Monocytes are a type of white blood cell . Like other white blood cells, monocytes play an important role in the immune system's ability to destroy invaders and facilitate healing and repair.
Monocytes are formed in the bone marrow and enter the peripheral blood, where they circulate for several days. They represent 5% to 10% of circulating leukocytes in healthy people.
Monocytes are probably best known for their role as a kind of reserve in the military. If necessary, some of them can be called upon to form precursors to two other types of white blood cells: tissue macrophages and dendritic cells .
But monocytes also play other roles in infections and diseases, some of which have nothing to do with tissue macrophages and dendritic cells.
Until recently, it was believed that the main function of monocytes is to perceive the environment and to replenish the pool of tissue macrophages and dendritic cells as needed. It is now known that subsets of monocytes have different protein markers or labels on the outside, and these subsets can also behave differently .
Now three types of human monocytes are described. Classic monocytes make up approximately 80% of the total monocyte population. The remaining 20 percent can be classified by their protein labels as non-classical monocytes and intermediate monocytes.
When it comes to the different types of monocytes and how they work in the immune system, researchers are still working out the details, and much more is currently known about mouse monocytes than human monocytes.
The terms "inflammatory" and "anti-inflammatory" are also used to describe human monocytes based on specific protein markers or receptors outside of these cells.
In humans, however, it is not yet clear what proportion of monocytes are mobile enough to enter and exit tissues, and evidence suggests that there may be species of monocytes that can ingest, digest, or phagocytize invaders without actively developing a inflammation.
In the spleen
Significant numbers of human monocytes are believed to migrate to tissues throughout the body, where they can reside or give rise to macrophages, which have important roles in fighting infection and removing dead cells.
The spleen contains all the major types of "mononuclear phagocytes," including macrophages, dendritic cells, and monocytes. Thus, the spleen can be an active site of the innate immune system .
Innate immunity refers to the immunity you were born with, rather than the more specific immunity you may develop after, for example, a vaccine or after recovering from an infectious disease. The innate immune system works through a variety of mechanisms, including phagocytosis and inflammation.
Macrophages can participate in phagocytosis, the process by which they absorb and destroy debris and invaders. In this way, they can also "remove" old and spent red blood cells.
Macrophages in the spleen help remove waste and old cells from the blood, but they can also help T lymphocytes recognize foreign invaders. When this happens, it is called antigen presentation.
This last part, antigen presentation, is where the innate immune system ends and the acquired or internalized immune response to a particular foreign invader begins .
How Monocytes Help Fight Infections
From the above, we know that some monocytes become macrophages in Pac-Man-like tissues, consuming bacteria, viruses, waste and any infected or diseased cells.
Compared to specialized immune infantry (T cells), macrophages are more available to recognize and attack a new threat. They can simply sit in their usual favorite spots, or they can quickly migrate to the site of inflammation, where they may be needed to fight infection.
Other monocytes become dendritic cells in tissues, where they work with T lymphocytes. Macrophages can also present antigens to T cells , but dendritic cells have traditionally been considered by those skilled in the art.
They accumulate debris from the breakdown of bacteria, viruses, and other foreign materials and present them to T cells for viewing and form an immune response to invaders .
Like macrophages, dendritic cells are capable of presenting antigens to T cells in a specific context, as if saying, "Hey, look at this, do you think we should do more with this?"
When you have a complete blood count (CBC) with a differential count, the monocytes of the white blood cells are counted and their number is indicated, as well as what percentage of the total white blood cells are monocytes.
- An increase in the monocyte count may be associated with an infection caused by bacteria, fungi, or viruses. It can also be a response to stress. An increased number of monocytes may be due to a problem with the production of blood cells. In some cases, the excess is due to a malignancy, as in certain types of leukemia.
- Low monocyte counts can occur after chemotherapy , usually due to a low total white blood cell count .
In humans, monocytes are implicated in a number of diseases, including microbial infections, rapid-onset organ damage and shock, osteoporosis, cardiovascular disease, metabolic diseases, and autoimmune diseases .
However, the way that different types of monocytes behave in different human diseases remains an active area of research.
Monocytes in Listeria
Listeria monocytogenes is a type of bacteria that causes listeriosis , an infamous foodborne illness. Precautions against listeria are one of several taken during pregnancy, as listeria can cause meningitis in newborns and also miscarriage; Pregnant women are often advised not to eat soft cheeses that may contain listeria.
It turns out that monocytes can help fight infection, but they can also turn into Trojan horses, transporting bacteria to the brain, and that's the problem with Listeria. The listeria enters the monocytes, but then the monocytes cannot kill the bacteria and multiply .
Monocytes for leukemia
The cell line that gives rise to monocytes can become disordered and multiply uncontrollably. Acute monocytic leukemia, or "M5 FAB subtype," according to one classification system, is a form of acute myelogenous leukemia . In M5, more than 80% of the disordered cells are monocytes.
In chronic myelomonocytic leukemia (CML), there are increased numbers of monocytes and immature blood cells in the bone marrow and circulating in the blood.
CMML has characteristics of two different blood disorders and is therefore classified as a combined entity, myelodysplastic syndrome / myeloproliferative neoplasia (MDS / MPN) according to the classification system of the World Health Organization. It progresses to acute myeloid leukemia in approximately 15 to 30% of patients.
Monocytes for lymphoma and other cancers
Researchers are discovering that monocytes can have undesirable effects on tumors and malignancies in the family of lymphocytes and white blood cells (these diseases are known as lymphoproliferative diseases).
The presence of macrophages and their activity in tumors has been associated with the ability of tumor cells to create a blood supply, enter and travel through the bloodstream. In the future, this discovery could lead to macrophage-targeted therapy to prevent metastasis and tumor growth.
For various diseases, some doctors begin to use the absolute monocyte count as an indicator of risk or worse prognosis before treatment.
Increases in monocyte counts above a certain threshold are associated with worse outcomes in patients with T-cell lymphomas and Hodgkin's disease . The lymphocyte-to-monocyte ratio can also help identify patients at high risk for diffuse large B-cell lymphoma. and untreated metastatic colorectal cancer .