The morphogenesis and classification of diffuse interstitial lung diseases: a clinicopathological approach, based on tissue reaction patterns.

This account gives a clinicopathological overview and classification of diffuse interstitial lung diseases, emphasising the interstitial pneumonias. The morphogenesis and progression of fibrosing alveolitis are discussed in relation to the normal cellular and extracellular components of the pulmonary interstitium, with the alveolar macrophage playing a pivotal role. So-called desquamative interstitial pneumonia is considered to represent an early steroid-responsive cellular exudative phase of fibrosing alveolitis, most of the intra-alveolar cells being macrophages rather than type II pneumocytes. The distinction between fibrosing alveolitis and diffuse alveolar damage, and their possible pathogenetic relationships to each other, are also discussed. Throughout this presentation it is stressed that inflammatory interstitial disorders such as the "desquamative" alveolopathies constitute stereotyped patterns of tissue reaction to a variety of injurious agents, which should always be taken into account in reaching a definitive clinicopathological diagnosis, especially in fibreoptic biopsies where histomorphological discriminants may not be present because of the small amount of tissue sampled.