Drug Hypersensitivity Reactions: Gell-Coombs Classification

Clinical meaning

Drug hypersensitivity reactions are immune-mediated adverse drug reactions that are unpredictable, not dose-dependent (in contrast to pharmacological side effects), and require prior sensitization (except for some Type I reactions mediated by pre-existing cross-reactive IgE). The Gell-Coombs classification organizes these reactions into four types based on the immune mechanism:

Type I — Immediate (IgE-mediated) hypersensitivity: The classic 'allergic' reaction. During initial exposure, the drug (or drug-protein hapten complex) stimulates B cells to produce drug-specific IgE antibodies, which bind to FcεRI receptors on mast cells and basophils (SENSITIZATION phase — no symptoms). Upon RE-EXPOSURE, the drug cross-links two adjacent IgE molecules on the mast cell surface, triggering immediate degranulation and release of preformed mediators (histamine, tryptase, prostaglandins, leukotrienes). Clinical manifestations occur within MINUTES to 1 HOUR: urticaria (hives), angioedema, bronchospasm, and in the most severe form, anaphylaxis (cardiovascular collapse, laryngeal edema, distributive shock). Most common causative drugs: beta-lactam antibiotics (penicillins, cephalosporins — the most common cause of drug-induced anaphylaxis), NSAIDs (via COX-1 inhibition causing leukotriene shunting — technically a pseudoallergic mechanism, not IgE-mediated), neuromuscular blocking agents, and latex.

Type II — Cytotoxic (IgG/IgM-mediated) hypersensitivity: Drug molecules bind to cell surface proteins (on red blood cells, platelets, or neutrophils), acting as haptens. IgG or IgM antibodies recognize the drug-cell surface complex, leading to cell destruction via complement activation (membrane attack complex), antibody-dependent cellular cytotoxicity (ADCC by NK cells), or opsonization and phagocytosis. Onset: hours to days. Clinical manifestations depend on the target cell: hemolytic anemia (penicillin, methyldopa, cephalosporins), thrombocytopenia (heparin-induced thrombocytopenia type II, quinidine, vancomycin), agranulocytosis (clozapine, methimazole, TMP-SMX), or interstitial nephritis. Classic example: methyldopa-induced autoimmune hemolytic anemia (positive direct Coombs test).

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Diagnosis & workup

Diagnostics & workup: - Serum tryptase level (for suspected Type I — anaphylaxis): peak tryptase occurs 60-90 minutes after mast cell degranulation; a level >11.4 ng/mL or >2x baseline + 2 confirms mast cell activation; draw within 1-4 hours of the reaction onset; tryptase is NOT elevated in non-IgE-mediated reactions (pseudoallergic NSAID reactions, red man syndrome) - Skin testing (Type I evaluation): skin prick testing (SPT) and intradermal testing (IDT) detect drug-specific IgE on cutaneous mast cells; most validated for penicillin (using penicilloyl polylysine/Pre-Pen as major determinant and penicillin G as minor determinant); positive predictive value ~50%; negative predictive value >97% — a negative penicillin skin test makes IgE-mediated penicillin allergy highly unlikely - Direct Coombs (direct antiglobulin) test (for Type II reactions): detects IgG or complement (C3) bound to the patient's red blood cell surface; positive in drug-induced autoimmune hemolytic anemia (methyldopa, penicillin, cephalosporins); also check indirect Coombs (detects circulating antibodies), reticulocyte count, haptoglobin (decreased in hemolysis), LDH (elevated in hemolysis), and peripheral smear for spherocytes - Complement levels — C3, C4, CH50 (for Type III reactions): immune complex deposition activates complement → decreased C3 and C4 levels; also check ESR, CRP (elevated), urinalysis (proteinuria, hematuria from glomerulonephritis), and ANA/anti-histone antibodies if drug-induced lupus is suspected - CBC with differential (for Type II and Type IV): thrombocytopenia (HIT type II, quinidine), agranulocytosis (clozapine — MANDATORY ANC monitoring), eosinophilia >1,500/mcL (DRESS syndrome — Type IVb), atypical lymphocytosis (DRESS) - Liver and kidney function panels: hepatitis (AST/ALT elevation — DRESS syndrome organ involvement), interstitial nephritis (creatinine elevation, eosinophiluria — Type II or Type IV), drug-induced liver injury (DILI — can be any type; ALT >3x ULN with symptoms or >5x ULN requires immediate drug discontinuation) - Patch testing (for Type IV — delayed reactions): apply the suspected drug to intact skin under occlusion for 48 hours; read at 48 and 96 hours; positive reaction (erythema, papules, vesicles) confirms T-cell-mediated hypersensitivity; useful for contact dermatitis causative agents and for identifying causative drugs in morbilliform eruptions; NOT validated for SJS/TEN (risk of triggering recurrence)

Risk factors: - Prior drug hypersensitivity reaction: strongest predictor of future reactions; a patient with a confirmed Type I (anaphylaxis) reaction to a beta-lactam has a 40-60% risk of reacting on re-exposure; cross-reactivity between drug classes must be assessed - HIV infection: 10-100 fold increased risk of drug hypersensitivity, particularly to TMP-SMX and antiretrovirals; immune dysregulation alters drug antigen presentation and T-cell reactivity - HLA genetic associations: HLA-B*5701 → abacavir hypersensitivity (MANDATORY testing before prescribing — eliminates ~50% of reactions); HLA-B*1502 → carbamazepine SJS/TEN in Southeast Asian and South Asian populations (FDA-mandated testing); HLA-B*5801 → allopurinol severe cutaneous reactions (test in Southeast Asian and African American populations) - Atopic history (asthma, eczema, allergic rhinitis): increases risk of Type I reactions, particularly to beta-lactams and NSAIDs; atopic patients have higher baseline IgE levels and mast cell reactivity - Female sex: approximately 65% of drug hypersensitivity reactions occur in women; hormonal influences on immune function and differences in drug metabolism contribute to this disparity - Polypharmacy: increased number of medications increases both the probability of drug interaction and the difficulty of identifying the causative agent when a reaction occurs - Concurrent viral infections: active EBV, CMV, or HHV-6 infections alter immune responses to drugs, increasing hypersensitivity risk — EBV + aminopenicillin interaction (70-100% rash rate) is the classic example; HHV-6 reactivation is implicated in DRESS syndrome pathogenesis

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