Malignant histiocytosis is a rare hereditary disease found in the Bernese Mountain Dog and humans, characterized by histiocytic infiltration of the lungs and lymph nodes.

Histiocytes are a component of the immune system that proliferate abnormally in this disease.

[2] A histiocyte is a differentiated tissue cell that has its origin in the bone marrow.

[3] Histiocytic disorders refer to diseases that are caused by abnormal behavior of these cells.

They include the following: Up to 25 percent of Bernese Mountain Dogs may develop malignant histiocytosis in their lifetime.