What Are Monocytes?

Monocytes Perform Immune System Functions

Monocyte cell
Monocyte cell. Science Picture Co/Collection Mix: Subjects/Getty Images

Definition of Monocyte

Monocytes are a type of white blood cell, and they are part of your immune system. They are produced in the bone marrow, and after about three or four days, they are released into the blood stream. Approximately 3-10% of the circulating blood cells in healthy individuals are monocytes.

When you have a CBC blood test done, monocytes are counted and the number is reported, as well as what percentage of total white blood cells are monocytes.

An increase in monocytes may be the result of an infection by a bacteria, fungus, or virus. It can also be a response to stress, or because of diseases such as certain types of leukemia. Low monocytes may be seen after chemotherapy, usually because your overall white blood cell count is low.

What Do Healthy Monocytes Do in the Body?

Most monocytes aren't circulating in the bloodstream. They stay in the blood only a few hours after they are released from the bone marrow. Instead, they migrate into tissues throughout your body where they perform essential functions to fight infection and clean up dead cells. About half of them end up in the spleen. That is a good site for them to perform their functions as part of the innate immune system, cleaning the blood of debris and helping the T-lymphocytes recognize foreign invaders.

Monocytes help to fight infection in two ways. Those that transform into macrophages in the tissues are like Pac-Man, gobbling up bacteria, viruses, debris, and any cells that have been infected or are sick.

They are the garbage disposal units of the body. They are non-specific in what they break down. They may simply be sitting in their usual favorite spots, or they may quickly migrate to a site of inflammation where they may be needed to fight an infection.

Other monocytes transform into dendritic cells in the tissues, where they work with the T lymphocytes.

They accumulate debris from the breakdown of bacteria, viruses, and other foreign material and present it to the T-cells so they can see it and form an immune response to the invaders. The dendritic cells are saying, "Hey look at this, do you think you should do something about this?" The dendritic cells don't attack things themselves, but they enable the T-cells to do so.

Monocytes in Leukemia

The cell line that forms monocytes can become disordered and multiply out of control. Acute monocytic leukemia, FAB subtype M5, is one of the forms of acute myelogenous leukemia. In M5, more than 80% of the disordered cells are monocytes. 

In chronic myelomonocytic leukemia (CMML) there are increased numbers of moncytes and immature blood cells in the bone marrow and circulating in the blood. It is usually grouped with myelodysplastic/myeloproliferative diseases. It can progress to acute myeloid leukemia in 15%-30% of patients.

Monocytes in Lymphoma and Cancer

Researchers are finding that monocytes may have undesirable actions in relation to tumors and lymphoproliferative diseases.

Macrophages working in areas where there are tumors are associated with allowing the tumor cells to build a blood supply and to invade and travel through the bloodstream. This may lead to therapy that targets macrophages to prevent metastasis and tumor growth.

Some are using the absolute monocyte count as an indicator of a worse prognosis before treatment. An increased number of monocytes above a certain threshold is associated with a poorer outcome in patients with T-cell lymphomas and Hodgkin disease. The lymphocyte-to-monocyte ratio may also help identify high-risk patients in diffuse large B-cell lymphoma and untreated metastatic colorectal cancer.


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