Definition of Histiocytosis:

Histiocytosis is a general name for a group of syndromes that involve an abnormal increase in the number of immune cells called histiocytes.

There are three major classes of histiocytoses:

• Langerhans cell histiocytosis, which is also called histiocytosis X
• Malignant histiocytosis syndrome (now known as T-cell lymphoma)
• Non-Langerhans cell histiocytosis (also known as hemophagocytic syndrome)

This article focuses only on Langerhans cell histiocytosis (histiocytosis X).

Causes, incidence, and risk factors:

Histiocytosis X has typically been thought of as a cancer-like condition. More recently researchers have begun to suspect that it is actually an autoimmune phenomenon, in which immune cells mistakenly attack the body, rather than fight infections. Extra immune cells may form tumors, which can affect various parts of the body including the bones, skull, and other areas.

Some forms of the disorder are genetic.

Histiocytosis X is thought to affect roughly 1 in 200,000 people each year. It is most often seen in children age 1 to 15. The rate peaks among children age 5 to10.

Pulmonary histiocytosis X is a specific type of this disorder that involves swelling of the small airways (bronchioles) and small blood vessels in the lungs. It is most common in adults. The inflammation leads to lung stiffening and damage. The cause is unknown. It most often affects those aged 30 to 40, usually cigarette smokers.