## Pertussis Toxin Mechanisms **Key Point:** The 'whooping' sound in pertussis results from the paroxysmal cough followed by the characteristic inspiratory stridor caused by laryngeal involvement and airway obstruction. ### Tracheal Cytotoxin (TCT) — The Culprit Tracheal cytotoxin is a peptidoglycan fragment that: - Directly damages ciliated epithelium of the respiratory tract - Causes loss of cilia and mucociliary clearance - Leads to mucus accumulation and airway obstruction - Results in the paroxysmal cough and post-tussive vomiting - Produces the characteristic 'whoop' sound during the inspiratory phase ### Other Pertussis Toxins (Comparative Context) | Toxin | Mechanism | Effect | |-------|-----------|--------| | **Pertussis toxin (PTX)** | ADP-ribosylates Gi proteins | Lymphocytosis, impaired immune response | | **Tracheal cytotoxin (TCT)** | Peptidoglycan fragment | **Ciliary damage → 'whooping' cough** | | **Adenylate cyclase toxin (ACT)** | Increases intracellular cAMP | Local tissue damage, impaired phagocytosis | | **Endotoxin (LPS)** | TLR-4 activation | Systemic inflammation, fever | **High-Yield:** TCT is the primary virulence factor responsible for the classic clinical presentation of pertussis (paroxysmal cough with 'whoop'). PTX is responsible for the lymphocytosis seen in the blood. **Clinical Pearl:** The 'whoop' occurs during the inspiratory phase after a prolonged expiratory cough, as air rushes past the narrowed, inflamed larynx. [cite:Park 26e Ch 24]
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