Langerhans cell histiocytosis pathophysiology

Pathophysiology
The disease spectrum results from clonal accumulation and proliferation of cells resembling the epidermal dendritic cells called langerhans cells, hence sometimes called dendritic cell histiocytosis. These cells in combination with lymphocytes, eosinophils and normal histiocytes form typical LCH lesion that can be found in almost any organ.

There are three types of histiocytoses 1. malignant (true histiocytic lymphomas) 2. "reactive" (benign histiocytoses) 3. Langerhans Cell Histiocytosis

"Reactive" in this context indicates that the abnormality may be due to a physiological reaction to infection. For example leukocytosis (proliferation of white blood cells) is a normal reaction to infection, and "histiocytes" are developmentally related to white blood cells (see article hematopoiesis).

Langerhans Cell Histiocytoses are traditionally divided into three groups:
 * Unifocal (aka "Eosinophilic granuloma"): a slowly progressing disease, characterized by an expanding proliferation of Langerhans cells in various bones, skin, lungs or stomach.
 * Multifocal unisystem: characterized by fever and diffuse eruptions, usually on the scalp and in the ear canals, as well as bone lesions. Mostly seen in children. In 50% of cases the stalk of the pituitary gland is involved, leading to diabetes insipidus. The triad of diabetes insipidus, proptosis, and lytic bone lesions is known as Hand-Schuller-Christian triad
 * Multifocal multisystem (Letterer-Siwe disease): a rapidly progressing disease where Langerhans cells proliferate in many tissues. It is mostly seen in children under age 2, and the prognosis is poor: even with aggressive chemotherapy, the 5-year survival is only 50%.

Pulmonary Langerhans cell histiocytosis (PLCH) is a smoking-related interstitial lung disease, used to be considered a benign condition in adults, long term complications like pulmonary hypertension is becoming increasingly recognized.