Characterised by the clonal proliferation of pathogenic Langerhans cells in single or multiple organs.
Exact aetiology and pathogenesis is unknown; however, it is thought to be either a malignant process or due to immune dysregulation.
Clinical presentation is heterogeneous and ranges from self-healing bone lesions to multi-system, life-threatening disease.
Tissue biopsy of lesional cells showing the presence of CD1a and langerin (CD207) is definitive for diagnosis in the correct clinical setting.
Multi-system disease is treated with systemic, multi-agent chemotherapy.
Treatment of relapsed disease and CNS neurodegenerative disease remains a challenge.
Prognostic factors include rapid response to therapy and risk-organ involvement.
Langerhans cell histiocytosis (LCH) is a rare disease characterised by the clonal proliferation of pathogenic Langerhans cells and cytokine over-production. This leads to inflammation and tissue destruction in different organs of the body.   It may affect either a single organ system or multiple organs, and commonly involves the bone, skin, lungs, liver, spleen, bone marrow, pituitary gland, and eyes. Less common sites involved include the lymph nodes, thyroid, ears, CNS, gastrointestinal system, and oral mucosa.   Because pathological Langerhans cells (dendritic cells found in the skin) are the cell type common to the various manifestations of the disease, the term 'LCH' is preferred over previous names such as 'histiocytosis X'.   It is the most common type of histiocytosis (i.e., syndrome characterised by the abnormal proliferation of histiocytes). 
Associate Professor of Pediatrics
University of Toronto
The Hospital for Sick Children
Chair of the Rare Histiocytoses Committee
OA declares that he has no competing interests.
Director of Pediatric Immunology, Hematology, Oncology, Bone Marrow Transplantation, and Autoimmune Diseases
Leiden University Medical Center
RME has previously worked with Dr Abla on a review of LCH. RME is an author of several references cited in this monograph.
Assistant Professor of Pediatrics
Keck School of Medicine
University of Southern California
Children's Hospital Los Angeles
RJ declares that she has no competing interests.
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