Monocyte Functions in the Body

White Blood Cells That Help Fight Infection

3D rendered Illustration of anatomically correct Monocyte immune system defense cells
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Monocytes are a type of white blood cell that's formed in the blood marrow and released into the blood. Like other white blood cells, monocytes are important in the immune system’s ability to destroy invaders like viruses and bacteria. They also help with healing and repair in the body.

This article will go over the types of monocytes and their functions. It also covers various health conditions that can cause too many or too few monocytes in your blood.

Monocyte Types

Researchers used to think that the main role of monocytes was sensing the environment and replenishing the pool of tissue macrophages and dendritic cells, which are two other types of white blood cells.

Now, we know that different kinds of monocytes have unique markers or protein tags on the outside of them. These different types of monocytes act differently in the body.

The three kinds of human monocytes are:

  • Classical monocytes account for about 80% of the total monocyte population in the body.
  • The remaining 20% can be classified by their protein tags as either non-classical monocytes or intermediate monocytes.

As for the role that different kinds of monocytes have in the immune system, researchers are still figuring that out. We know a lot more about mouse monocytes than we do about human monocytes.

"Inflammatory” and “anti-inflammatory” are also applied to human monocytes. Whether they have these terms applied to them is based on the protein tags (or receptors) that are found on the outside of these cells.

However, we don't know exactly what proportion of monocytes in humans can move well enough to go in and out of the body's tissues. Research suggests that some monocytes can "eat" (engulf) and digest (phagocytize) invaders without actively promoting inflammation.

What Is the Normal Range for Monocytes?

The normal range for monocytes in healthy adults is 4% to 8%. The normal range for the absolute number of monocytes is 200-800 per cubic millimeter.

However, the lab that processes your blood sample might use different ranges to report your monocyte count. If you aren't sure if your levels are normal, ask your provider.

Monocyte Functions

Monocytes form in the bone marrow, then get released into the blood where they will circulate for several days.

Monocytes are best known for their role as "reserve forces" in the body, as they can be called on to form the beginnings (precursors) of two other types of white blood cells: tissue macrophages and dendritic cells. Monocytes also have other roles in infection and disease.

Monocytes in the Spleen

A good number of human monocytes move into the tissues of your body where they help make the macrophages that fight infection and clean up dead cells.

The spleen has all major types of mononuclear phagocytes in it, including macrophages, dendritic cells, and monocytes. The spleen is an active site for the innate immune system.

Monocytes and Innate Immunity

Innate immunity is the immunity that you are born with. It's not the same as the targeted immunity you get from a vaccine or after recovering from an infectious illness. The innate immune system works in a different way, including phagocytosis and inflammation.

Macrophages can use phagocytosis—a process by which they "eat" (engulf) and destroy debris and invaders. They can also "retire" old, worn-out red blood cells through this process.

Macrophages in the spleen clean up debris and old cells from the blood, but they also help T-lymphocytes recognize foreign invaders—what's called antigen presentation.

Antigen presentation is where the innate immune system ends and the acquired or learned immune response to a specific foreign invader starts.

Fighting Infection

Some monocytes turn into macrophages in the tissues and act sort of like Pac-Man, gobbling up bacteria, viruses, debris, and any cells that have been infected or are sick.

Compared to T-cells, macrophages are more immediately available to recognize and attack a new threat. They can do this just by sitting in their usual spot in the body or quickly moving to a site of inflammation where they are needed to fight infections.

Other monocytes turn into dendritic cells in the tissues where they work with T lymphocytes. Macrophages can also present antigens to T-cells. However, dendritic cells are considered the "specialists" at this task. 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.

Like macrophages, dendritic cells are able to present antigens to T-cells in a certain context, as if to say, "Do you think we should be doing more about this?"

Associated Conditions

Certain health conditions can cause you to have too few or too many monocytes in your blood.

When you have a complete blood count (CBC) blood test done with a breakdown of the different cells (differential count) the white blood cell monocytes are counted and the number is reported. You will also see what percentage of total white blood cells are monocytes.

  • Monocytosis is an increase in monocytes. Infection by bacteria, fungus, or viruses can cause high monocytes. It can also be a response to stress. High monocyte counts can also be caused by a problem with blood cell production. Malignancy—such as certain types of leukemia—can also cause high monocytes.
  • Monocytopenia is a low number of monocytes. It can happen after chemotherapy because your overall white blood cell count is low.

Researchers are still learning but they think that in humans, monocytes might also be involved in certain diseases including microbial infection, shock, organ injuries, osteoporosis, cardiovascular disease, metabolic diseases, and autoimmune diseases.

Monocytes in Listeria

Listeria monocytogenes is a species of bacteria that can cause listeriosis, a type of foodborne illness. Listeria precautions are one of several given during pregnancy because Listeria can cause meningitis in newborns as well as pregnancy loss. People who are pregnant are often advised not to eat soft cheeses, which can have Listeria in them.

Monocytes can help fight infection, but they can also become “Trojan horses,” by transporting the bacteria into the brain. When Listeria gets inside the monocytes, the cells are unable to kill the bacteria and they are able to multiply.

Monocytes in Leukemia

If the line of cells that make monocytes get disordered, the cells may start making copies of themselves out of control.

Acute monocytic leukemia is a form of acute myelogenous leukemia. In this type, more than 80% of the disordered cells are monocytes.

In chronic myelomonocytic leukemia (CMML), there are increased numbers of monocytes and immature blood cells in the bone marrow and circulating in the blood.

CMML has features of two different blood disorders. Therefore, the World Health Organization (WHO) classifies it as myelodysplastic syndrome/myeloproliferative neoplasm (MDS/MPN). It progresses to acute myeloid leukemia in about 15% to 30% of patients.

Monocytes in Lymphoma and Other Cancers

Monocytes may have actions related to tumors and cancerous behaviors of white blood cells called lymphocytes. These diseases are known as lymphoproliferative diseases.

The presence of macrophages and their activities in tumors have been associated with enabling the tumor cells to build a blood supply and to invade and travel through the bloodstream. Knowing this might help researchers create cancer therapies that target macrophages to prevent metastasis and tumor growth.

Some providers use the absolute monocyte count as an indicator of risk or a worse prognosis before treatment in people with cancer.

For example, an increased number of monocytes above a certain threshold is associated with worse outcomes in patients with T-cell lymphomas and Hodgkin's disease.

The lymphocyte-to-monocyte ratio may also help identify high-risk patients in diffuse large B-cell lymphoma and untreated metastatic colorectal cancer.

Summary

Monocytes are a type of white blood cell. The function of monocytes is to be involved in the immune system. Monocytes are also linked to certain medical conditions, including cancer. You can find out if your monocyte levels are too high or too low by having a blood test.

Frequently Asked Questions

  • Are monocytes high in COVID-19?

    Infections can cause monocytes to increase. Some people with viral illnesses like COVID may have higher than normal levels of white blood cells in their blood, including monocytes.

  • What does monocytopenia mean?

    Monocytopenia means you have lower-than-normal numbers of monocytes in your blood.

  • What does monocytosis mean?

    Monocytosis means the number of monocytes in your blood is higher than normal.

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Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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Additional Reading

By Karen Raymaakers
Karen Raymaakers RN, CON(C) is a certified oncology nurse that has worked with leukemia and lymphoma patients for over a decade.