This chapter summarizes the clinical spectrum of the histiocytic disorders—Langerhans cell histiocytosis (LCH), haemophagocytic lymphohistiocytosis (HLH), and some uncommon histiocytic disorders, including juvenile xanthogranuloma (JXG) and Rosai–Dorfman disease—as well as the current diagnostic and therapeutic approaches in these diseases. Multiple activating mutations in the RAS–RAF–MEK–ERK pathway have recently been described in LCH. Their role in the pathophysiology of the disorder and in targeted therapy is reviewed. This chapter explains the differences between primary and secondary HLH, and reviews the genetic abnormalities playing a role in both forms of HLH.
Fine-needle cytology of cutaneous juvenile xanthogranuloma and langerhans cell histiocytosis
