Histiocytosis is an umbrella term for the proliferative disorders of dendritic cells

Histiocytosis is an umbrella term for the proliferative disorders of dendritic cells or macrophages.

Langerhan cell histiocytosis is proliferation of a special type of histiocyte called as Langerhan cell
It is considered to be a neoplastic proliferation because of the mutation in an activating valine to glutamate substitution at residue 600 in BRAF which is present in 55-60% of cases.

MORPHOLOGY
Regardless of the clinical picture the proliferating Langerhans cells have abundant often vacuolated cytoplasm and vescicular nuclei containing linear grooves or folds.
The presence of Birkbeck’sgranules in the cytoplasm is characteristic.
Birbeck granules are pentalaminar tubules often with a dilated terminal end producing a Tennis racket shaped appearancewhich contain the protein langerin.