Monocytes are cells of the human blood. They belong to the white blood cells (leukocytes]) and play a role in immune defense.
What are monocytes?
Monocytes are part of the human blood. They belong to the leukocyte cell group and thus play a role in defense. Like many other leukocytes, monocytes can leave the blood and migrate into tissues. There they develop into macrophages. Macrophages are scavenger cells. They remove cell debris, destroy tumor cells, eat bacteria, other pathogens and foreign bodies, and serve to heal wounds.
Anatomy and structure
Monocytes are highly variable in their external appearance. They range in diameter from 4 to 21 µm. This makes them among the largest blood cells within the leukocyte cell group. About three to eight percent of all leukocytes are monocytes. As their name suggests, they have a single nucleus. This is quite large and usually bean-shaped. Compared to other cells and to 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 combinations of different surface markers, three subpopulations can be distinguished in monocytes. 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. During maturation, the cells pass through different stages. From the hemocytoblast, they develop through the monoblast and the promonocyte to the final monocyte. Both monocytes and neutrophil granulocytes develop from the bipotent stem cell CFU-GM. Only at a later stage of differentiation do the cell lineages 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 circulate in the blood for only about 12 to 48 hours, after which they usually migrate to surrounding tissues for further differentiation into various cell forms. The most important storage site of monocytes is the spleen. From here, they can be released in large numbers when acutely needed.
Function and tasks
During the short time that monocytes circulate in the blood, their main task is phagocytosis. Inside them, the cells have numerous lysosomes. Lysosomes are cell organelles that contain digestive enzymes. If the monocytes now encounter a pathogen or foreign body, they take it up into their cell interior. There it is rendered harmless by the lysosomes and digested. The monocytes belong to the non-specific cellular defense. 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 capable of presenting some of the material they phagocytose on their surface. This is also referred to as antigen presentation. The lymphocytes recognize these presented antigens and produce antibodies in response. This allows more of these pathogens to 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 take up these foreign proteins as part of phagocytosis and break them down intracellularly. They also release chemical attractants to attract more macrophages and other defense cells. They also release cytokines that cause local inflammation. Antigen presentation to macrophages is through the MHC-II molecule. However, macrophages do not only take care of foreign materials, they also eliminate old or defective cells of their own body. If the infection has been successfully fought, the macrophages are also involved in healing. They promote the formation of scar tissue and the formation of new blood vessels. Some macrophages have special functions in organs.For example, macrophages reside in the testis and secrete a subtance that neighboring cells need to make testosterone.
Diseases
If the number of monocytes in the blood is decreased, the condition is called monocytopenia. The lower normal limit in this case is 200 cells per microliter of blood. Monocytopenias usually occur in the context of leukemias. 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, monocytosis occurs in systemic histoplasmosis or leishmaniasis. One disease in which monocytes play an important role is tuberculosis. In tuberculosis, the pathogen, Mycobacterium tuberculosis, enters the lungs via the respiratory tract. There, macrophages take up the pathogen. However, the pathogens have a protective layer so that they cannot be digested conclusively by the macrophages. In order to protect the body from the bacteria anyway, more monocytes are taken from the blood. These transform into so-called epithelioid cells and surround the macrophage with the bacterium like a protective wall. The cells within this protective wall die, 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 even years after the initial infection and cause reinfection.
