Definition of commonly used antimicrobial hypersensitivity terms

‘Hypersensitivity’ is a broad term for unpredictable adverse drug reactions, which can be caused by immune-mediated or other mechanisms; ‘allergy’ refers to an immune-mediated mechanism. In this topic, ‘hypersensitivity’ and ‘allergy’ are used interchangeably. Common terms used when discussing antimicrobial hypersensitivity are defined in Common terminology used when discussing antimicrobial hypersensitivity.

Table 1. Common terminology used when discussing antimicrobial hypersensitivity
Term	Definition or example
hypersensitivity	broad term used to describe unpredictable adverse drug reactions, which can be caused by immune-mediated allergy or other mechanisms
Type A reactions (often referred to as ‘drug intolerance’)	pharmacologically predictable adverse drug reactions (eg gastrointestinal intolerance)
Type B reactions	unpredictable reactions mediated by immunological or other mechanisms – most common types of immune-mediated reactions are immediate IgE-mediated (Type B-I) and delayed T-cell mediated (Type B-IV) reactions
immediate hypersensitivity	immediate reactions are typically from IgE-mediated or non–IgE-mediated mast cell activation occur soon after exposure to a drug, typically within minutes to 2 hours. Not all immediate hypersensitivity reactions are severe
delayed hypersensitivity	delayed reactions, typically presumed secondary to T-cell mediated reactions occur after more than one dose of a drug, with an onset days after starting treatment; however, delayed hypersensitivity can occur more rapidly on rechallenge (within 6 hours)
severe hypersensitivity	examples of severe hypersensitivity include anaphylaxis, compromised airway, airway angioedema, hypotension, collapse, severe cutaneous adverse drug reactions (eg drug rash with eosinophilia and systemic symptoms [DRESS], Stevens–Johnson syndrome / toxic epidermal necrolysis [SJS/TEN], severe blistering or desquamative rash), and significant internal organ involvement (eg acute interstitial nephritis)
nonsevere hypersensitivity	examples include urticaria, mild immediate rash, benign childhood rash and maculopapular rash
direct delabeling	removal of a patient’s reported antibiotic allergy based on the patient’s history alone
delabeling following allergy testing	removal of a patient’s reported antibiotic allergy following allergy testing (eg oral challenge, skin testing)
oral challenge (also called provocation testing, drug challenge, graded challenge or test dosing)	the controlled administration of a drug in order to diagnose hypersensitivity
desensitisation	a process of administering fractional doses of a drug in order to induce a temporary state of drug tolerance. Very low doses are administered and slowly increased to the desired dose