Histological features of the airways in asthma and COPD.

Asthma and chronic obstructive pulmonary disease (COPD) are complex conditions with imprecise definitions which make definitive morphological comparisons difficult. Broadly, the airways in asthma are occluded by tenacious plugs of exudate and mucus, there is fragility of airway surface epithelium, thickening of the reticular layer beneath the epithelial basal lamina and bronchial vessel congestion and oedema. There is increased inflammatory infiltrate comprising 'activated' lymphocytes and eosinophils with release of granular content in the latter, and there is enlargement of bronchial smooth muscle particularly in medium sized bronchi. Three conditions contribute to COPD. In chronic bronchitis there is mucous hypersecretion with enlargement of tracheo-bronchial submucosal glands and a disproportionate increase of mucous acini. In small (peripheral) airways disease, there is inflammation of bronchioli, mucous metaplasia and hyperplasia, with increased intralumenal mucus, increased wall muscle, fibrosis and airway stenoses. Respiratory bronchiolitis is a critically important early lesion which may predispose to the development of centrilobular emphysema. The severity of destruction of alveolar wall in emphysema appears to be the most important determinant of chronic deterioration of airflow.