ABSTRACT
The subsequent fate of the particle and nature of the disease process depends at least in part on interactions of the so-conditioned particle with pulmonary alveolar macrophages free on the alveolar surface. Macrophages can phagocytose a surfactant-coated quartz particle and subsequent cellular digestive processes may modify or remove the prophylactic surfactant coating with consequent restoration of cytotoxic activity. A particle may be cleared to the ciliated airways and out of the lung (Fig. 1, path a). Conversely the macrophage may die and the particle, perhaps associated with cellular residue, will be available again in the alveolar space for possible recoating and possible reingestion by other recruited macrophages, establishing a continuous ingestion-reingestion cycle with accumulation of the free particles in the lung (Fig. 1, path b). During these cycles the activated macrophage will secrete a large amount of substances such as cytokines, reactive oxygen species (ROS), reactive nitrogen intermediates (RNI), arachidonic acid metabolites and growth factors, which cause persistent inflammation and may severely damage epithelial target cells (Fig. 1, path c).
