Monocytes and macrophages
Monocytes are the largest white blood cells circulating in the blood. When they move into the tissues, they can become macrophages, cells that “eat” germs and clean up, or dendritic cells. This topic explains what these cells do, what monocytes mean on a blood test and how they relate to some leukemias.
01Short answer
02Simple explanation
A monocyte is a large white cell that travels in the blood. It stays in the blood only for a short time, then passes through the vessel wall into the tissues.
In the tissues, a monocyte can become a macrophage. “Macrophage” means “big eater”: this cell swallows and digests germs, dead cells and debris, acting as the body’s “cleaner”.
A monocyte can also become a dendritic cell. A dendritic cell picks up a piece of a germ, carries it to a lymph node and shows it to T cells: “Learn to recognise this enemy!”
Macrophages live in many organs: in the spleen they clear old blood cells and germs, in the lungs they trap dust and bacteria, and in the liver they filter the blood.
The link with leukemia: in some leukemias, abnormal cells of the monocyte line multiply (CMML, monocytic types of AML). A rise or fall in monocytes on a blood test can give a doctor a clue, but on its own it is not a diagnosis.
03Scientific explanation
From monocyte to tissue cell: according to the NCI dictionary, a monocyte is made in the bone marrow, travels in the blood to the tissues and there becomes a macrophage or a dendritic cell. According to Cleveland Clinic, monocytes are the largest white blood cells, about twice the size of a red blood cell.
Phagocytosis: according to the NCI and InformedHealth, macrophages are phagocytes: they surround and kill germs, remove dead cells and activate other immune cells. The remains of a swallowed germ are moved to the cell surface, where the adaptive immune system can recognise them.
Presenting antigens: according to the NCI, dendritic cells are found in tissues such as the skin and boost immune responses by showing antigens on their surface to other immune cells. Macrophages are also antigen-presenting cells.
A newer scientific view: according to a review by Ginhoux and Jung, research done mainly in animals shows that most tissue macrophages form before birth from embryonic precursors and maintain themselves by self-renewal, while monocytes mainly come to help during inflammation.
04What happens in the body?
Monocytes on a blood test
According to MedlinePlus, monocytes usually make up 2–8% of white blood cells; according to Cleveland Clinic, this is about 200–800 cells per microlitre in healthy adults. Normal ranges vary slightly between laboratories. A raised monocyte count is called monocytosis, and a low count monocytopenia. In both cases, the doctor looks for the cause based on other signs and tests. Your doctor always interprets the result together with your other results.
How does a macrophage fight a germ?
When a macrophage recognises a germ, it wraps its membrane around it and pulls it inside. Inside the cell, the germ is broken down by enzymes and toxic substances. At the same time, the macrophage releases signalling substances that call neutrophils and other cells to the site of inflammation. According to InformedHealth, tissue cells and immune cells die in this fight along with the germs; together, their remains form a yellowish fluid called pus.
Dendritic cells: the messengers
Dendritic cells keep watch in the skin, the mucous membranes and other tissues. After picking up a piece of a germ, they travel to a lymph node, find matching T cells there and activate them. In this way, innate immunity “passes the message” to adaptive immunity.
Monocytes and leukemia
According to the NCI, in CMML the bone marrow makes too many monocytes and myelocytes. According to the American Cancer Society, AML includes acute myelomonocytic and monocytic types; some types of AML can spread to the gums, causing swelling and bleeding. In hairy cell leukemia, by contrast, low monocytes are typical.
During treatment
Because chemotherapy suppresses the bone marrow, monocytes can fall just as neutrophils do. When white cells are low, the risk of infection rises; if you get a fever, call your team at once. Your team follows the recovery of your blood counts with regular tests.
05The path of a monocyte
From the bone marrow to the tissues (simplified).
06Monocyte, macrophage and dendritic cell
| Cell | Where it is found | Main job |
|---|---|---|
| Monocyte | In the blood | Move into the tissues and become a macrophage or dendritic cell |
| Macrophage | Almost all tissues (spleen, lungs, liver and others) | Swallow germs and dead cells, call other cells |
| Dendritic cell | Skin, mucous membranes, lymph nodes | Show antigens to T cells |
| Neutrophil (for comparison) | In the blood and at sites of inflammation | Swallow bacteria quickly, as the first line |
* Neutrophils are phagocytes too; we cover them in detail in the topic on neutropenia.
