Agranulocytes – What Are They, And What Do They Do?

What Is The Definition Of Agranulocytes?

Agranulocytes, which also includes monocytes as well as lymphocytes, are a specific type of white blood cell, which differentiate themselves from granulocytes by not having any visible granules.

Because of this they have an entirely clear cytoplasm which makes the nucleus much more clearly visible.

As well as the lack of any visible granules, agranulocytes also are defined by having a large single nucleus that is not lobed. Because of this they can be called mononuclear leukocyte, and this differentiates them from granulocytes which are multi-lobed.

The role of agranulocytes within the body is to cope with adaptive immunity, which means acquiring immunity, and because of this they use their immunological memory in fighting against any pathogens that could be invading the body.

To make it simpler some of the main functions which cover what agranulocytes cover is:

• Protecting your body against microbes like viruses and bacteria.
• Destroying any cancerous cells.
• They are capable of activating any other cells involved in immunity.
• They can also destroy any damaged or old cells inside the body.

How Are Agranulocytes Produced?

Agranulocytes have a similar production to other types of white blood cells. Similar to white blood cells they are produced through a process that is called leukopoiesis.

The process of leukopoiesis begins by differentiating the hemocytoblast (the pluripotent hemopoietic stem cell).

Monocytes will be produced using the myeloid lineage, whereas lymphocytes are made using the lymphoid lineage.

The Production Of Monocytes

When monocytes are being produced, there are hemocytoblasts that are able to differentiate by producing myeloid stem cells. These will be under the influence of both interleukins 3 and 5, and also stimulating factors of agranulocytic-colony (AG-CSF).

The produced myeloid stem cells will then differentiate and form a monoblast which will then be committed to producing monocyte (which is committed progenitor cells.)

In this the monoblast will then further develop until it is a promonocyte which will then produce a monocyte.

Granulocytes are also made through the myeloid stem cell lineage, however this will be influenced by interleukins 3 as well as 5 on top of any granulocytic stimulating factors.

After the myeloid stem cell leave this circulation they will then enter into tissues, monocytes will then become macrophages which are often described by some as ‘big eaters’.

These macrophages are known by a large variety of more specific names that all depend on which tissues they end up residing in. Some of the more common examples of these are:

• Microglia, which can be found within the central nervous system
• Alveolar macrophages, which can be found in the lung
• Kupffer cells, which are found within the liver
• Osteoblasts, which are found within the bone

The Characteristics Of Monocytes

Monocytes will make up somewhere between 2 to 8% of the total leukocyte count. Also, these monocytes can measure anywhere between 12 and 20um in diameter and this makes them the largest blood cells that can be found anywhere in the body.

They are unique for their characteristic horseshoe-shaped nucleus, in spite of this, their nucleus is not lobed

The Production Of Lymphocytes

When lymphocytes are produced, the hemocytoblast will first differentiate which will produce a lymphoid stem cell.

After this, the produced lymphoid stem cells will differentiate under the same influence of both interleukins 3 as well as 5, and agranulocytic colony-stimulating factors. These will then produce the lymphoblast.

After this there will be specific differentiation as well as proliferation, and then the lymphoblast will be able to produce prolymphocyte which will then be able to produce and develop lymphocytes.

In this stage of the process there will be 2 main types of lymphocyte present, these are the B and the T lymphocytes.

The B lymphocytes are the mature lymphocytes, so being the more mature of the lymphocytes, the B lymphocytes will then leave the bone marrow and proceed into the bloodstream using the sinusoidal capillaries and then will finally reach the lymphoid organs (these secondary lymphoid organs include the spleen, lymph nodes, and more and this is where they will be able to mature further).

On the other hand the T lymphocytes are not mature enough and therefore not functional. They will need to be able to reach a lymphatic organ or gland (like the thymus gland) to make it, so they can mature to a stage in which they can be usable. 

T lymphocytes migrating between the bone marrow and the thymus can be influenced by different chemokines that will be produced by the thymus gland. These can include thymosin, thymopoetin, thymotaxin, as well as any other thymic factors.

Also, while lymphocytes as well as monocytes are made through different specific lineages, their productions are both influenced by the similar styles of factors including; IL-3, IL-5 as well as AG-CSF.

