Table 1.

Classification of Histiocytic disorders.

HD type	Examples
L group	LCH Indeterminate cell histiocytosis ECD Mixed ECD and LCH
C group	Cutaneous non-LCH histiocytoses Cutaneous non-LCH histiocytoses with a major systemic component
M group	Primary malignant histiocytosis Secondary malignant histiocytosis
R group	Nodal RDD Extranodal RDD Neoplasia-associated RDD Immune disease-associated RDD
H group	Primary HLH Secondary HLH

ECD, Erdheim–Chester disease; HD, histiocytic disorder; HLH, haemophagocytic lymphohistiocytosis; LCH, Langerhans cell histiocytosis; RDD, Rosai–Dorfman disease.