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INTRODUCTION

  • Monocytes in the blood are in transit. They function in the tissues, where they mature into macrophages and participate in:

    — Inflammation, including granulomatous reactions, atheroma formation, and tissue repair

    — Immunologic reactions, including delayed hypersensitivity

    — Reactions to neoplasia and allografts

  • The need for macrophages in tissues also can be met by local proliferation of macrophages, not requiring increased transit of blood monocytes.

  • In normal subjects, a mean of 87% of blood monocytes intensely express CD14 (lipopolysaccharide receptor) but not CD16 (Fc receptor) (CD14++/CD16–), the classical subset, designated MO1.

  • A mean of 4% of blood monocytes have weaker expression of CD14 and strongly express CD16 (CD14+CD16++), the intermediate subset, designated MO2.

  • A third monocyte subset, at a mean of 9% of blood monocytes, have weak expression of CD14 and strong expression of CD16 (CD14low/CD16++), the nonclassical subset, designated MO3.

  • Distinctions among monocyte subsets have been validated by gene expression profiling.

  • Older persons have a striking decrease in the proportion of classical CD14++/CD16– monocytes to intermediate CD14+/CD16++ monocytes compared with younger persons.

  • Disorders rarely produce abnormalities of monocytes alone in the absence of other blood cell abnormalities.

NORMAL BLOOD MONOCYTE CONCENTRATION

  • The monocyte count averages 1.0 × 109/L in neonatal life, gradually decreasing to a mean of 0.4 × 109/L in adult life.

  • Monocytosis (in adults) is a count greater than 0.8 × 109/L.

  • Monocytopenia is a count less than 0.2 × 109/L.

HEMATOLOGIC DISORDERS ASSOCIATED WITH MONOCYTOSIS

TABLE 35–1DISORDERS ASSOCIATED WITH MONOCYTOSIS

Neoplastic or Clonal Monocytic Proliferations

  • Myelodysplastic syndrome

  • Acute myelogenous leukemia (myelomonocytic or monocytic types)

  • Chronic myelomonocytic leukemia

  • Juvenile myelomonocytic leukemia

  • Unusual type of BCR-ABL (p190)–positive chronic myelogenous leukemia with monocytosis

Reactive (Nonclonal) Monocytic Proliferations

  • Neutropenic states: cyclic neutropenia; ...

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