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Author: Dr Amy Stanway, Dermatology Registrar, Nottingham, United Kingdom, 2005. Revised by Dr David Lim, Dermatology Registrar, Auckland, New Zealand, August 2011.

What are histiocytoses?

Histiocytoses are diseases 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.

Classification of histiocytosis

There are many different types of histiocytosis. They 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
Non-cancerous (‘reactive’) increase in number of Langerhans' cells (histiocytes within the epidermis)
Class IIa: Dermal dendritic cell histiocytosis
Non-cancerous (‘reactive’) increase in number of non-Langerhans' cell histiocytes.
Class IIb: Non-Langerhans' cell, non-dermal dendritic cell histiocytosis
Reactive increase in number of (non-Langerhans’ and non-dermal dendritic) histioctyes
Class III: Malignant histiocytoses
Cancerous change in histiocytes
  • Monocytic leukaemia
  • Malignant histiocytosis
  • True histiocytic lymphoma

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