In medicine, histiocytosis refers to an excessive number of histiocytes, (tissue macrophages), and is typically used to refer to a group of rare diseases which share this as a characteristic. Occasionally and confusingly, the term 'histiocytosis' is sometimes used to refer to individual diseases. According to the Histiocytosis Association, 1 in 200,000 children in the United States are born with histiocytosis each year. HAA also states that most of the people diagnosed with histiocytosis are children under the age of 10. The University of California, San Francisco, states that the disease usually occurs from birth to age 15. Histiocytosis (and malignant histiocytosis) are both important in veterinary as well as human pathology. There are competing systems for classifying histiocytoses. According to the 1999 classification proposed by the World Health Organization, they can be divided into three categories.
Contributions by Arcadian, Histiocytosis Association, and Davidruben.