological shock, often having a fatal outcome, caused by an extreme immunological reaction to antigens. It is a severe form of anaphylaxis{2}, and is characterized by smooth muscle contraction and capillary dilation throughout the body, initiated by antibodies of the IgE class. It is usually seen when an antigen to which a person has become hypersensitized is injected intravenously or subcutaneously. It is also called generalyzed anaphylaxis or systemic anaphylaxis, and sometimes anaphylaxis.

WordNet 3.0

1. n. a severe and rapid and sometimes fatal hypersensitivity reaction to a substance (especially a vaccine or penicillin or shellfish or insect venom) to which the organism has become sensitized by previous exposure