A 28-year-old woman presents to the emergency department with acute onset of facial angioedema, lip swelling, and difficulty breathing 15 minutes after receiving an intramuscular injection of penicillin G for acute streptococcal pharyngitis. She has no prior history of drug allergy. On examination, she is anxious, with visible urticaria on her trunk and neck, blood pressure 92/58 mmHg, and respiratory rate 28/min. Serum tryptase level is elevated. Which of the following best explains the immunological mechanism underlying this acute reaction?

Explanation

## Immunological Classification This clinical presentation is a classic **Type I hypersensitivity reaction (immediate hypersensitivity)** occurring within minutes of drug exposure. ### Mechanism of Type I Hypersensitivity **Key Point:** Type I hypersensitivity is IgE-mediated and occurs in two phases: 1. **Sensitization phase**: Initial exposure to antigen (penicillin-protein complex) → B cell activation → differentiation into plasma cells → IgE antibody production → IgE binds to high-affinity receptors (FcεRI) on mast cells and basophils 2. **Re-exposure/Challenge phase**: Antigen cross-links IgE on mast cell/basophil surface → rapid degranulation → release of preformed mediators (histamine, tryptase, chymase) and newly synthesized mediators (leukotrienes, prostaglandins, platelet-activating factor) ### Clinical Features Correlating with Mechanism | Feature | Mediator | Effect | |---------|----------|--------| | Angioedema, urticaria | Histamine | Increased vascular permeability | | Bronchospasm, dyspnea | Leukotrienes (LTC4, LTD4, LTE4) | Smooth muscle contraction | | Hypotension | Histamine, PAF | Vasodilation | | Elevated tryptase | Mast cell degranulation | Diagnostic marker | **High-Yield:** Elevated serum tryptase within 15 minutes of symptom onset is a hallmark finding confirming mast cell/basophil degranulation and confirms Type I hypersensitivity. **Clinical Pearl:** The rapid onset (minutes), presence of urticaria, angioedema, respiratory compromise, and hemodynamic instability are pathognomonic for anaphylaxis, which is the most severe manifestation of Type I hypersensitivity. ### Why This Is Not Other Types - **Type II (cytotoxic)**: Would involve IgG-mediated cell destruction; typically presents with hemolytic anemia, thrombocytopenia, or hemolytic transfusion reactions over hours - **Type III (immune complex)**: Serum sickness-like reaction; occurs 7–14 days after drug exposure, not within minutes - **Type IV (delayed)**: T cell-mediated; manifests 24–72 hours after exposure (e.g., contact dermatitis, drug rash with eosinophilia) **Mnemonic: ABCDE of Anaphylaxis (Type I)** — Airway compromise, Breathing difficulty, Cardiovascular collapse, Dermal manifestations (urticaria, flushing), Edema (angioedema)