The Characteristics Of Lymphocytes

One of the defining characteristics of lymphocytes is that these are the only blood cells that are created by the lymphoid stem cells. These cells also undergo reproduction within the lymphatic tissues (specifically lymphocytes can undergo some further maturing within these lymphatic tissues).

Lymphocytes also comprise 20 to 30% of the bodies total leukocyte count, which makes them the second-biggest group of leukocytes that is present within the blood.

Based on the size of lymphocytes, they can be divided into 3 main groups, the largest of them measure somewhere between 14 and 17um in diameter.

While the smaller cells are between 6 and 9um and are characterized by their particularly large nucleus, finally the relatively middling cells will measure between 10 and 14um and compared to the smaller have a small cytoplasm to nucleus ratio.

Functions Of Agranulocytes

Functions Of Monocytes

Once a monocyte migrates and then enters the bloods’ circulation, they will be able to differentiate. Through this process they will be able to differentiate into either macrophages or dendritic cells, this depends on the type of infection they will be tackling.

Because of this, if you identify the functions that these different cells have, you will be able to have an improved understanding of the functions of monocytes.

Macrophages

After the presence of a microbial (for example a bacterial) infection has had its presence recognized, your innate immune system will begin to work on localizing the infection using a process that is called inflammation.

The foal of this is to stop any infection from getting to different parts of your body. This inflammation will lead to the dilation of the blood vessel, and this will lead to more blood being brought into this area.

As this happens, monocytes will leave circulation and then enter the affected tissues, they will then differentiate and form macrophages and these will then generally reside within different body tissues.

In the site of the infection, the macrophages will engulf any invading pathogens present and destroy them using a process called phagocytosis.

After the invading pathogen has been ingesting or engulfed by the macrophages, the macrophage will create a vacuole which is called a phagosome and this will contain lysosomes.

These lysosomes contain proteolytic digestive enzymes and these will then act on destroying the pathogen.

As this digestion of the invading pathogen continues, the macrophages which are antigen-presenting cells, will start to form an MHC class II complex (known as a major histocompatilibility molecules) and these will then be put onto the cell membrane.

As well as this, the macrophage will also put a small peptide (an antigen) of the invading pathogen onto this MHC molecule.

This will make it so any other immune cells (particularly T lymphocytes which hold CD4 glycoproteins) to be able to identify and then be able to bind onto this type of antigen which will make initiating an immune response and then releasing the correct activating B cells and chemicals much quicker for an improved immune response.

So on top of just being able to help in destroying the invading pathogens through the process of phagocytosis, the macrophages will also work to stimulate any other immune cells to be able to act against any similar pathogens.

This means the macrophages are involved in the adaptive as well as the innate immune response systems.

Dendritic Cells

Similarly to the macrophages, the dendritic cells also have differentiated monocytes that are present within the tissue and like macrophages, they are also capable of doing phagocytosis.

These dendritic cells mainly act as antigen-presenting cells and because of this are often called professional antigen-presenting cells.

For most situations, dendritic cells will usually be located within parts of the body which are close to external areas. Because of this they are usually located in areas like skin and lungs and other similar tissues.

Within these specific areas, the dendritic cells are in a good position to interact with potential invading pathogens that are just entering the body.

When this happens they will be able to quickly capture and subsequently phagocytose these pathogens using the lysosomes for destroying them.

Similarly to macrophages, they will use a small peptide from the invading pathogen and then express it on the MHC complex for improved immune response.

Dendritic cells are characterized by being motile which is a useful ability as it makes it, so they can move to other tissue like the spleen where they will be able to directly present the antigen to any other immune cells activating their ability to properly respond to it.

Because of this dendritic cells have shown that they can work well with T lymphocytes within the immune system in protecting the body towards any potential invading microorganisms.

Functions Of Lymphocytes

B Lymphocytes 

These are the lymphocytes that are produced and are immediately functional from within the bone marrow. They are known as B cells and are mainly involved in humoral immunity, which is also called body fluid immunity.

Because of this they are able to produce as well as store antibodies for when there is a chance that the body could become reinfected.

If there is chance of an infection, the pathogens will release their antigens which can be identified by the receptors located on the B lymphocytes.

This will usually then start the process of engulfing the pathogen using immune cells like dendritic cells or macrophages that will then present the antigen to the B cells.

The antigens which are presented (being peptide molecules) will be able to float around the body and then be able to get to the tissues where they will be identified.

