Histiocytosis is a rare medical condition that primarily affects children. It is characterized by an abnormal increase in the number of immune cells, called histiocytes, that are found in various tissues and organs. This can lead to inflammation, organ dysfunction, pain, and other complications. Histiocytosis can be classified into several subtypes, including Langerhans cell histiocytosis, hemophagocytic lymphohistiocytosis, and malignant histiocytic disorders. Each has distinct clinical features and outcomes.