07Key terms
- Monocyte
- A white blood cell from the bone marrow that becomes a macrophage or dendritic cell in the tissues.
- Macrophage
- An immune cell that swallows and kills germs and clears away dead cells.
- Dendritic cell
- A cell that boosts immune responses by showing antigens to other immune cells.
- Phagocyte
- A cell that can swallow and destroy germs and foreign material.
- Phagocytosis
- The process of a cell swallowing and digesting particles.
- Antigen-presenting cell
- A cell that activates other immune cells by showing an antigen on its surface.
- Monocytosis
- A higher-than-normal number of monocytes in the blood.
- Monocytopenia
- A lower-than-normal number of monocytes in the blood.
- CMML
- Chronic myelomonocytic leukemia: a disease in which the bone marrow makes too many monocytes.
- Cytokine
- A protein released by immune cells to “talk” to each other.
08Key points
- Monocytes are large white blood cells made in the bone marrow.
- In the tissues, they can become macrophages or dendritic cells.
- Macrophages swallow and clear away germs and dead cells.
- Dendritic cells activate T cells.
- These cells are a bridge between innate and adaptive immunity.
- Monocytes usually make up 2–8% of white cells.
- CMML and some types of AML arise from the monocyte line.
- Always discuss blood test results with your doctor.
09Frequently asked questions
My monocytes are slightly high. Should I worry?
According to Cleveland Clinic, monocytes can rise with infections, inflammatory and autoimmune diseases and other causes. A single slight rise is usually not dangerous, but your doctor will look further into lasting changes.
My monocytes are low. What does that mean?
Anything that lowers white blood cells, including chemotherapy, can also lower monocytes. Your doctor decides on the cause and what it means.
Do macrophages fight leukemia?
Macrophages can also swallow tumour cells, but sometimes the tumour environment weakens them. This is an active area of research; for now, your treatment is based on proven methods chosen by your team.
How are monocytes different from neutrophils?
Both are phagocytes. Neutrophils are the fastest and most numerous “first responders” against bacteria, while monocytes turn into long-lived tissue macrophages that both clean up and link up with the rest of the immune system.
What should I eat to raise my monocytes?
There are no proven foods that specifically raise monocytes. A balanced diet supports your general health; during treatment, follow food safety rules.
Is CMML a leukemia or a different disease?
CMML belongs to a mixed group with features of both myelodysplastic and myeloproliferative diseases. According to the NCI, its signs can include fever, weight loss and feeling very tired. Your hematology team plans its treatment and follow-up individually.
10Questions for your doctor
Take these questions to your appointment. The ones you save are collected on the “My questions” page and stay on this device only.
Is my monocyte level normal?
Is the change in my monocytes related to my illness?
What does monocyte recovery tell us?
How will this value be followed during treatment?
12Sources
- National Cancer Institute (NCI). NCI Dictionary of Cancer Terms: monocyte
- National Cancer Institute (NCI). NCI Dictionary of Cancer Terms: macrophage
- National Cancer Institute (NCI). NCI Dictionary of Cancer Terms: dendritic cell
- National Cancer Institute (NCI). NCI Dictionary of Cancer Terms: phagocyte
- National Cancer Institute (NCI). NCI Dictionary of Cancer Terms: antigen-presenting cell
- Cleveland Clinic. Monocytes: Function, Range & Related Conditions
- MedlinePlus (U.S. National Library of Medicine). Blood differential test
- InformedHealth.org (IQWiG). The innate and adaptive immune systems
- Nature Reviews Immunology, 2014. Ginhoux F, Jung S. Monocytes and macrophages: developmental pathways and tissue homeostasis
- National Cancer Institute (NCI), SEER Training. Spleen
- National Cancer Institute (NCI). Myelodysplastic/Myeloproliferative Neoplasms Treatment (PDQ®) – Patient Version
- American Cancer Society. Acute Myeloid Leukemia (AML) Subtypes and Prognostic Factors
- American Cancer Society. Signs and Symptoms of Acute Myeloid Leukemia (AML)
- Blood, 2017. Grever MR et al. Consensus guidelines for the diagnosis and management of patients with classic hairy cell leukemia