|MadSci Network: Immunology|
It is true that the membrane of the T helper cell should not be
antigenic by itself. However, the penicillin binds to many sites on the
membrane. The arrangment of these determinants on the
membrane is what the immune system reacts to. At this point, the
small penicillin molecule appears to the immune system to be a
large, multi-determinant antigen, which, of course, is fully capable of
elliciting an immune response. Below is an excerpt from a recent
review of drug allergies:
Drug metabolism typically occurs as 2 different steps, phase I and phase II reactions. Phase I reactions are oxidation-reduction or hydrolysis reactions, and phase II reactions are conjugation reactions that result in the formation of inactive compounds that are easily excreted. Most often, a reactive metabolite formed by phase I metabolism is promptly detoxified and eliminated. However, reactive drug metabolites that are not detoxified may bind to proteins or nucleic acids, causing cellular necrosis or the production of an altered gene product, respectively, as part of a directly toxic effect. Reactive metabolites of sulfonamides have been shown to act in this manner. 11,12 Alternatively, reactive metabolites may act as haptens that bind covalently with cellular macromolecules such as serum proteins 6,7 or cell-surface membranes. 13 Such binding results in the formation of large, multivalent immunogens that may initiate an immune response. It is thought that hypersensitivity reactions to antibiotics may occur by way of this mechanism.
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