[Recent advances in histiocytosis]

"Histiocytosis" is the term currently used to describe the group of diseases characterized by activation and proliferation of monocytic-mononuclear cells. Some of these are "reactive" to well-known causes, mycobacteriae, viral and parasitic infections, or chronic storage of minerals. Wider and more intriguing is the group of histiocytosis secondary to unknown causes: sinus histiocytosis with massive lymphadenopathy; histiocytosis in the course of systemic illnesses such as rheumatoid arthritis, SLE, Crohn disease, ulcerative colity, sarcoidosis, Weber-Christian disease, Wegener granulomatosis. Histiocitytosis X is the most frequent type of histiocytosis. Hematophagocytosis is a paraphysiologic phenomenon; however, when enormously increased it is characteristic of both the virus-associated hemophagocytic syndrome and Farquhar syndrome. In some cases of severe combined immunodeficiencies (SCID) histiocytic proliferation has been observed. Finally, during the past decade the morphologic approach has led to definition of the X-linked lymphoproliferative disease (XLP) and its erroneous classification as histiocytosis. These conditions are reviewed and some clinical cases are reported.