<unk>  cell<unk>  arises from epidermal dendritic cells<unk>  cells), which are normally involved in the process of antigen presentation to lymphocytic cells.<unk>  cells originally arise from the bone marrow, then the cells migrate to other organs such as the skin, lymph nodes, and lungs. The exact pathogenesis of<unk>  cell<unk>  is not fully understood. It is thought that<unk>  cell<unk>  is the result of either a true neoplastic process or a reactive immune condition. Genes commonly involved in the pathogenesis of<unk>  cell<unk>  include BRAF gene and MAP2K1 gene. On gross pathology, scaly erythema, red papules, and extensive eruptions located on the scalp are characteristic finding of<unk>  cell<unk> . Characteristic findings of<unk>  cell<unk>  on light microscopy may include clusters of dendritic cells with kidney-shaped nucleus and abundant foamy cytoplasm. On electron microscopy<unk>  cell<unk>  is characterized by Birbeck granules, which are electron dense, cytoplasmic, tennis racket-like bodies. On immunohistochemistry<unk>  cell<unk>  is characterized by positivity to CD1a, S100, and CD207 (langerin).
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