Drug hypersensitivity reaction
Drug hypersensitivity reactions are immune-mediated adverse drug reactions classified by the Gell-Coombs system into immediate IgE-mediated (type I) and delayed T-cell-mediated (type IV) forms, ranging from urticaria and anaphylaxis to severe cutaneous reactions. Diagnosis rests on the reaction phenotype and timing, and management is stopping the culprit, treating by mechanism, and documenting the drug to prevent re-exposure.
- Topic reviewThe full article, organized the way the exam asks it.
- ABIM-format MCQsVignettes with an explanation for every wrong answer.
- Key factsThe numbers and thresholds worth memorizing, up front.
- AI tutorMedBrain answers the question you're stuck on.
What this article covers
- Overview
Definition: a drug hypersensitivity reaction is an immune-mediated adverse drug reaction, distinct from dose-related toxicity, mediated by antibodies or T cells against the drug.
- Epidemiology
Common cause of adverse drug events: antibiotics (beta-lactams, sulfonamides) and anticonvulsants are frequent culprits.
- Risk factors
Prior reaction to the drug or its class: the strongest predictor.
- Pathophysiology
Type I (immediate): drug-specific IgE on mast cells triggers degranulation, causing urticaria, angioedema, and anaphylaxis.
- Clinical features
Immediate reactions: urticaria, flushing, angioedema, bronchospasm, and hypotension within minutes to a few hours of exposure.
- Differential diagnosis
Non-immune (dose-related) drug toxicity: predictable, dose-dependent effects rather than hypersensitivity.
- Diagnosis
Classify the reaction by phenotype and timing, using the Gell-Coombs framework: the diagnosis rests on identifying whether the reaction is immediate (IgE-mediated: urticaria, angioedema, anaphylaxis…
- Management
First, stop the culprit drug and avoid the cross-reacting class until evaluation is complete.
- Complications
Anaphylaxis and death: from immediate reactions.
- Prognosis
Most reactions are mild and resolve after stopping the drug.
- Prevention
Document the drug, class, and reaction type clearly in the record after any reaction.
Read the whole article
Subscribing to Internal Medicine opens every article, the full question bank and MedBrain, on the web, iPhone and Android.
Related articles
- Anaphylaxis
Anaphylaxis is an acute, potentially fatal systemic hypersensitivity reaction from massive mast cell and basophil mediator release, presenting within minutes as skin, respiratory, cardiovascular, and gastrointestinal involvement after allergen exposure. It is a clinical, treat-first diagnosis, and intramuscular epinephrine is the immediate life-saving therapy.
- Urticaria
Urticaria is transient, pruritic, edematous wheals from mast cell histamine release, classified by duration into acute (under 6 weeks) and chronic (6 weeks or longer, usually spontaneous/idiopathic). Diagnosis is clinical with minimal testing, and treatment is stepwise second-generation H1 antihistamines up-titrated as needed, adding omalizumab for refractory chronic disease.
- Angioedema
Angioedema is self-limited, non-pitting swelling of the deep dermis and submucosa that follows one of two mechanisms: mast cell (histamine) mediated, which occurs with urticaria and responds to epinephrine and antihistamines, or bradykinin mediated, which occurs without urticaria and does not. Distinguishing the two determines whether treatment is allergic therapy or bradykinin-targeted therapy, and airway involvement is the central danger.