Pseudoallergic Drug Reactions

Reactions that clinically resemble allergic reactions but lack an immune basis have been referred to as "pseudoallergic." They include almost the entire range of immediate hypersensitivity clinical patterns. Pseudoallergic reactions range in significance from the alarming but trivial anxiety or vasovagal reactions caused by local dental anesthetics to sometimes fatal reactions to ionic radiocontrast media. However, cytotoxic reactions, serum sickness, and severe dermatologic reactions (e.g., StevensJohnson syndrome and toxic epidermal necrolysis) are immunologic processes that are not likely to be mimicked by nonimmune processes seen with pseudoallergic reactions.

Pseudoallergic reactions are important in patient counseling and management considerations. The reactions represent common biological functions (direct histamine release by vancomycin, opiates), whereas immunologic (allergic) reactions are based on the structure of the drug. Even a mild drug allergy may carry significant potential for anaphylaxis on readministration. In contrast, pseudoallergic reactions tend to remain constant whether mild or severe and are dose-related.

Pseudoallergic reactions, then, are reactions where the tools of the immune system are used in exactly the same way, but without the "learning" response by T cells and generally without the much greater danger that true immunologic sensitization implies. Pseudoallergic reactions may be thought of as a subtype of idiopathic reactions, rather than an activation of the patient's immune system. The pathophysiology of pseudoallergic reactions is generally unknown, but indicators of immune activation are not seen when they occur.

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