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Micro & Immunology · patient case with 2 questions

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.

Question 1

Which molecule mediates this reaction?

  1. Histamine
  2. Epinephrine
  3. Serotonin
  4. Bradykinin
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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Question 2

What is the immediate management?

  1. Intramuscular epinephrine into the anterolateral thigh
  2. A salbutamol inhaler alone
  3. An oral antihistamine and observation
  4. An intravenous corticosteroid as the first drug
Show answer and explanation

Correct answer: A. Intramuscular epinephrine into the anterolateral thigh

  • A. CORRECT. Anaphylaxis is treated with EPINEPHRINE without delay — 0.3–0.5 mg of 1:1000 intramuscularly into the anterolateral thigh (vastus lateralis), repeated every 5–15 minutes as required. Adrenaline reverses bronchoconstriction, restores vascular tone and stabilises mast cells. Call emergency services and give oxygen alongside it.
  • B. Salbutamol may help bronchospasm as an adjunct but does not address the vascular collapse.
  • C. An oral antihistamine is far too slow and treats only the cutaneous features.
  • D. Corticosteroids have a role in preventing the biphasic late-phase reaction, but they take hours to act and must never precede adrenaline.

Note: Route matters: IM into the anterolateral thigh gives faster, more reliable absorption than subcutaneous or deltoid injection.

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