Anaphylaxis

the immediate intense kind of immunologic (allergic) reaction characterized by contraction of smooth muscle and dilation of capillaries due to release of pharmacologically active substances (histamine, bradykinin, serotonin, and slow-reacting substance).

Description
Anaphylaxis is classically initiated by the combination of an antigen (allergen) with mast-cell-fixed, cytophilic antibody (mainly IgE); the reaction can also be initiated by relatively large quantities of serum aggregates (antigen-antibody complexes, and other immune complexes) that activate complement leading to production of anaphylatoxin, a reaction sometimes termed “aggregate anaphylaxis.