TABLE 1.

Types and classifications of HSRs^b

HSR classification	Reaction type	Immunologic mechanism	HSR name/symptom
I	IgE-mediated, immediate-type hypersensitivity	Antigen exposure causes IgE-mediated activation of mast cells and basophils with release of allergic mediators (histamine, prostaglandins, and leukotrienes).	Anaphylaxis, angioedema, bronchospasm, urticaria (hives)
II	Antibody dependent	An antigen or hapten on the cell binds to antibody (IgG), leading to cell or tissue injury.	Hemolytic anemia, thrombocytopenia, neutropenia
III	Immune complex disease	Antigen-antibody (IgG) complexes cause damage by complement activation and/or recruitment of neutrophils.	Serum sickness
IV	Cell-mediated, delayed-type hypersensitivity	Antigen exposure activates T cells, which then mediates tissue injury.	Maculopapular rash, organ-specific reactions,a SJS/TEN, DRESS syndrome

^aOrgan-specific reactions include acute interstitial nephritis, an immune-mediated nephritis, and immune-mediated hepatitis.

^bData are from references 2 to 4. Abbreviations: HSR, hypersensitivity reaction; SJS/TEN, Stevens-Johnson syndrome/toxic epidermal necrolysis; DRESS, drug rash eosinophilia and systemic symptoms.