A 28-year-old woman presents to the emergency department with acute onset of angioedema involving the lips, tongue, and throat 15 minutes after receiving intravenous penicillin for a urinary tract infection. She reports a prior episode of similar swelling after consuming shellfish 2 years ago. On examination, she is anxious, with audible stridor and mild wheezing. Blood pressure is 95/60 mmHg, heart rate 118/min. Within minutes of receiving intramuscular epinephrine, her symptoms begin to resolve. What is the immunological mechanism underlying this acute reaction?

Explanation

## Immunological Classification This patient presents with **Type I hypersensitivity (immediate hypersensitivity)**, characterized by rapid onset (minutes), IgE-mediated mast cell and basophil degranulation, and dramatic response to epinephrine. ### Key Pathophysiological Features **Key Point:** Type I hypersensitivity involves cross-linking of high-affinity IgE receptors (FcεRI) on mast cells and basophils by allergen-specific IgE, triggering exocytosis of preformed mediators (histamine, tryptase, chymase) and synthesis of newly-formed mediators (leukotrienes, prostaglandins, platelet-activating factor). ### Clinical Clues in This Vignette 1. **Temporal onset**: 15 minutes post-exposure — classic for Type I (occurs within seconds to minutes) 2. **Symptoms**: Angioedema, stridor, wheezing — all due to histamine-induced vasodilation, increased vascular permeability, and smooth muscle contraction 3. **Prior sensitization**: History of shellfish reaction suggests prior IgE production against cross-reactive epitopes 4. **Dramatic response to epinephrine**: Confirms mast cell/basophil degranulation (α-adrenergic inhibition of further mediator release, β-adrenergic smooth muscle relaxation) ### Mediators Released | Mediator | Source | Effect | |----------|--------|--------| | Histamine | Preformed (mast cell granules) | Vasodilation, increased vascular permeability, smooth muscle contraction | | Tryptase | Preformed (mast cell granules) | Marker of mast cell activation; serum levels peak at 15–30 min | | Leukotrienes (LTC₄, LTD₄, LTE₄) | Newly synthesized | Potent bronchoconstrictors, chemotaxis | | Prostaglandin D₂ | Newly synthesized | Vasodilation, platelet aggregation | | Platelet-activating factor (PAF) | Newly synthesized | Platelet aggregation, increased vascular permeability | **High-Yield:** Serum tryptase measured within 15–30 minutes of symptom onset is a diagnostic marker for mast cell activation and supports the diagnosis of anaphylaxis. ### Why Epinephrine Works Epinephrine acts via: - **α₁-adrenergic effects**: Vasoconstriction, reduced angioedema - **β₂-adrenergic effects**: Smooth muscle relaxation (bronchodilation), reduced mast cell degranulation - **β₁-adrenergic effects**: Increased cardiac output and perfusion **Clinical Pearl:** The dramatic and rapid response to IM epinephrine (within minutes) is pathognomonic for Type I hypersensitivity and distinguishes it from other hypersensitivity types. ### Mnemonic for Hypersensitivity Types **CITE** — **C**ytotoxic (Type II), **I**mmune complex (Type III), **T**ype I (IgE-mediated), **E**xaggerated cellular (Type IV). [cite:Robbins 10e Ch 6]