Opsonin-independent phagocytosis by human alveolar macrophages: augmentation by human plasma fibronectin.

A trypsin-sensitive membrane recognition unit that mediates phagocytosis of particulate activators of the human alternative complement pathway is present on human alveolar macrophages. Fragmented human plasma fibronectin selected by affinity chromatography with a monoclonal antifibronectin antibody augments this capacity. These data suggest a nonimmune mechanism for the clearance of some microorganisms from the opsonin-deficient microenvironment of the lung in which the alveolar macrophage is the principal resident phagocyte.