Drug allergies are immune-mediated adverse drug reactions that occur when the immune system recognizes a medication as a foreign antigen and mounts a specific immune response against it. Unlike predictable, dose-dependent toxic effects, allergic reactions involve prior sensitization and are not directly related to the pharmacologic action of the drug. They account for approximately 5 to 10 percent of all adverse drug reactions but are disproportionately responsible for severe, life-threatening events. The Gell and Coomb’s classification system divides hypersensitivity reactions into four types based on the immune mechanism involved.
Type I (immediate hypersensitivity) reactions are mediated by drug-specific IgE antibodies bound to mast cells and basophils. Upon re-exposure to the offending drug, cross-linking of surface IgE triggers rapid degranulation with release of histamine, leukotrienes, and prostaglandins. Clinical manifestations range from mild urticaria, pruritus, and angioedema to severe anaphylaxis with bronchospasm, laryngeal edema, hypotension, and cardiovascular collapse. Symptoms typically appear within minutes to one hour of drug administration. Penicillins are the most common cause of IgE-mediated drug allergy, affecting approximately 1 to 2 percent of treated patients. The classic type I reaction to penicillin can be life-threatening, and patients with a history of immediate hypersensitivity should undergo skin testing before rechallenge.
Type II (cytotoxic hypersensitivity) reactions involve IgG or IgM antibodies directed against drug-modified cell surface antigens, leading to complement activation, antibody-dependent cell-mediated cytotoxicity, and phagocytosis. This mechanism underlies drug-induced hemolytic anemia, where drug-antibody complexes bind to red blood cells, causing their destruction. Cephalosporins, penicillins, and methyldopa are among the drugs implicated. Drug-induced thrombocytopenia and neutropenia also involve type II mechanisms. The onset is typically days to weeks after drug initiation, and recovery follows drug discontinuation.
Type III (immune complex-mediated) hypersensitivity occurs when drug-antibody complexes form in the circulation and deposit in tissues, triggering complement activation and inflammatory cell recruitment. This mechanism produces serum sickness, characterized by fever, rash, arthralgias, lymphadenopathy, and nephritis appearing one to three weeks after drug exposure. Classic causative agents include heterologous antisera, but drug-induced serum sickness is also seen with non-protein drugs including cefaclor, minocycline, and bupropion. Symptoms typically resolve over days to weeks after drug discontinuation, though systemic corticosteroids may be required in severe cases.
Type IV (delayed-type hypersensitivity) reactions are mediated by drug-specific T lymphocytes rather than antibodies. These reactions typically appear 48 to 72 hours after exposure in sensitized individuals and encompass a wide clinical spectrum. Contact dermatitis from topical medications represents the classic type IV reaction. More severe forms include drug rash with eosinophilia and systemic symptoms (DRESS), Stevens-Johnson syndrome, and toxic epidermal necrolysis, where cytotoxic T lymphocytes induce widespread keratinocyte apoptosis. Anticonvulsants, allopurinol, sulfonamides, and NSAIDs are common triggers.
Common allergenic drugs include penicillins, cephalosporins, sulfonamides, NSAIDs, and anticonvulsants. Cross-reactivity occurs when structurally related drugs are recognized by the same immune mediators. Cross-reactivity among penicillins is universal, while cross-reactivity between penicillins and cephalosporins is approximately 10 percent and most relevant for first-generation cephalosporins. Sulfonamide antibiotics and non-antibiotic sulfonamides generally do not cross-react because the antigenic determinants differ.
Diagnosis of drug allergy requires a detailed history establishing a temporal relationship between drug exposure and symptom onset, characterization of the reaction pattern, and assessment of the likelihood of IgE mediation. Skin testing, in vitro specific IgE assays, and drug provocation testing may be employed depending on the clinical scenario. Management involves immediate withdrawal of the offending drug, symptomatic treatment with antihistamines and corticosteroids for mild reactions, and epinephrine for anaphylaxis. Desensitization protocols may allow temporary reintroduction of essential medications in patients with confirmed allergies when no suitable alternatives exist.
