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Histiocytoses are a group of disorders, which have in common accumulation of histiocytes – the old term for macrophages – in tissues. Histiocyte accumulations may result from several morbid processes. Some infectious typically cause accumulation of histiocytes and other inflammatory cells to form granulomas, like in tuberculosis, in atypical mycobacterial infection, which occur in a context of elective immune deficiency, or in some rare infections (like whipple disease). Other morbid processes are secondary to a metabolic impairment like in storage diseases (e.g. Gaucher disease).

Rarely, Histiocytoses present the natural history and features of true malignant processes. Such rare cancers are named histiocytic sarcoma, or interdigitating sarcoma. Another group of diseases historically assigned to the histiocytosis, are the different forms of hemophagocytic lymphohistiocytosis (Ref cure hlh), which embrace the genetic form caused by impaired cytotoxicity and an acquired form, the so called macrophage activation syndrome (systemic hyperinflammation secondary to another underlying disease, e.g. rheumatic diseases and some malignancies).

Finally, some histiocytic diseases are characterized by the ‘sole’ tissue infiltration by histiocytes without relation with a documented infectious, metabolic or underlying malignant disease. Those diseases form the group of the true histiocytosis, like Langerhans cell histiocytosis (LCH), Erdheim-Chester Disease (ECD), Rosai-Dorfman Disease (RDD), and Juvenile Xanthogranuloma (JXG).