Monocytes are a type of white blood cell that play an important role in the immune system by helping fight infections caused by bacteria, viruses, and other harmful agents. These cells are produced in the bone marrow and circulate in the blood before moving into tissues, where they become macrophages.
Monocyte levels can be measured with a complete blood count (CBC), which helps determine if levels are normal, high, or low. Normal monocytes typically make up a small percentage of total white blood cells, but changes in these levels can be linked to infections, autoimmune diseases, or blood disorders.
High monocytes, known as monocytosis, may be associated with conditions like chronic infections, inflammatory diseases, or certain cancers. Low monocytes, or monocytopenia, can indicate a weakened immune system and may occur with infections, medication use, or bone marrow problems.
Reference ranges
Reference ranges for monocytes vary by laboratory. In general, monocytes make up about 2% to 10% of total leukocytes, or approximately 200 to 1,000 cells per microliter (µL) of blood.
Abnormal monocyte levels do not usually cause symptoms. Instead, symptoms are typically related to the underlying condition affecting monocyte levels and are often only identified through a blood test.
High monocytes
An increased number of monocytes (monocytosis) is usually associated with:
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Chronic infections, such as tuberculosis
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Ulcerative colitis
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Protozoan infections
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Hodgkin’s disease
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Myelomonocytic leukemia
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Multiple myeloma
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Autoimmune diseases, such as lupus and rheumatoid arthritis
High monocyte levels typically do not cause symptoms on their own and are usually detected only through a blood test. Any symptoms a person has are generally due to the underlying condition causing monocytosis and should be evaluated and treated by a doctor.
Low monocytes
Low monocyte levels (monocytopenia) usually indicate a weakened immune system. According to the National Cancer Institute, monocytes help kill microorganisms and support immune responses.
They can occur with blood infections, chemotherapy, and bone marrow diseases such as aplastic anemia and leukemia.
Low monocytes may also be seen with skin infections, corticosteroid use, and HPV infection. It is rare for monocyte levels to be close to zero. In these cases, doctors may investigate monoMAC syndrome, a genetic condition in which the bone marrow does not produce enough monocytes.
Treatment may include medications to prevent or treat infections, such as antibiotics, and in some cases a bone marrow transplant to address the underlying genetic disease.