Drug allergy

Table 2 Classification of allergic drug reactions: mechanisms, clinical manifestations, and timing of reactions [10,11,12,13].

Immune reaction	Mechanism	Clinical manifestations	Timing of reaction
Type I (IgE-mediated)	Drug-IgE complex binding to mast cells with release of histamine, inflammatory mediators	Anaphylaxis^a, urticaria^a, angioedema^a, bronchospasm^a	Minutes to hours after drug exposure
Type II (cytotoxic)	Specific IgG or IgM antibodies directed at drug-hapten coated cells	Anemia, cytopenia, thrombocytopenia	Variable
Type III (immune complex)	Tissue deposition of drug-antibody complexes with complement activation and inflammation	Serum sickness, vasculitis, fever, rash, arthralgia	1 to 3 weeks after drug exposure
Type IV (delayed, cell mediated)^b	MHC presentation of drug molecules to T cells with cytokine and inflammatory mediator release; may also be associated with activation and recruitment of eosinophils, monocytes, and neutrophils	Contact dermatitis, delayed morbilliform reactions, organ damage	2 to 7 days after drug exposure; can be up to 8 weeks

AGEP: acute generalized exanthematous pustulosis; DRESS: drug reaction with eosinophilia and systemic symptoms; IgE: immunoglobulin E; IgG: immunoglobulin G; IgM: immunoglobulin G; MHC: major histocompatibility complex
^aThese reactions may also be non-immunologically mediated
^bType IV reactions can be further classified into the following subtypes: type IVa which involve macrophages (e.g., contact dermatitis); type IVb which involve eosinophils (e.g., DRESS syndrome); type IVc which involve CD4 + or CD8 + T cells (e.g., maculopapular); and type IVd which involve neutrophils (e.g., AGEP)

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