TNFSF11 and Langerhans cell histiocytosis: Serum from LCH patients has elevated levels of the soluble interleukin‐2 (IL‐2) receptor, a T‐cell activation marker, and receptor activator of nuclear factor kappa‐B ligand (RANKL), an osteoclast activator, by reflecting cytokines/chemokines secretion from the lesion site.17