<prompt> Birbeck granules are seen in the cytoplasm ofMast cellsLangerhans cellsThrombocytesMyelocytes  <response>: Langerhans cell histiocytosis LCHLangerhans cell is a dendritic cell which is an antigen presenting cellLCH is a clonal histiocytic disorderBirbeck granules are present in the cytoplasm of Langerhans cell.Characteristic features of LCHPresence of Birbeck granules - Electron microscopy Tennis racquet appearancePresence of enzymes like 1. a- Naphthyl Acetate Esterase 2. a- Naphthyl Butyrate Esterase 3. Acid phosphatasePresence of markers S-100, CD1a positivity HLA- DR expressionThree clinicopathological entities1.Acute disseminated LCH2.Multifocal LCH3.Unifocal LCH Earlier Histiocytosis x term was used. This includes-Letterer-Siwe disease generalized disease-Hand - Schuller- Christian disease-Eosinophilic granuloma localized lesion However now the term LCH is preferred.Acute disseminated LCH Letterer- Siwe disease - Affects children 2 yrs-Seborrhoeic type of skin rash on front back of trunk and scalp-Moderate hepatomegaly-Few palpable lymph nodes-Pulmonary lesions- uncommon-Bone- Lytic lesions 80-Moderate degree fever-Otitis media mastoiditis may be seen-CNS involvement due to contiguous spread-Hematologic findings: Pancytopenia-Bone marrow: Infiltration by histiocytesMultifocal LCH - Multiple lesion involving multiple systems-Prognosis better than disseminated LCH-Fever rash-Involvement of posterior pituitary- Diabetes insipidus-Typical Hand- Schuller Christian disease Triad of exopthalmus diabetes insipidus calvarial defectUnifocal LCHFocal lytic lesion in boneExcellent prognosisRef: Robbins 8e p631, 7e p701, 702 