Allergic reactions and allergic diseases are the most common human disorders of immune regulation. Diseases may include localized responses in the skin and various portions of the airway, or systemic responses characterized by extensive skin involvement, severe airway compromise, or cardiovascular collapse. Mechanisms include mast cell or basophil activation by the cross-linking of allergen-specific homocytotropic IgE, cellular infiltration following mast cell or basophil mediator release, complement activation, the deposition of immune complexes in susceptible tissues, or the infiltration of activated T-lymphocytes. The degree of impairment from allergic disease varies widely, with most reactions posing minor inconvenience, but with rare episodes requiring intensive—and sometimes unsuccessful—interventions to prevent death.