Monocytes - Definition, Structure, Types, Functions

What are Monocytes? – Definition of Monocytes

Monocytes are a type of white blood cell or leukocyte. They are the most abundant form of leukocyte in the circulation and are capable of differentiating into macrophages and dendritic cells derived from monocytes.

As part of the innate immune system of vertebrates, monocytes also impact adaptive immunological responses and perform tissue repair. Human blood contains at least three subclasses of monocytes based on their phenotypic receptors.
The bone marrow generates monocytes from antecedents known as monoblasts, which are bipotent cells that developed from hematopoietic stem cells.

Typically, monocytes circulate in the blood for one to three days before migrating to tissues throughout the body and differentiating into macrophages and dendritic cells.
Monocytes are a type of white blood cell that protects the body against bacteria, viruses, and fungus. Monocytes are the largest type of immune system white blood cell. Initially, they are created in the bone marrow before being released into the blood and tissues. As specific pathogens enter the body, they rush to the site of attack. Furthermore, monocytes are the largest leucocytes.
In the tissues, monocytes can also divide into dendritic cells. Dendritic cells are immune cells that digest antigens and transmit them to the immune system (defense system). They are therefore classified as antigen-presenting cells.

Monocytes respond to inflammatory signals in the body and can arrive rapidly (within 8 to 12 hours) at sites of infection or tissue injury, where they split into macrophages and dendritic cells to offer additional immune response.
The lowest phase or surface of a monocyte is not as smooth because it has particular proteins that enable it to bind with various types of virus or bacteria cell.
The primary purpose of phagocytosis is to defend the organism against attack by dangerous pathogens and to eliminate all damaged or dead blood cells. Monocytes are extremely adaptable cells in that they can change in response to environmental signals.

In contrast, the presence of certain proteins in the blood that bind to the virus or bacterium makes it easier for immune system cells to recognize invaders.

Structure of Monocytes

The monocytes are the biggest cells found in the blood’s periphery. They have a diameter that falls between 14 and 20 micrometers.
The cell has an uneven appearance and has a kidney- or oval-shaped nucleus as well as cytoplasmic material. The ratio of nucleus to cytoplasm in monocytes is relatively high, at 3:1.

The monocyte’s nucleus is highly visible and folded. The cytoplasm contains a significant number of granules.
These monocyte granules are referred to as cytoplasmic granules. The concentration of these granules is greatest towards the cell membrane.
A chromatin network is present in the nucleus. Moreover, chromatin aggregates are seen within the nuclear membrane’s interior. There are functionally significant blebs and ruffles located on the surface of the monocyte.

Due to the phagocytic and mobile nature of monocytes, the surface’s uneven structure and ruffles reduce the repulsive forces, with negatively charged groups approaching the cell.
Many mitochondria are present in the cytoplasm and are tiny and somewhat elongated. We may also observe a golgi complex and a centrosome within the nucleus.
In addition, the presence of many microvilli on the cell membrane facilitates the mobility and adhesion of monocytes to other cells.

The diameter of the granules in the cytoplasm ranges between 0.05 and 0.2 micrometers. Nonetheless, they are thick and homogeneous.

Subsets/Types of Monocytes

They are formed in the bone marrow and circulate in the blood for about 1-3 days before migrating into tissues, where they differentiate into macrophages or dendritic cells. Monocytes can be divided into several subsets based on their phenotype and function:

Classical Monocytes (CD14++CD16-): These are the most abundant subset of monocytes in the circulation, accounting for around 80-90% of all monocytes. They express high levels of CD14 and low levels of CD16, and are involved in the clearance of pathogens and dead cells.

Non-classical Monocytes (CD14+CD16++): These monocytes express high levels of CD16 and low levels of CD14, and are involved in the surveillance of blood vessels for damage or infection. They have also been shown to play a role in the recruitment of T cells to sites of inflammation.
Intermediate Monocytes (CD14++CD16+): This subset of monocytes expresses high levels of both CD14 and CD16, and has been shown to have a pro-inflammatory phenotype. They produce high levels of cytokines and chemokines, and are involved in the pathogenesis of several inflammatory disorders.

The classification of monocyte subsets is still an area of active research, and new subsets are constantly being discovered.

Functions of Monocytes

Phagocytosis: Monocytes are phagocytic cells, which means they can engulf and digest invading pathogens such as bacteria, viruses, and fungi.
Antigen presentation: Monocytes can present antigens, or pieces of the pathogen, to T cells, which helps to stimulate a more specific and targeted immune response.
Cytokine secretion: Monocytes can release cytokines, which are signaling molecules that help to activate other immune cells, such as T cells and B cells, to fight the infection.

Tissue repair: Monocytes can also play a role in tissue repair and wound healing by releasing growth factors and other molecules that promote tissue regeneration.
Immune regulation: Monocytes can regulate the immune response by either activating or suppressing other immune cells.

FAQ

What are monocytes?

Monocytes are a type of white blood cell that play a key role in the immune system by helping to fight off infections and other diseases.

What do monocytes look like?

Monocytes are the largest of the white blood cells and have a distinctive kidney-shaped nucleus. They are round or oval in shape and are typically about 12-20 micrometers in diameter.

What is the function of monocytes?

Monocytes are phagocytic cells, which means they are able to engulf and destroy pathogens, such as bacteria and viruses. They also help to activate other immune cells and produce cytokines, which are important for regulating immune responses.

Where are monocytes produced?

Monocytes are produced in the bone marrow, the spongy tissue inside bones that is responsible for the production of blood cells.

How do monocytes differ from other white blood cells?

Monocytes are a type of granulocyte, which means they do not contain granules or vesicles in their cytoplasm like other white blood cells such as neutrophils and eosinophils. Monocytes also have a longer lifespan than other white blood cells.

What is a monocyte count?

A monocyte count is a blood test that measures the number of monocytes in the blood. This test is often performed as part of a complete blood count (CBC).

What does a high monocyte count indicate?

A high monocyte count can indicate an infection or inflammation in the body. It can also be a sign of a blood disorder, such as leukemia.

What does a low monocyte count indicate?

A low monocyte count can indicate a weakened immune system or a bone marrow disorder.

How are monocytes produced in the body?

Monocytes are produced in the bone marrow from stem cells. They then travel through the bloodstream to various tissues and organs where they mature into macrophages or dendritic cells.

What are some common monocyte disorders?

Some common monocyte disorders include monocytosis (high monocyte count), monocytopenia (low monocyte count), and monocytic leukemia (cancer of the monocytes).

Can monocyte levels be influenced by lifestyle choices?

Yes, lifestyle choices such as diet and exercise can affect monocyte levels. For example, a diet high in antioxidants can help to reduce inflammation and lower monocyte levels.

How are monocyte disorders treated?

The treatment for a monocyte disorder depends on the underlying cause. In some cases, antibiotics or antiviral medications may be prescribed to treat an infection. In other cases, chemotherapy or bone marrow transplants may be necessary to treat leukemia.

References

Van Furth, R., & Beekhuizen, H. (1998). Monocytes. Encyclopedia of Immunology, 1750–1754. doi:10.1006/rwei.1999.0443

Espinoza VE, Emmady PD. Histology, Monocytes. [Updated 2022 Apr 28]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2023 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK557618/
https://www.biologyonline.com/dictionary/monocyte
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