Monocytes are a type of white blood cell Overview of White Blood Cell Disorders White blood cells (leukocytes) are an important part of the body’s defense against infectious organisms and foreign substances ( the immune system). To defend the body adequately, a sufficient... read more 
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 Components of the Immune System The immune system is designed to defend the body against foreign or dangerous invaders. Such invaders include Microorganisms (commonly called germs, such as bacteria, viruses, and fungi) Parasites... read more . 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 Gaucher Disease Gaucher disease is a type of lysosomal storage disorder called a sphingolipidosis. It is caused by a buildup of glucocerebrosides in tissues. Children who have the infantile form usually die... read more and Niemann-Pick disease Niemann-Pick Disease Niemann-Pick disease is type of lysosomal storage disorder. Types A and B are sphingolipidoses and are caused by a buildup of sphingomyelin in the tissues. Type C is a lipidosis that is caused... read more ).
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.
Increased number of monocytes
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 Sarcoidosis Sarcoidosis is a disease in which abnormal collections of inflammatory cells (granulomas) form in many organs of the body. Sarcoidosis usually develops in people aged 20 to 40 years, most often... read more 
, and Langerhans cell histiocytosis Pulmonary Langerhans Cell Histiocytosis Pulmonary Langerhans cell histiocytosis is a disorder in which cells called histiocytes and eosinophils (types of white blood cells) proliferate in the lungs, often causing scarring. People... read more .
Low number of monocytes
A low number of monocytes in the blood (monocytopenia) can be caused by anything that decreases the overall white blood cell count (see also Neutropenia Neutropenia Neutropenia is an abnormally low number of neutrophils (a type of white blood cell) in the blood. Neutropenia, if severe, significantly increases the risk of life-threatening infection. Neutropenia... read more 
and Lymphocytopenia Lymphocytopenia Lymphocytopenia is an abnormally low number of lymphocytes (a type of white blood cell) in the blood. Many disorders can decrease the number of lymphocytes in the blood, but viral infections... read more ), such as a bloodstream infection Sepsis and Septic Shock Sepsis is a serious bodywide response to bacteremia or another infection plus malfunction or failure of an essential system in the body. Septic shock is life-threatening low blood pressure ... read more , chemotherapy, or a bone marrow disorder.
MonoMAC syndrome
This rare genetic disorder, also called GATA2 deficiency affects the bone marrow, causing a very low monocyte count and also decreasing the number of certain kinds of lymphocytes Lymphocytes One of the body's lines of defense ( immune system) involves white blood cells (leukocytes) that travel through the bloodstream and into tissues, searching for and attacking microorganisms and... read more 
. The disorder increases the risk of infection with certain organisms, including a group of bacteria known as Mycobacterium avium complex Infections Caused by Bacteria Related to Tuberculosis (TB) Many species of mycobacteria exist. The species Mycobacterium tuberculosis is the one that causes tuberculosis. The other mycobacteria species that cause disease are discussed here. These... read more (MAC) that are related to tuberculosis, human papillomavirus Human Papillomavirus (HPV) Infection Human papillomavirus (HPV) can be sexually transmitted and causes changes in cells, which can lead to genital warts or to precancer or cancer of the cervix, vagina, vulva, anus, or throat. Different... read more 
(HPV), and certain fungi. People are also at risk of developing some types of leukemia Overview of Leukemia Leukemias are cancers of white blood cells or of cells that develop into white blood cells. White blood cells develop from stem cells in the bone marrow. Sometimes the development goes awry... read more .
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.
Antibiotics are given to treat infections, and a stem cell transplant Stem Cell Transplantation Stem cell transplantation is the removal of stem cells (undifferentiated cells) from a healthy person and their injection into someone who has a serious blood disorder. (See also Overview of... read more may cure the disorder. Unvaccinated people should be given HPV vaccination Human Papillomavirus (HPV) Vaccine The human papillomavirus (HPV) vaccine helps protect against infection by the strains of HPV that are most likely to cause the following: Cervical cancer, vaginal cancer, and vulvar cancer in... read more .
