Читать книгу Clinical Pharmacology and Therapeutics - Группа авторов - Страница 161

Drug allergy is a subset of type B adverse drug reaction. Allergy is a reproducible, immune‐mediated adverse reaction. While allergies can be mild, a patient who is allergic to a drug should avoid taking it again as repeated exposure can prompt more severe allergic reactions.

Allergies can be classified based on the Coombs and Gell classification of hypersensitivity reactions.

 Type 1 hypersensitivity is relatively common. This is IgE mediated, immediate type allergy which can be recognised by rapid onset (usually within 1 hour) of itchy rash, swelling (particularly airway and tongue swelling), wheeze and if particularly severe anaphylactic shock. Common causes of type 1 drug allergy are penicillins and non‐steroidal anti‐inflammatory drugs.

 Type 2 hypersensitivity is caused by antibodies binding to cells or tissues within the body. Such adverse reactions are much less common than type 1. Examples include haemolytic anaemia secondary to the antibiotic rifampicin.

 Type 3 hypersensitivity is caused by antibodies binding to soluble factors within the body and generating immune complexes that can inflame small blood vessels and cause an illness known as serum sickness. This is another uncommon mechanism of drug allergy and no drug commonly causes it. Potential drug causes include penicillin or cephalosporin antibiotics and allopurinol.

 Type 4 hypersensitivity causes most delayed‐type cutaneous drug reactions. It is mediated by the action of T lymphocytes within the skin rather than antibodies. Such reactions are not usually severe but in unusual cases severe cutaneous or systemic illness can arise. Antibiotics are common causes of delayed‐type rashes. Severe reactions such as Stevens–Johnson syndrome or DRESS (drug reaction with eosinophilia and systemic symptoms) are rare but can be caused by almost any drug.

While drug allergy should be taken seriously, it is also overdiagnosed. It is important to carefully record the circumstances of a possible drug allergy and consider seeking specialist advice if the situation is unclear. This is particularly important if the drug is felt to be particularly useful or important in the patient's management. It is a consistent finding that around 10–15% of any population will report having an allergy to penicillin but only around 10% of these patients will actually have evidence of an allergy on detailed testing.