ABSTRACT
Acute systemic anaphylaxis is a type I IgE-mediated hypersensitivity reaction (Fig. 32.1) in which the response is so rapid and overwhelming as to be life-threatening. As with any type I hypersensitivity reaction, the first exposure to the allergen generates allergen-specific IgE antibodies, which become bound to Fc receptors (FceRI) on the surface of mast cells. Reexposure to the same allergen, usually by the same route, leads to an allergic reaction due to cross-linking of the IgE antibodies by the allergen. Antibody cross-linking induces the mast cells to release a variety of chemical mediators, particularly histamine, which increases the permeability of blood vessels, and leukotrienes, which affect smooth muscle, causing bronchospasm (see Fig. 33.4).
