A 28-year-old woman presents with sudden onset of urticaria, angioedema, and bronchospasm 15 minutes after receiving intravenous penicillin for a urinary tract infection. Which of the following is the most common immunological mechanism underlying this acute hypersensitivity reaction?

## Mechanism of Type I Hypersensitivity **Key Point:** Type I hypersensitivity (immediate hypersensitivity) is the most common immunological mechanism in acute drug reactions presenting within minutes to hours of exposure. ### Pathophysiology The reaction occurs in two phases: 1. **Sensitization phase**: IgE antibodies bind to high-affinity IgE receptors (FcεRI) on mast cells and basophils. 2. **Re-exposure phase**: Cross-linking of IgE-receptor complexes by antigen triggers rapid degranulation, releasing preformed mediators (histamine, tryptase, heparin) and newly synthesized mediators (leukotrienes, prostaglandins, bradykinin). ### Clinical Presentation The rapid onset (15 minutes in this case) of urticaria, angioedema, and bronchospasm is pathognomonic for IgE-mediated mast cell degranulation. Penicillins are among the most common drug causes of Type I hypersensitivity reactions, occurring in 1–2% of exposed individuals with prior sensitization. **High-Yield:** The **biphasic response** may occur: immediate reaction (minutes) followed by recurrence 4–12 hours later due to newly synthesized mediators and recruitment of eosinophils and neutrophils. ### Comparison with Other Hypersensitivity Types | Type | Mediator | Onset | Example | |------|----------|-------|----------| | Type I | IgE | Minutes to hours | Anaphylaxis, acute urticaria | | Type II | IgG/IgM | Hours to days | Drug-induced hemolytic anemia | | Type III | Immune complexes | 3–10 days | Serum sickness, vasculitis | | Type IV | T cells | 24–72 hours | Contact dermatitis, delayed drug reaction | **Clinical Pearl:** Penicillin metabolites (particularly the penicilloyl moiety) act as haptens, binding to carrier proteins and triggering IgE responses in sensitized individuals. This is why a negative skin test to penicillin does not completely exclude IgE-mediated allergy. **Mnemonic: FAST** — **F**ast onset (minutes), **A**ntigen-IgE interaction, **S**ensitization required, **T**ype I hypersensitivity.