Histiocytosis is a generic name for a group of syndromes characterized by an abnormal increase in the number of certain immune cells called histiocyte cells. These include monocytes, macrophages, and dendritic cells.

There are three major classes of histiocytoses, but this article focuses on Langerhans cell histiocytosis, which is also called histiocytosis X.

The other two classes are non-Langerhans cell histiocytosis (also known as hemophagocytic syndrome) and malignant histiocytosis syndrome (now known as T-cell lymphoma).

Most cases of histiocytosis X affect children between ages 1 and 15 years old. The incidence peaks amongst children between 5 and 10. Histiocytosis X is thought to affect roughly 1 in 200,000 people each year.

The extra immune cells may form tumors, which can affect various parts of the body. In children, histiocytosis X usually involves the bones (80%) and may consist of single or multiple sites. The skull is frequently affected.

The tumors produce a punched-out appearance on bone X-ray. Tumors in weight bearing bones, such as the legs or spine, may fracture spontaneously. There is often systemic involvement as well, which may affect the whole body and cause rashes, lung problems, gum infiltration, lymph gland swelling, hormonal problems, enlargement of the spleen and liver, and anemia. Not surprisingly, with a disease that affects so many systems and organs, histiocytosis may be deadly.

Children over 5 years old often have only bone involvement, however. Unfortunately, those surviving for long periods often continue to have problems related to the condition. Young children, especially infants, are more likely to have systemic involvement and a fatal outcome.

Histiocytosis X has typically been thought of as a cancer-like condition, but more recently researchers have begun to suspect that it is actually an autoimmune phenomenon, in which rogue immune cells attack the body, rather than fighting infections. Some forms are genetic.

Another type, pulmonary histiocytosis X, is characterized by inflammation of the small airways (bronchioles) and the small blood vessels in the lungs. It is most common in adults. The inflammation leads to stiffening (fibrosis) and destruction of the walls of the alveoli (parts of the lung). The cause is unknown. People 30 to 40 years old are affected most often. 90% of patients are cigarette smokers. Spontaneous pneumothorax occurs frequently in this condition.