The histiocytoses: clinical presentation and differential diagnosis.

The histiocytoses, whether reactive or neoplastic, can be related histologically and immunophenotypically to their normal counterparts within the histiocytic system. This system has two subsets: The dendritic (antigen-presenting) cells and the phagocytic histiocytic (antigen-processing) cells. Dermatopathic lymphadenitis and Langerhans cell histiocytosis (histiocytosis X) are reactive proliferative disorders of dendritic cells. Malignancies of dendritic cells exist, but they are very rare. Benign proliferations of phagocytic histiocytes include the hemophagocytic syndromes, both familial and reactive, as well as sinus histiocytosis with massive lymphadenopathy (Rosai-Dorfman disease) and histiocytic necrotizing lymphadenitis (Kikuchi's disease). Neoplasms of phagocytic histiocytes include acute monocytic leukemia and the very rare entities, malignant histiocytosis and true histiocytic lymphoma. The latter must be distinguished from sinusoidal, large cell anaplastic lymphomas.