In medicine, histiocytosis is an excessive number of histiocytes[1] (tissue macrophages), and the term is also often used to refer to a group of rare diseases which share this sign as a characteristic.

[2] HAA also states that most of the people diagnosed with histiocytosis are children under the age of 10, although the disease can afflict adults.

[6][7] However, the classifications in ICD10 and MeSH are slightly different, as shown below: Alternatively, histiocytoses may be divided into the following groups:[8]: 714–724 Lymphohistiocytosis is a similar immune system disease characterized by the inappropriate activation of natural killer cells, CD8+ cytotoxic T-cells, and macrophages, involving principally the liver, spleen and central nervous system and associated with severe lymphoid atrophy.

The modalities used may include:[11] Patients and families can gain support and educational materials from the Histiocytosis Association.

[12][13] The North American Consortium for Histiocytosis (NACHO) is a group of institutions that collaborate on scientific and clinical research for histiocytic diseases.