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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.[1] They account for approximately 5% of circulating nucleated cells in normal adult blood.[2] The half-life of circulating monocytes is approximately one to three days.[3] Monocytopenia, a decrease in circulating monocytes, is a common finding in myelodysplastic syndromes.[4] 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.[1] When monocytosis is persistent and unexplained, the diagnosis of chronic myelomonocytic leukemia merits investigation.[5]

The role of monocytes is versatile and related to various types of disorders and infectious processes, and inflammation. Monocytosis and monocytopenia have been identifying factors in multiple hematologic disorders, such as leukemias and myelodysplastic syndromes.[22] Monocytosis is present in numerous inflammatory and immune disorders, such as connective tissue diseases, such as rheumatoid arthritis, systemic lupus erythematosus, and sarcoidosis.[23][24][25] Monocytes play a crucial role in the immune response to acute infectious and inflammatory processes, such as mycobacterial infections and varicella-zoster virus.[26][27] Monocytosis also occurs in myocardial infarction and is often related to the serum creatinine kinase and extent of infarction.[28] Monocytes can remove the debris and help with remodeling following myocardial infarctions.[29]