Within the body, these B cells use specific receptors to be able to detect and then bind to the specific antigens of the pathogens and this will lead to the cell going through the process of endocytosis while cell-mediated.

This is where the cell membrane will invaginate, so it will be able to fully engulf the invading antigen and let it be acted upon by the cell’s digestive enzymes.

After this the antigen will be fully broken down, and it will start to create an MHC class II which alongside the molecules from the antigen (the antigenic epitope), will be placed upon the cell’s membrane.

After the MHC class II complex has been formed (and will then bind the antigenic epitope) which will be on the surface of the B cells, it will interact and bind with the help of any T cells (using their receptors and CD4 glycoprotein) which will then result in the release of chemicals called lymphokines which will start to influence the creation of more B cells.

After this the newly created B cell clones will be able to find and then bind to more antigens and then differentiate to plasma cells which can create antibodies that are used for binding of antigens.

As well as memory B cells (which are used for retaining the memory of the invading antigens which will make it easy for destroying them upon future invasion from the same pathogen).

T Lymphocytes

Different from the previous B lymphocytes, the T cells which were released from the bone marrow will not be active or functional upon production.

Because of this they will have to move onto the thymus which is where they will undergo the further development as well as maturation which they need. At this stage they are called naive as they are not able to be activated for use.

Within the immune system the T lymphocytes have effector as well as regulatory functions.

Types Of T Cells

Helper T Cells

As has been previously mentioned, the T helper cells will be activated once they have interacted with an antigen-presenting cell like a dendritic cell.

On the surface of helper T cells, they have T cell receptors which are able to bind onto the aforementioned MHC class II complex which is located on the antigen-presenting cells which will then lead to them finally being activated.

Similarly to the B cells, the activation of helper T cells will lead to more being created through cloning. These clones will be used to help with immune response.

However, helper T cells also produce memory T cells which will retain the memory of the attacking pathogen and the antigens it used.

While memory T cell will last longer, so they can identify the pathogen for the future, the helper T cell clones will be able to release cytokines which are able to stimulate and increased creation of more immune cells like more macrophages or killer T cells which will help with response to invading pathogens.

The cytokines which are released by these T cells will also help with the maturation of the B cells turning them into plasma or memory B cells.

Cytotoxic T Cells (Or Killer T Cells)

Within the body, the role of the cytotoxic cells is for the destruction or lysis of any infected or potentially cancerous cells (for example cells which have been infected by viruses).

Unlike the cells which present as antigens which will contain the MHC class II), all the cells within the body with nuclei carry an MHC class I complex on their surfaces which is used in case a cell is damaged.

In the case of cancer cells, the abnormal proteins will be presented by the MHC class I complex on the surface.

This will also happen when a cell has become infected by something like a virus.

When this happens the viral proteins will be presented by the MHC class I complex and then the proteins will be identified using the specific receptors which are found on the cytotoxic T cells which will then lead to binding.

Then this will lead to the cytotoxic T cells being activated and then differentiating into either effector cytotoxic T cells as well as memory cytotoxic T cells.

The effector cells are then able to destroy the infected or damaged cells through released perforins which are used to perforate the infected or damaged cell which will destroy it, or alternately make proteins called granzymes which will promote the cell destruction. Cytotoxic T cells are a good example of a natural killer cell.

The Differences Between Agranulocytes And Granulocytes

Both granulocytes as well as agranulocyes are white blood cells which play very important functions int he bodies immune system.

Both have differences in their presence of cytoplasmic granules and the nucleus shape, as well as the type of immune cells which they use. However, they both are used for the same purpose within animals.

The biggest noticeable difference is that while the granulocytes hold visible granules, they are not as readily visible as they are in agranulocytes. Also, granulocytes are polymorphonucleate, while agranulocytes are mononuclear leukocytes.

While granulocytes are differentiated by their lobed nucleus, agranulocytes stand out having just a single large nucleus. For some lymphocytes the nucleus is so large that it does not leave enough room for the cytoplasm.

Within the blood granulocytes consists of neutrophils and eosinophils will make up most of the white blood cells being 65% while the agranulocytes will only be the remaining 35%.

They all come from the bone marrow while some agranulocytes will mature and then develop into lymphoid organs.

Granulocytes are mainly used in innate immunity whereas agranulocyte is mainly used for adaptive immunity, however neither are exclusive with both getting involved in the other.

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