Mechanisms of anaphylaxis and pseudoallergic syndromes

Proposed mechanism(s)	Characteristics
IgE-mediated activation of mast cells and basophils Examples: Reactions to penicillin and other beta-lactams and to neuromuscular-blocking agents	Can be evaluated with skin tests and/or in vitro tests for allergen-specific IgE Requires prior exposure to allergen or cross-reacting allergen Amenable to desensitization in some situations
Immunologic mechanisms NOT involving IgE (mediated by IgG or IgM, antigen:antibody complexes, and/or complement) Examples: Reactions to dextran, protamine	Formerly called anaphylactoid reactions Skin testing and in vitro IgE testing are NOT useful Pretreatment with glucocorticoids and/or antihistamines may be helpful
Direct (nonimmunologic) activation of mast cells and basophils Examples: Reactions to radiocontrast agents, opioids, and to neuromuscular-blocking agents	Formerly called anaphylactoid reactions Skin testing and in vitro IgE testing are NOT useful Pretreatment with glucocorticoids and/or antihistamines may be helpful Slow administration of responsible agent generally minimizes reactions

IgE: immunoglobulin E; IgG: immunoglobulin G; IgM: immunoglobulin M.

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