A 32-year-old woman presents to the emergency department with a 6-hour history of severe right lower abdominal pain, fever (38.5°C), and vomiting. On examination, she has rebound tenderness and guarding in the right iliac fossa. Blood investigations show WBC 14,500/μL with left shift, and imaging confirms acute appendicitis. During surgical exploration, the appendix is found to be acutely inflamed with fibrinopurulent exudate on the serosal surface. Which of the following mediators is primarily responsible for the increased vascular permeability that led to the exudation observed in this patient?

Explanation

## Acute Inflammation and Vascular Permeability ### Mechanism of Increased Vascular Permeability In acute inflammation, increased vascular permeability is the hallmark feature that allows plasma proteins and cells to exit the microvasculature. This occurs primarily at the level of post-capillary venules. **Key Point:** Histamine and bradykinin are the primary immediate mediators responsible for increased vascular permeability in acute inflammation, acting within minutes of injury. ### Mediators and Their Roles | Mediator | Source | Onset | Duration | Effect on Permeability | |----------|--------|-------|----------|------------------------| | Histamine | Mast cells, basophils | Immediate (minutes) | Brief (15–30 min) | Marked increase | | Bradykinin | Plasma kinin system | Immediate–early | Brief | Marked increase | | Leukotrienes (C4, D4, E4) | Mast cells, neutrophils | Early | Prolonged | Marked increase | | Complement C3a, C5a | Complement cascade | Early | Variable | Moderate increase | | Substance P | Nerve terminals | Variable | Variable | Mild increase | | Prostaglandins (PGE2, PGI2) | Endothelial cells, macrophages | Early–delayed | Prolonged | Mild increase | | TNF-α, IL-1 | Macrophages, endothelial cells | Early–delayed | Prolonged | Moderate increase | ### Histamine Mechanism 1. Released from preformed granules in mast cells and basophils within seconds of injury 2. Acts on H1 and H2 receptors on endothelial cells 3. Causes endothelial cell contraction, widening intercellular junctions 4. Effect is rapid but short-lived (15–30 minutes) 5. Responsible for the immediate phase of increased permeability ### Bradykinin Mechanism 1. Generated from high-molecular-weight kininogen via the contact (Hageman) factor system 2. Acts on B2 receptors on endothelial cells 3. Causes sustained increase in permeability 4. Also produces pain (hence the name "kinin" = pain-producing substance) 5. Contributes to the early inflammatory response **Clinical Pearl:** In acute appendicitis, the fibrinopurulent exudate observed on the serosal surface is the direct result of histamine and bradykinin-mediated increase in vascular permeability, allowing fibrin and inflammatory cells to accumulate. **High-Yield:** Histamine is the fastest-acting mediator of increased permeability (within seconds), while bradykinin provides more sustained effects. Together, they account for the immediate and early phases of acute inflammation. ### Why Other Mediators Are Secondary - **Complement C3a and C5a** are important but act slightly later and have additional roles (chemotaxis, opsonization) - **Prostaglandins** primarily regulate vasodilation and pain, not the primary permeability increase - **TNF-α and IFN-γ** are cytokines that enhance inflammation over hours, not the immediate mediators - **Leukotrienes** are more potent than histamine but are generated later from arachidonic acid metabolism