In septic shock, which cytokine is primarily responsible for the initial systemic vasodilation and increased vascular permeability?

Explanation

## Septic Shock: Cytokine Cascade **Key Point:** TNF-α is the **primary initiator** of the inflammatory cascade in septic shock. It is released by macrophages in response to bacterial lipopolysaccharide (LPS) and triggers the systemic inflammatory response. ### Role of TNF-α in Septic Shock **High-Yield:** TNF-α directly causes: 1. **Vasodilation** — via induction of nitric oxide (NO) synthase, leading to excessive NO production 2. **Increased vascular permeability** — endothelial dysfunction and loss of fluid into interstitium 3. **Myocardial depression** — direct negative inotropic effect 4. **Activation of coagulation cascade** — tissue factor expression, leading to DIC ### Cytokine Hierarchy in Sepsis | Cytokine | Primary Effect | Timing | | --- | --- | --- | | **TNF-α** | Initiates cascade; vasodilation, permeability | Early (0–2 hrs) | | **IL-1** | Amplifies TNF-α effects; fever, adhesion molecules | Early–Mid | | **IL-6** | Systemic inflammation marker; acute phase response | Mid–Late | | **IFN-γ** | T-cell activation; less central to acute shock | Late | **Mnemonic:** **TNF = The Notorious First** — TNF-α is the first and most potent trigger of septic shock. ### Clinical Pearl Anti-TNF-α monoclonal antibodies (infliximab) have been studied in sepsis but showed limited benefit, suggesting that blocking a single cytokine cannot reverse the full cascade once initiated. This highlights the **redundancy and complexity** of the inflammatory response. [cite:Robbins 10e Ch 4; Harrison 21e Ch 325]