Langerhans cells are the pathognomonic cells for LCH and are located throughout the reticuloendothelial system. They express HLA-DR, S-100, and CD1a (Fig. 1A and B). The pathological hallmarks of these cells are "Birbeck" granules, which are rod-like structures with dilated terminal ends identified by electron microscopy (EM). The immunohistochemical identification of HLA-DR, S-100, and CD1a receptors has replaced EM identification. LCH appears to be a myeloproliferative disease due to changes in gene structure. The inciting events are unclear, but infection could be a contributing factor (3).

FIGURE 1 (A) Langerhans cell histiocytosis. H&E stain. (B) Langerhans cell histiocytosis. CD1 stain. (C) Langerhans cell histiocytosis. S-100 stain.
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