Table of Contents
Definition of Monocytes
A leukocyte (white blood cell), also called an agranulocyte, that has a single-lobed nucleus and contains no granules in its cytoplasm.
During the first phase, the monocyte circulates in the blood and the lymph, functioning as a phagocyte that consumes pathogenic particles in the circulation.
After about 24 hours the monocyte migrates into the tissue to enter its second phase of life.
Once in the tissue the monocyte matures, becoming a fixed phagocytic cell called a macrophage that may acquire a specific name, depending on its location. About half of the body’s macrophages migrate to the lymphatic structures. Most of the remainder reside in the liver, where they are called Kupffer cells. Macrophages that settle in the layers of the skin are Langerhans cells, and those that inhabit the bone are osteoclasts.
Two to 8 percent of the body’s leukocytes are monocytes; a normal monocyte count is 200 to 1100 monocytes per microliter of whole blood.
The number of monocytes in circulation may increase with infection, leukemia, lymphoma, many other types of cancer, and autoimmune disorders in which there is active inflammation and autoimmune activity. The number of monocytes in circulation may decrease in aplastic anemia and with steroid medications.
For further discussion of blood and lymph structure and function please see the overview section “The Blood and Lymph.”