Destruction of respiratory epithelium and diminished mucociliary function occurs. Persistent cough and mucous plugging result. Expectorated sputum or phlegm can become purulent from the influx of phagocytes into mucus. On the other hand, chronic bronchitis is characterized by chronic bronchial inflammation with mucosal thickening with excessive airway mucus plugging. Acinar enlargement from alveolar wall damage can occur.
Inflammation can eventually lead to fibrosis and narrowing of small airways.
Proliferation and hypertrophy of airway mucus glands is characteristic and most importantly, the normally sterile tracheobronchial tree becomes colonized with bacteria. The most common infectious causes of acute bronchitis are viruses, mostly rhinoviruses and adenoviruses. Bacteria such as S. pneumoniae and H. influenzae are the least common infectious cause of acute bronchitis in an otherwise healthy person. Although viruses are also the most common cause of infectious exacerbations of COPD, bacteria assume a more important causative role. Mycoplasma pneumoniae and Chlamydiae pneumoniae account for far fewer cases of COPD exacerbation than in acute bronchitis in otherwise healthy persons.