A monocyte is a type of phagocytic leukocyte which plays an important role in the human immune system. Monocytes replenish resident macrophages and dendritic cells under normal states, and in response to inflammation signals, monocytes move quickly, between 8 to 12 hours, to sites of infection in the tissues and divide and differentiate into macrophages and dendritic cells to elicit an immune response. Half of monocytes are stored in the spleen. Monocytes are usually identified in stained smears by their large bilobate nucleus.
Monocytes are produced from myelo-monocytic stem cells in the bone marrow. When they then go into blood stream, they circulate for a few days, then move into tissues. In the tissue they further mature into macrophages. Monocytes make up about 8% of the white blood cells. They are closely related to neutrophils. Monocytes process foreign antigens and present them to the immunocompetent lymphocytes. They are also capable of phagocytosis.