Cutaneous Histiocytoses

Abstract Skin involvement is common in all forms of histiocytic disorders and is seen in all ages. In general, for convenience, the histiocytoses were divided into those involving dendritic cells (DC disorders) and those involving the macrophage–monocyte lineage (or non‐DC disorders). Recent molecular insights, however, have led to a new classification system in which disorders that commonly harbour mutations in the MAPK pathway are classified together as the L group, while disorders that mainly involve skin and mucosa form the C group, the various forms of Rosai‐Dorfman disease constitute the R group, the malignant histiocytoses represent the M group, and finally the family of haemophagocytic lymphohistiocytoses comprise the H group. This chapter will provide an overview of the histiocytic disorders including a brief discussion of possible pathogenetic mechanisms, followed by an in‐depth discussion of the skin manifestations, pathology and therapy of the histiocytic disorders.