Monocytes are a type of white blood cell that fight certain infections and help other white blood cells remove dead or damaged tissues, destroy cancer cells, and regulate immunity against foreign substances.
Monocytes are produced in the bone marrow and then enter the blood, where they account for about 1 to 10% of the circulating white blood cells (200 to 600 monocytes per microliter of blood [0.2 to 0.6 × 109 per liter]). After a few hours in the blood, monocytes migrate to tissues (such as spleen, liver, lungs, and bone marrow tissue), where they mature into macrophages.
Macrophages are the main scavenger cells of the immune system. Certain genetic abnormalities affect the function of monocytes and macrophages and cause buildup of fatty (lipid) debris within the cells. The resulting disorders are the lipid storage diseases (such as Gaucher disease and Niemann-Pick disease).
Low or high numbers of monocytes do not usually cause symptoms. However, people may have symptoms of the disorder that caused the change in monocyte number.
Diagnosis is by blood testing (complete blood count) done when a person has signs or symptoms of an infection or autoimmune disorder. Sometimes the condition is discovered by chance when a complete blood count is done during a routine physical or for the evaluation of another condition.
Treatment for monocyte disorders depends on the cause.
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. An increase in the number of macrophages in parts of the body other than in the blood (such as the lungs, skin, and other organs) can occur in response to infections, sarcoidosis, and Langerhans cell histiocytosis.
A rare genetic disorder that affects the bone marrow, causes a very low monocyte count and also decreases the number of certain kinds of lymphocytes. The disorder increases the risk of infection with certain organisms, including a group of bacteria known as Mycobacterium avium complex (MAC) that are related to tuberculosis, human papillomavirus (HPV), and certain fungi. People are also at risk of developing some types of leukemia.
Symptoms vary depending on the specific microorganism that causes the infection but often involve the lungs or skin.
Diagnosis is by a blood test that shows the absence of monocytes and by genetic testing.