Two major subsets of histiocytic cells are recognized within the lymphoreticular system: the dendritic cells or antigen-presenting cells and the phagocytic histiocytes or antigen-processing cells. Reactive and proliferative lesions of histiocytes can be related to these functional subsets. Reactive proliferations with a major dendritic cell component include dermatopathic lymphadenitis and Langerhans cell histiocytosis. Rare neoplasms derived from dendritic cells have been described. Benign proliferative lesions of phagocytic histiocytes include sinus histiocytosis with massive lymphadenopathy, the hemophagocytic syndromes including familial erythrophagocytic lymphohistiocytosis, Kikuchi's disease, and the various granulomatous lesions of lymph nodes. Although the hemophagocytic syndromes were often interpreted in the past as a variant of malignant histiocytosis (histiocytic medullary reticulosis), the reactive nature of this process is now accepted. The inciting event is usually an infection, and it is hypothesized that the histiocytic proliferation may result from exaggerated lymphokine production in an immunocompromised host. Reactive lesions of histiocytes are much more common than their rare neoplastic counterparts. True histiocytic lymphomas must be distinguished from the more common sinusoidal large cell immunoblastic lymphomas which usually express the CD30 antigen.