Monocytes are a type of white blood cell that fights off bacteria, removes dead or damaged tissues, destroys cancer cells, and regulates immunity against foreign substances.

Monocytes are produced in the bone marrow and then enter the bloodstream, where they account for about 1 to 10% of the circulating leukocytes (200 to 600 monocytes per microliter of blood). After a few hours in the bloodstream, they migrate to tissues (such as spleen, liver, lungs, and bone marrow tissue), where they mature into macrophages, the main scavenger cells of the immune system.

Genetic abnormalities that affect the function of monocytes and macrophages and cause buildup of debris within the cells result in the lipid storage diseases (such as Gaucher disease and Niemann-Pick disease)

An increased number of monocytes in the blood (monocytosis) occurs in response to chronic infections, in autoimmune disorders, in blood disorders, and in certain cancers. A proliferation of macrophages in tissues can occur in response to infections, sarcoidosis, and Langerhans cell histiocytosis

A low number of monocytes in the blood (monocytopenia) can occur in response to the release of toxins into the blood by certain types of bacteria (endotoxemia), as well as in people receiving chemotherapy.