Anatomy & Organs

Monocytes – structure, function & diseases

Monocytes

Monocytes are cells of human blood . They belong to the white blood cells ( leukocytes ) and play a role in the immune system .

What are monocytes?

Monocytes are part of human blood. They belong to the cell group of leukocytes and thus play a role in defense. Like many other leukocytes, monocytes can leave the blood and migrate into the tissues .

There they develop into macrophages . Macrophages are scavenger cells. They remove cell residues, destroy tumor cells , eat bacteria , other pathogens and foreign bodies and serve to heal wounds .

Anatomy & Structure

Monocytes are very variable in their external appearance. They have a diameter of 4 to 21 µm. This makes them one of the largest blood cells within the cell group of leukocytes. About three to eight percent of all leukocytes are monocytes.As their name suggests, they have a single cell nucleus . This is quite large and usually bean-shaped. Compared to other cells and for its size, it contains relatively little cytoplasm . Monocytes are not homogeneous, which means that there are different subgroups. Typically, the cells carry the surface marker CD14 on their surface. However, there are also monocytes that carry the surface marker CD16 in addition to the CD14 marker. Based on the combination of different surface markers, three subpopulations of the monocytes can be distinguished. These are the “classical monocytes” (CD14++CD16-), “intermediate monocytes” (CD14++CD16+) and “non-classical monocytes” (CD14+CD16++).

Monocytes are formed in the bone marrow as part of monocytopoiesis. Monocytopoiesis is a part of hematopoiesis . Cells go through different stages during maturation. They develop from the hemocytoblast via the monoblast and the promonocyte to the finished monocyte. Both the monocytes and the neutrophilic granulocytes develop from the bipotent stem cell CFU-GM. Only at a later stage of differentiation do the cell lines of monocytes and granulocytes split. The formation of the cells is influenced by the growth factors GM-CSF (granulocyte-macrophage colony-stimulating factor) and M-CSF (monocyte colony-stimulating factor).

Monocytes only circulate in the blood for about 12 to 48 hours, after which they usually migrate to the surrounding tissues, where they further differentiate into different cell forms.

The most important storage location for monocytes is the spleen . From here they can be released in large numbers in the event of an acute need.

Function & Tasks

During the short time that the monocytes are circulating in the blood, their main function is phagocytosis . Inside the cells there are numerous lysosomes. Lysosomes are cell organelles that contain digestive enzymes. When the monocytes encounter a pathogen or foreign body, they absorb it into their cell interior. There it is rendered harmless and digested by the lysosomes.

The monocytes belong to the non-specific cellular defence. They not only eat pathogens and foreign substances, but also produce cytokines, chemokines, growth factors and complement factors. Most of these substances play a role in immunological reactions and in inflammatory processes within the body. They are therefore also referred to as mediators.

Monocytes are also able to present part of the material they have phagocytosed on their surface. This is also referred to as an antigen presentation. The lymphocytes recognize these presented antigens and then produce antibodies. This means that more of these pathogens can be rendered harmless more quickly. When the monocytes have migrated into the tissue, they are called macrophages.

Macrophages recognize foreign proteins in the tissue. They also absorb these foreign proteins during phagocytosis and break them down intracellularly. They also release chemical attractants to attract more macrophages and other immune cells. They also release cytokines that cause local inflammation. The antigen presentation to the macrophages is carried out by the MHC-II molecule.

But the macrophages not only take care of foreign materials, they also eliminate old or defective cells from their own body. If the infection has been successfully combated, the scavenger cells are also involved in the healing process. They promote the formation of scar tissue and the formation of new blood vessels.

Some macrophages have special functions in organs. For example, there are macrophages in the testes that secrete a substance that neighboring cells need in order to be able to produce testosterone.

Diseases

If the number of monocytes in the blood is reduced, this is called monocytopenia. The lower normal limit is 200 cells per microliter of blood. Monocytopenias usually occur in the context of leukemia . An increase in monocytes is called monocytosis. Monocytosis is a subtype of leukocytosis.Monocytosis is found in chronic inflammation, necrosis and disease processes with increased phagocytosis. For example, systemic histoplasmosis or leishmaniasis leads to monocytosis.

A disease in which the monocytes play an important role is tuberculosis . In tuberculosis, the causative agent, Mycobacterium tuberculosis, enters the lungs via the airways . There, the macrophages absorb the pathogen. However, the pathogens have a protective layer so that they cannot be finally digested by the macrophages. In order to still protect the body from the bacteria , more monocytes are taken from the blood.

These transform into so-called epitheloid cells and surround the macrophages with the bacterium like a protective wall. The cells within this protective wall die off, but the pathogens remain trapped. It only becomes problematic when the protective wall can no longer be maintained due to an immune deficiency . The pathogens can then be released years after the initial infection and cause reinfection.

Lisa Newlon
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Hello! I am Lisa Newlon, and I am a medical writer and researcher with over 10 years of experience in the healthcare industry. I have a Master’s degree in Medicine, and my deep understanding of medical terminology, practices, and procedures has made me a trusted source of information in the medical world.