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INTRODUCTION

  • Histiocyte is an archaic term for macrophages. The histiocytoses include diseases arising from all cells of the mononuclear phagocytic system, with diseases characterized by presumed lineage and biology into dendritic cell (DC) disorders, macrophage-related disorders, and malignant histiocytic disorders (Table 36–1).

  • Historically, the histiocytic disorders have been characterized by the phenotype of disease-specific histiocytes (Table 36–2).

  • Now, recurrent mitogen-activated protein kinase (MAPK) pathway mutations in clonal myeloid cells in Langerhans cell histiocytosis (LCH), Erdheim-Chester disease (ECD), juvenile xanthogranuloma (JXG), Rosai-Dorfman disease (RDD), and malignant histiocytic diseases clearly separate these neoplastic diseases of inflammatory cells from hemophagocytic lymphohistiocytosis (HLH), characterized by polyclonal reactive macrophages.

TABLE 36–1HISTORICAL CLASSIFICATION OF HISTIOCYTIC DISORDERS
TABLE 36–2CHARACTERISTIC IMMUNOPHENOTYPE OF PATHOLOGIC CELLS IN HISTIOCYTIC DISORDERS

CLONAL HISTIOCYTOSES

Langerhans Cell Histiocytosis

Definition

  • The term Langerhans cell histiocytosis includes disorders previously called histiocytosis X (eosinophilic granuloma, Letterer-Siwe disease, Hand-Schüller-Christian disease), self-healing histiocytosis, and Langerhans cell granulomatosis.

  • The hallmark LCH DC has abundant eosinophilic to amphophilic cytoplasm and a nucleus that is kidney-shaped, deeply indented, or grooved.

Epidemiology

  • In pediatric patients, the incidence of LCH has been estimated to be 2 to 10 cases per 1 million.

  • The male-to-female ratio is close to 1.

  • Median age ...

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