Langerhans' cells are white blood cells in the immune system that normally play an important role in protecting the body against viruses, bacteria and other invaders. They are found in the skin, lymph nodes, spleen, bone marrow and lungs.
In Langerhans' cell histiocytosis (formerly known as histiocytosis X), Langerhans' cells multiply abnormally. Instead of helping to protect the body, these cells, in massive numbers, cause tissue injury and destruction, especially in the bones, lungs and liver. Although this overgrowth of cells may be like cancer, most researchers do not consider Langerhans' cell histiocytosis to be a form of cancer. In fact, the cells appear to be normal. There are just too many of them. Instead, it seems to be disease of the immune system, in which immune cells multiply abnormally and promote inflammation and damage of surrounding tissues.
Langerhans' cell histiocytosis can develop in only one site or organ, or it can involve several different sites and organs. In most cases, Langerhans' cell histiocytosis that affects many body systems typically occurs in children younger than 2, whereas single-site disease may occur in people of any age.
The cause of the disorder is unknown. However, researchers are studying the possibility that some common substance in the environment — perhaps a viral infection — may trigger the disease by causing the immune system to overreact. However, researchers have not yet identified an infection or substance that triggers the disease. One intriguing observation is that almost all people with the disease whose lungs are involved are current or former cigarette smokers. However, even among smokers, this condition is rare, and smoking seems to play no role in the disease when it affects parts of the body outside the lungs. Other theories suggest that the primary cause of Langerhans' cell histiocytosis is an abnormal immune system.
Langerhans' cell histiocytosis is rare, affecting one in 250,000 children and one in 1 million adults in the United States. Although it can strike at any age, it is most common in children and young adults, with about 70% of cases occurring before age 17. During childhood, Langerhans' cell histiocytosis peaks between ages 1 and 3.
Langerhans' cell histiocytosis includes disorders previously considered separate diseases: diffuse reticuloendotheliosis, eosinophilic granuloma, Hand-Schüller-Christian disease and Letterer-Siwe disease.