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Disorders of the mononuclear phagocyte system that exclusively result in abnormalities of either monocytes, macrophages, or dendritic cells are uncommon. Abnormalities of blood monocyte concentration, ie, monocytosis or monocytopenia, are rarely isolated; they are usually accompanied by altered concentrations of granulocytes or lymphocytes. Monocytes are critical sources for proinflammatory and inflammatory cytokines and, when inappropriately activated, can result in the lymphohistiocytic hemophagocytic syndrome with fever, intravascular coagulation, and organ pathology. A variety of hematopoietic neoplasms may have a phenotype characterized by a large proportion of leukemic monocytes, promonocytes or monoblasts. Clonal monocytosis is a rare initial manifestation of a myelodysplastic syndrome. Some cases of myelogenous leukemia have progenitor cells that mature preferentially into leukemic monocytes, including acute monoblastic or monocytic leukemia, chronic myelomonocytic leukemia, and juvenile myelomonocytic leukemia. Two acquired diseases, hairy cell leukemia and aplastic anemia, result in a severe depression of blood monocytes (along with other blood cell types). Mutations in GATA2 are associated with profound monocytopenia (amonocytosis) and mycobacterial infections (the MonoMAC syndrome). Inherited disorders affecting white cells, such as chronic granulomatous disease and Chédiak-Higashi syndrome, result in impaired monocyte function. Monocyte dysfunction may accompany a variety of severe illnesses, such as sepsis, trauma, and cancer. Monocytes also contribute to a variety of diseases, such as Crohn disease and rheumatoid arthritis, by virtue of their being a principal source of tumor necrosis factor. Monocytes play a pathogenetic role in other complex, acquired disorders such as thrombosis and atherogenesis. Macrophage or dendritic cell abnormalities principally involve tissues, where the diseases are referred to pathologically as histiocytosis. These disorders can be inherited, such as familial hemophagocytic lymphohistiocytosis; inflammatory, such as infectious hemophagocytic lymphohistiocytic syndrome; or clonal (neoplastic), such as Langerhans cell histiocytosis. They can result from an inherited enzyme insufficiency in macrophages that lead to exaggerated storage of macromolecules and subsequent tissue injury, such as in Gaucher disease. Table 69–1 catalogues the major categories of qualitative and quantitative abnormalities of monocytes, macrophages, and dendritic cells. Chapter 70, Table 70–1, provides a more detailed list of disorders associated with monocytopenia or monocytosis.

Acronyms and Abbreviations

CD, cluster of differentiation; GM-CSF, granulocyte-macrophage colony-stimulating factor; HLA-DR, human leukocyte antigen-D related; IL, interleukin; MonoMAC syndrome, monocytopenia and Mycobacteriaum avian complex infections syndrome; MPS, mononuclear phagocyte system; TNF, tumor necrosis factor.

Table 69–1.Disorders of Monocytes, Macrophages, and Dendritic Cells

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