## Diphtheria Toxin and Pseudomembrane Formation ### Mechanism of Pseudomembrane **Key Point:** Diphtheria toxin is a lysogenic phage-encoded exotoxin that causes local tissue necrosis and inflammation, leading to the characteristic gray-white pseudomembrane in the pharynx, larynx, or nasopharynx. ### Toxin Structure and Function The diphtheria toxin is an A-B toxin: - **A subunit (catalytic):** Catalyzes ADP-ribosylation of elongation factor 2 (EF-2), inhibiting protein synthesis - **B subunit (binding):** Binds to heparin-binding EGF-like growth factor precursor (HB-EGF) receptor on host cells ### Pathophysiology of Pseudomembrane 1. Local invasion of *Corynebacterium diphtheriae* at mucosal surfaces 2. Toxin production → epithelial cell necrosis and hemorrhagic edema 3. Fibrin deposition and inflammatory exudate → gray-white adherent membrane 4. Membrane composed of: fibrin, dead epithelial cells, bacteria, and inflammatory cells **Clinical Pearl:** The pseudomembrane is **adherent and bleeds when forcibly removed** — a key clinical sign distinguishing it from streptococcal pharyngitis. ### High-Yield Fact **High-Yield:** Only *lysogenic* strains of *C. diphtheriae* carrying the β-phage produce toxin. Non-lysogenic strains cause mild or no disease. [cite:Park 26e Ch 10] 
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