Inflammatory giant cells.

Giant cells are commonly seen in granulomas produced by a wide variety of known and unknown agents. It is widely accepted that giant cell formation results from fusion of mononuclear phagocytes. Both experimental and circumstantial evidence suggests that fusion takes place following the attachment of more than one macrophage to the same endocytic material. This view is in keeping with the consistent observation of giant cell formation in granulomas, where macrophages are actively ingesting material in close apposition to other macrophages. The experimental evidence for this view derives from the finding that if more than one macrophage is attached simultaneously to the same endocytic material in vitro, fusion results. Study of the phagocytic capacity of giant cells formed in vivo suggests that the same mechanism of fusion operates in granulomatous inflammation. Giant cell formation in granulomas thus appears to be the incidental result of macrophages ingesting material in close apposition to other macrophages. The material responsible for fusion may either be the visible particulate cause of granuloma formation or the denatured macromolecules formed by the inflammatory process.