Monocytes are white blood cells that derive from the bone marrow. A monocyte is part of the innate immune response and functions to regulate cellular homeostasis, especially in the setting of infection and inflammation. They account for approximately 5% of circulating nucleated cells in normal adult blood. The half-life of circulating monocytes is approximately one to three days. Monocytopenia, a decrease in circulating monocytes, is a common finding in myelodysplastic syndromes. While monocytosis, an increase in circulating monocytes, is a common finding in the peripheral blood, especially in association with infection, trauma, medications, autoimmune disease, and some malignancies. When monocytosis is persistent and unexplained, the diagnosis of chronic myelomonocytic leukemia merits investigation.
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