The term drug allergy refers to one type of untoward drug reaction that usually occurs within minutes to 2 or 3 months after medication is administered. Most adverse drug reactions are not allergic or immunologic, and the distinction should be made between an adverse drug effect and an immunologic drug reaction. It is often not possible or advisable to rechallenge a patient with an incriminated drug, so patients must carry a label of probable drug allergy. Nevertheless, it is helpful to classify untoward drug reactions to assess the likelihood of safe use of the drug in the future. Fortunately, advances in basic and clinical understanding of drug allergies and their prevention have made it possible to attempt essential readministration of previously incriminated therapeutic agents.
There are eight main types of untoward drug reactions: Allergic, immunologic, idiosyncrasy, intolerance, side effects, secondary effects, drug - drug interactions, coincidence and phobic reaction.
An allergic or immunologic reaction is a direct response to the drug, classically after previous exposure. The earlier exposure may have been uneventful except for immunologic sensitization. The sensitization becomes manifest when a medication is readministrated.
The mechanism of an allergic or immunologic drug reaction can vary from IgE mediated as in the case of penicillin, to mast cell histamine release from vancomycin, to unknown agents as from radiographic contrast materials.
Many drugs have low molecular weights, so they might bind to carrier proteins to be antigenic. As best defined for penicillin, the parent drug or commonly some of its metabolites serve as haptens in IgE mediated allergic reactions. Some drugs are of high molecular weight and could serve as immunogens without carrier proteins e.g. insulin.
To demonstrate an IgE mediated drug reaction, test for immediate cutaneous reactivity or for serum anti drug IgE is recommended.
General principles of patient assessment & management of drug allergy:
- The history, supplemented with review of available medical records, provides a basis for determining whether an adverse drug reaction appears to be immunologic or allergic.
- When medical records are not available or the patient or family cannot provide useful details, it is advisable to consider the patient allergic to the suspected drug.
- Medical records may reveal that several medications were being used, and it is not possible to be certain that the incriminated drug in fact caused the reaction.
- Testing for presence of total IgE antibodies can indicate the immunologic status, if an additional therapeutic agent is indicated.
- If a certain drug is indicated, detection of anti drug IgE helps to define the level of risk of another allergic reaction.
N.B. * Immediate allergic reactions to insulin are rare but they occur despite use of human recombinant insulin
* Preliminary data suggest possibility of the familial drug allergy.
IgE specific for the following drugs is now available:
2) Penicilloyl G
3) Penicilloyl V
5) Insulin human