What are Monocytes?
What are Monocytes? Monocytes are cells of the immune system that are immune effector cells with chemokine receptors and pathogen recognition receptors that circulate through the blood or remain localized in lymphoid organs.
- These cells makeup about 5% of the total circulating nucleated cells in human blood and have a half-life of three days.
- Monocytes are essential for both the innate immune system and the adaptive immune system, as they differentiate into macrophages and dendritic cells.
- Monocytes are one of the main cells of the immune system because they can perform a group of different functions performed by different immune cells.
- These can recognize danger signals or stimuli via their pattern recognition receptors, which leads to phagocytosis. These can also present antigens to other cells and secrete chemokines and cytokines.
- Monocytes can be defined as circulating blood cells that develop in the bone marrow from the common myeloid progenitor cells.
- However, monocytes are known for their developmental plasticity, as they can differentiate into osteoclasts or other immune cells depending on the inflammatory reaction.
- Monocytes are found in all vertebrates, but the concentration of monocytes in the bloodstream differs between species.
- Monocytes are considered to be cells of the innate immune system, as they are involved in reactions against viral, bacterial, fungal, or parasitic infections.
- These are mononuclear phagocytic cells of the immune system, which are described by different morphological and physiological properties depending on the differentiation stage of the cells.
Structure of Monocytes
- Monocytes are the largest cells in peripheral blood, between 14 and 20 µm in diameter.
- The morphological features of the cells include an irregular cell shape, an oval or kidney-shaped nucleus, cytoplasmic vesicles, and a high nucleus to cytoplasm ratio (3: 1).
- The core is prominent and remains folded rather than multi-lobed. The cytoplasm, in turn, contains a large number of cytoplasmic granules, which are usually numerous towards the cell membrane.
- The core contains a characteristic chromatin network with strands that bridge tiny clumps of chromatin. The chromatin clumps are located on the inside of the nuclear membrane.
- The surface of a monocyte contains frills and surface vesicles that are of functional importance.
- Monocytes are motile and phagocytic; Thus, reducing the curvature of the cell by creating ruffles reduces the repulsive forces as negative charge groups approach the cell.
- The cytoplasm contains mitochondria that are numerous, small, and elongated. The Golgi complex is also present in the nucleus along with the centrosome.
- The cell membrane is also characterized by numerous microvilli that aid in locomotion and attachment to other cells.
- The cytoplasm also has small cytoplasmic granules (0.05-0.2 µm in diameter) that are dense and homogeneous with a limiting membrane.
Subsets of Monocytes
- Monocytes found in humans can be divided into two subgroups based on the expression of CD14 and CD16. CD14 is a component of the lipopolysaccharide receptor complex, while CD16 is the TcγRIII immunoglobulin receptor.
- Both monocytes express different chemokines, immunoglobulins, adhesion proteins, and scavenger receptors.
- The first subgroup, CD14highCD16– monocytes, also called CD14 + monocytes, is 18 µm in diameter and makes up about 80-90% of the total circulating monocytes.
- The next subgroup, CD16highCD14–, also called CD16 + monocytes, is smaller with a diameter of 16 µm and makes up about 10% of the total circulating monocytes in humans.
- The CD16 + subset of monocytes produces high tumor necrosis factor and low IL-10 levels in response to stimulation by toll-like receptors. These are also known as proinflammatory monocytes.
- In addition, human blood contains another smaller subgroup of monocytes, which are characterized by the expression of surface molecules.
- CD14 + CD16 + CD64 + monocytes are a subgroup of highly phagocytic monocytes that express high levels of MHC class II. This subgroup is also known as transitional monocytes, which can activate T cells.
How do Monocytes work against pathogens?
- Monocytes have great receptors on cell membranes that can interact with pathogen-associated molecular patterns (PAMPs) that appear on invading pathogens.
- The binding creates a signal that causes monocytes to migrate from the bone marrow to the peripheral bloodstream within 12 to 24 hours.
- To get to the affected areas, monocytes should first attach to the endothelium and then move through the vascular surface.
- Finally, the monocytes adhere to the endothelium and finally pass through the endothelial cells through the process of diapedesis. The monocytes can then penetrate the endothelial basement membrane and migrate to the area of inflammation.
- Differentiation of monocytes occurs at the site of inflammation, and differentiation depends on the growth factors and cytokines produced during the process.
- Monocytes in the area of inflammation can also act as phagocytic cells, engulfing microorganisms, foreign substances, and dead and damaged cells.
- Some monocytes release cytokines, which help in recruiting other cells and connections into the affected area and causing further inflammation.
Functions of Monocytes
The following are some of the functions of monocytes:
- Monocytes are one of the most important components of the innate immune system as they differentiate into populations of dendritic cells and macrophages, which are involved in the regulation of cellular homeostasis.
- Monocytes regularly patrol the body for pathogens and regulate an immune response during infection and inflammation.
- Monocytes act as phagocytic cells and antigen-presenting cells in the peripheral blood to remove microorganisms, antigens, and dead or damaged cells.
- Different subsets of monocytes produce different cytokines that recruit additional cells and proteins into the affected areas to generate an effective immune response.
- Monocytes are highly plastic and heterogeneous, as they can change their functional phenotypes depending on environmental stimuli.
- A specific subgroup of monocytes called transitional monocytes are involved in the activation of T cells.