A 28-year-old woman presents with acute angioedema and urticaria 30 minutes after receiving intravenous penicillin for a urinary tract infection. She has no prior drug allergies. On examination, she is hemodynamically stable with normal airway patency. Which is the drug of choice for immediate management of this IgE-mediated hypersensitivity reaction?

## Management of Type I Hypersensitivity (IgE-Mediated) Reaction **Key Point:** Epinephrine is the first-line and gold-standard drug for acute anaphylaxis and severe IgE-mediated hypersensitivity reactions, including acute angioedema and urticaria with systemic involvement. ### Mechanism of Action Epinephrine acts as a non-selective adrenergic agonist: - **α-adrenergic effects:** Vasoconstriction → reduces angioedema and stabilizes mast cells - **β-adrenergic effects:** Bronchodilation, increased cardiac output, inhibition of mediator release from mast cells and basophils ### Dosing in Acute Anaphylaxis - **IM route:** 0.3–0.5 mg (1:1000 solution) in the anterolateral thigh — preferred for rapid absorption and safety - **IV route:** Reserved for profound hypotension or cardiac arrest (0.1 mg of 1:10,000 solution, titrated) ### Adjunctive Therapy (After Epinephrine) 1. **Antihistamines** (H1-blockers: diphenhydramine, cetirizine) — reduce urticaria and pruritus 2. **Corticosteroids** (hydrocortisone, methylprednisolone) — prevent biphasic reactions and late-phase inflammation 3. **H2-blockers** (ranitidine, famotidine) — reduce gastric acid secretion and potentiate H1-blocker effect **Clinical Pearl:** In this case, the patient is hemodynamically stable with patent airway, so IM epinephrine is appropriate. IV access should be established for adjunctive medications. **High-Yield:** Epinephrine must NEVER be delayed while awaiting IV access or other investigations. IM administration is faster and safer in the acute setting. ### Why Other Options Are Incorrect | Drug | Role in Anaphylaxis | Limitation | |------|---------------------|------------| | Hydrocortisone | Adjunct (prevents biphasic reaction) | Slow onset (30–60 min); does NOT replace epinephrine | | Diphenhydramine | Adjunct (antihistamine) | Slower than epinephrine; does NOT address airway/cardiovascular threat | | Omeprazole | Not indicated | No role in acute hypersensitivity; PPI has no immunomodulatory benefit | **Warning:** A common exam trap is selecting antihistamines or corticosteroids as "first-line." These are adjuncts only. Epinephrine is ALWAYS the first drug given in anaphylaxis, regardless of severity.