Micro & Immunology · patient case question
Which molecule mediates this reaction?
Patient case: Breathlessness after a prescribed antibiotic
Patient: Adult patient with a documented penicillin allergy
Chief complaint: Attended for treatment of a dental infection.
Background: Documented allergy to penicillin. Cephalexin was prescribed.
Current findings: Thirty minutes after the dose he becomes short of breath.
Show answer and explanation
Correct answer: A. Histamine
- A. CORRECT. This is a Type I IgE-mediated hypersensitivity reaction. Antigen cross-links IgE on mast cells and basophils, which degranulate and release HISTAMINE. Histamine produces bronchoconstriction, vasodilatation, increased vascular permeability and urticaria — the breathlessness described here.
- B. Epinephrine is the TREATMENT, not the mediator. This is the commonest error on this item.
- C. Serotonin plays a minor role in some species but is not the principal human mediator.
- D. Bradykinin mediates ACE-inhibitor angio-oedema, a distinct mechanism that does not respond well to adrenaline.
Note: Cross-reactivity between penicillins and cephalosporins is real but far lower than the traditionally taught 10% — modern estimates are around 1–2%, and are driven by side-chain similarity rather than the beta-lactam ring itself.
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