An Overview of Immunological Reactions to Drugs

Drug hypersensitivity reactions (DHRs) are immunologically mediated responses and a part of the spectrum of adverse drug reactions (ADRs). ADRs account for about 5–7% of all hospital admissions, and DHRs constitute about <10% of all ADRs. Detailed history, physical examination, and review of hospital records relating to an index episode are key to DHR workup. Skin tests and in vitro tests for immediate and non-immediate DHRs are useful although positive and negative predictive values have not been elucidated for the majority of drugs. Hence, these tests must be carefully interpreted by an experienced specialist in drug allergy in the context of clinical presentation. Drug eruptions may resemble viral exanthemata, making diagnosis particularly challenging in children. A drug provocation test is the “gold standard” in the diagnosis of DHRs although this intervention is mainly employed to demonstrate clinical tolerance and is undertaken after performing a careful risk–benefit analysis. Management of DHRs involves immediate cessation of exposure to possible culprit/s and supportive management. When an alternative drug is under consideration for clinical use, potential cross-reactivity needs to be considered. Rapid drug desensitization is an intervention employed to induce temporary immunological tolerance in patients with an immediate DHR and where there are no suitable alternatives. This procedure must only be undertaken under the guidance of a specialist allergist in a safe clinical environment with immediate access to critical care management.