A histiocytosis is a disease in which there are too many histiocyte cells in the skin and other organs.
Normal histiocyte cells are part of the immune system, alerting infection-fighting cells to the presence of foreign material such as bacteria (antigen presentation). They also perform a waste-disposal function, getting rid of waste products in the tissues (phagocytosis). Normal histiocyte cells are originally formed within the bone marrow and then migrate throughout the body via the bloodstream.
Most forms of histiocytosis are rare. They may occur at any age including during infancy and childhood. Symptoms range from none to severe. Some forms of histiocytosis resolve without treatment, but others may have a fatal outcome.
Cutaneous histiocytoses are classified according to the type of histiocyte within the skin.
It may be possible to tell which type of histiocytosis is present by the appearance of the skin lesion or rash, but a skin biopsy is usually necessary. Each of the different types has a different microscopic appearance and staining pattern.
Class I Langerhans' cell histiocytosis is a reactive increase in the number of Langerhans' cells (histiocytes within the epidermis).
Class IIa dermal dendritic cell histiocytosis is a reactive increase in the number of non-Langerhans' cell histiocytes.
Class IIb non-Langerhans' cell, non-dermal dendritic cell histiocytosis is a reactive increase in the number of non-Langerhans’ and non-dermal dendritic histiocytes.
Class III malignant histiocytoses are:
