Malignant histiocytosis is a hereditary disease found in the Bernese Mountain Dog characterized by histiocytic infiltration of the lungs and lymph nodes. The liver, spleen, and central nervous system can also be affected. Histiocytes are a component of the immune system that proliferate abnormally in this disease. In addition to its importance in veterinary medicine, the condition is also important in human pathology.^[1]

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A histiocyte is a differentiated tissue cell that has its origin in the bone marrow. The source for histiocytes is the monocyte/macrophage line. Monocytes (found in the blood) and macrophages (found in tissue) are responsible for phagocytosis (ingestion) of foreign material in the body. Langerhans cells are dendritic cells found in the skin and function by internalizing antigens (foreign particles) and presenting them to T cells. They arise from monocytes.^[1] Histiocytic disorders refer to diseases that are caused by abnormal behavior of these cells. They include the following:

• Reactive diseases of Langerhans cells
  • Histiocytomas
  • Cutaneous histiocytosis
  • Systemic histiocytosis
• Reactive diseases of macrophages
  • Hemophagocytic syndrome – a condition where macrophages phagocytose myeloid and erythroid precursors (similar to hemophagocytic lymphohistiocytosis in humans)
• Malignant diseases of Langerhans dells
  • Malignant histiocytosis
  • Diffuse histiocytic sarcoma
  • Localized histiocytic sarcoma
• Malignant diseases of macrophages
  • Histiocytic lymphoma

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Because of the rapid progression of this aggressive disease, the prognosis is very poor.^[1]