anaphylactic shock

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Anaphylaxis in which systemic vasodilation results in shock.

from the GNU version of the Collaborative International Dictionary of English.

• noun (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.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

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

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun 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