Mononuclear phagocytic and dendritic cell systems

This review presents a classification scheme for disorders of the mononuclear phagocyte and dendritic cell systems. Previous classifications, based primarily on cytomorphology, are now obsolete. Electron microscopy, enzyme histochemistry, and, especially, immunocytochemistry have revealed the histiocyte identified by hematoxylin-and-eosin staining to be not one but several cell types. Currently, two broad groups of histiocytic cells are recognized, the monocyte-macrophage group and the dendritic cells. Dendritic cells are further subclassified as Langerhans cells, indeterminate cells, interdigitating cells, and dermal dendrocytes. Many disorders of the skin and other organs are characterized by proliferation of "histiocytic"-appearing cells. Some of these diseases clearly behave in a malignant fashion, whereas others follow a benign or variable course. At present, the proliferating cell type has been identified in only a few of these disorders by ultramicroscopy and cytochemical and immunocytochemical methods. Nevertheless, the pieces of the puzzle are beginning to fall into place. In general, those disorders that typically behave in a malignant fashion have been more extensively studied. The benign histiocytic disorders of children and adults are a heterogeneous group of diseases. Currently, some of these disorders are unclassified or classified with uncertainty. For example, how should severe histiocytosis with massive lymphadenopathy be classified? There is a proliferation of nonlymphoid mononuclear cells that are positive for S-100 protein and negative for CD1 and that exhibit lymphophagocytosis. Is this then a mononuclear phagocytic or dendritic cell disorder? Juvenile xanthogranuloma is a disorder of macrophages. Self-healing reticulohistiocytosis and solitary Langerhans cell histiocytoma are benign proliferative disorders of Langerhans cells.(ABSTRACT TRUNCATED AT 250 WORDS)