anaphylaxis

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

• noun A sudden, usually systemic allergic reaction, characterized by vasodilation resulting in decreased blood pressure, smooth muscle contraction resulting in shortness of breath, and hives, caused by hypersensitivity induced by a secondary exposure to a foreign substance, such as a drug or protein, after an initial sensitizing exposure.

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

• noun (Med.) hypersensitivity (to a protein or drug) resulting from prior contact with a substance.
• noun (Med.) an immediate but transient allergic reaction to an antigenic substance to which a person has become hypersensitized; it is characterized by the contraction of smooth muscle and capillary dilation. These reactions are caused by the release of certain substances such as histamine, bradykinin, serotonin, and slow-reacting substance in response to the reaction of the antigen with a mast cell-fixed antibody, chiefly IgE. This term is sometimes used to refer to the severe form of the reaction called anaphylactic shock.

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

• noun Extreme sensitivity to a substance such as a foreign protein or drug.
• noun A severe and rapid systemic allergic reaction to an allergen, causing a constriction of the trachea, preventing breathing; anaphylactic shock.

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

• noun hypersensitivity reaction to the ingestion or injection of a substance (a protein or drug) resulting from prior contact with a substance