Despite the frequency of adverse reactions to drugs, allergic reactions are relatively uncommon. About 80% of adverse reactions are type A,or predictable,drug reactions that are explicable from the known pharmacology of the drug. Of the type B, or unpredictable, reactions many are due to drug intolerance (usually occurring at low doses of the drug), idiosyncratic reactions or pseudoallergy that is produced by the direct release of mediators from cells such as mast cells and basophils.The remainder of type B reactions have an immunological basis which is the requirement for true drug allergy.The term drug allergy is used loosely by the lay population, but also by health professionals.While most adverse reactions to drugs are not immunologically mediated, they are often recorded as 'allergy' in medical notes. This is a particular problem for antimicrobial drugs, especially penicillins, when a label of allergy is given in childhood following an unspecified adverse event.
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