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INTRODUCTION

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  • The terms histiocyte and macrophage are synonyms for the mature cell of the monocyte-macrophage system. The latter is the preferred term in discussions of cell biology and immunology, but “histiocytosis” continues to be used by pathologists and in textbooks for diseases of macrophages because of its entrenchment in the medical literature.

  • A classification of the histiocytoses most relevant to hematologists is shown in Table 36–1. They have been classified based on whether they are monocyte-derived dendritic cell–related, monocyte-macrophage–related, or neoplastic transformations of dendritic cells or macrophages.

  • Distinctions among the diseases of macrophages are made based on clinical findings, histopathology, immunophenotyping of surface antigen, cytochemistry, and cytogenetic or genetic features (Table 36–2).

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TABLE 36–1CLASSIFICATION OF HISTIOCYTIC DISORDERS
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TABLE 36–2DIFFERENTIATING CHARACTERISTICS OF HISTIOCYTES
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CLONAL HISTIOCYTOSES

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Langerhans Cell Histiocytosis

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Definition and History
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  • 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.

  • Langerhans cells are macrophages with irregularly shaped nuclei present in epidermis, mucosa, lymph nodes, thymus, and spleen.

  • As all macrophages types, they originate in marrow from a multipotential hematopoietic cell.

  • Identified by unique racquet-shaped ultrastructural inclusions (Birbeck bodies) and by immunoreactivity to neuroprotein S-100, neuronal specific enolase, and surface antigen CD1a and CD207 (Langerin). Birbeck bodies, CD1a expression, and strong CD207 expression are pathognomonic for Langerhans cells.

  • The principal function of Langerhans cells is to process antigen and present it to T cells.

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Etiology and Pathogenesis
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  • Although long thought to be ...

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