Phagocytosis – A Definition And Full Explanation

Defining Phagocytosis

In its most simple definition; phagocytosis is a type of endocytosis. It is a type of endocytosis where a cell engulfs particulate solid/matter cells/particles.

Phagocytosis plays a variety of differing roles within different types of cells. This can include anything as simple as food ingestion to something as specific as destroying specific cells or particulate matter.

There are a variety of cells that utilize phagocytosis, these include; macrophages, neutrophils, protozoa, epidermal cells, as well as a few different types of vascular endothelial cells.

Generally speaking, phagocytes are split into 2 groups, these 2 groups are ‘professional phagocytes’ which includes WBCs (white blood cells) and then ‘non-professional phagocytes’ which include epithelial cells.

What Is The Process Of Phagocytosis?

As previously defined, phagocytosis is a form of endocytosis in which a cell will be engulfing a cell and then subsequently digesting and then eventually expelling the waste products that are created.

Living cells will take in a variety of different substances and materials across the cell membrane. Mostly, these materials will include ions, oxygen, and fluids.

These can usually pass through the membrane simply through processes like osmosis or ion pumps. Some matter living cells could come in contact with include particles similar to viruses.

This matter tends to be too big to pass through the membrane through the aforementioned simple methods. Because of this phagocytosis is needed to get materials like this inside cells.

The process of this will involve invagination of the cell’s membrane which will then allow the cell to be able to engulf the material or object that was previously too big to enter.

Dependent on the type of cell which is being dealt with as well as the mechanism which is used for engulfing these materials, the endocytosis may be divided into pinocytosis, receptor-mediated endocytosis, and of course; phagocytosis. 

What makes phagocytosis different from these other processes it that the process of phagocytosis possesses some special proteins on the surface that will give the process the ability to identify specific particles pre-ingestion and allow them to be bound to.

Phagocytosis as a form of endocytosis depends on the process of binding the cell to its target material which is attempted to be engulfed.

What Are The Main Steps Of Phagocytosis?

First Step – The Activation Or Actuation

The first step for phagocytosis is when the cell which is going to engulf will come into the correct proximity of the object, material, or particle that is too large to be absorbed through more simple methods.

An example of this is when phagocytes will come close to cells like bacteria, this will trigger the phagocytes and allow it to begin the binding process.

This first step involves the process of chemotaxis, this is when the cells start to move to the area where there is a high concentration of particles, molecules, or cells that are foreign.

These cells are getting chemically stimulated due to the presence of the foreign substances.

Second Step – The Binding

This is when the surface receptors that are part of the phagocyte will bind or adhere onto the surface of the particle that is getting engulfed. This is the most important step of phagocytosis that leads to the eventual engulfing of the molecule.

Dependent on the cell which is doing the engulfing, there will be different versions of the surface receptors and these have a different but still important role in the binding of the phagocytosis.

These different types of receptors include; scavenger receptors, opsonin receptors, antibodies, and toll-like receptors. 

Scavenger receptors will bind onto the different materials being engulfed onto its surface. Opsonin receptors are one of the most closely studied types of these receptors, and they work to bind onto molecules that specifically have immunoglobulin G present on their surface.

Antibodies are produced by some of the cells that are capable of phagocytosis, and they make it, so it is possible to attach to and subsequently engulf certain different types of antigens.

The binding of receptors is one of the most important steps of phagocytosis as it allows the cell to identify the material they plan to engulf and then react to it accordingly.

Third Step – The Ingestion

This is when the cell that is capable of phagocytosis (a phagocyte) will begin to expand and then surround the material that is going to be ingested. This process involves the creation of a vesicle or a vacuole that surrounds the molecule that will eventually get fully ingested.

Fourth Step – The Digestion

For some phagocytes, the enzymes that are inside the vesicle (for example lysosome) will break down the molecule into more simple components.

Then the waste materials which are produced and can not be used will get removed from the cell in a process called exocytosis.

In spite of this there are some phagocytes that are involved in the process of immunity, for these there are structures known as peroxisomes that are made which trap and then remove the toxic molecules.

What Are Some Examples Of Phagocytosis

Some good examples of the process of phagocytosis are; the phagocytosis of apoptotic cells (known as efferocytosis), the phagocytosis of protozoa, and the phagocytosis of bacteria which is done by professional phagocytes. 

The aforementioned efferocytosis is in reference to the process in which apoptotic cells (these are dead cells) get removed using phagocytic cells.

This is actually one of the more important processes that is done by multicellular organisms. This is because it contributes towards tissue homeostasis as well as being possible to resolve inflammation present in infection sites.

Both professional as well as non-professional phagocytes are capable of ingesting dead cells.

What Are The Steps Within Efferocytosis?

Efferocytosis is the phagocytosis of specifically apoptotic cells, which are dead cells.

First Step – The Activation

Efferocytes (cells capable of phagocytosis of apoptotic cells) can be activated by a variety of different molecules that would be released by cells that are either dead or dying.

These molecules will release signals being the “find me” signal (being triphosphate nucleotides) and the “eat me” signals (which are phosphatidylserine).

There have been other molecules that have displayed the ability to activate these phagocytes which include some proteins and carbohydrates.

The phagocytes will use chemotaxis to reach the cell they are engulfing.

Second Step – The Binding And Recognition

The phagocyte will then use its specialized receptors which are on their surface.

These interact with the signaling molecules from the previous step and this helps them recognize the cells, this can also be done through the bridging of molecules.

For a healthy cell there is a molecule known as transmembrane protein CD47 which has been seen to stop a cell from receiving phagocytosis but when the cell eventually dies, this molecule will be lost as well.

Third Step – The Engulfment

After the molecule is attached, the membrane f the phagocyte will undergo a modification known as cytoskeletal rearrangement which will make it, so it can surround its target cell and engulf it.

Fourth Step – The Degradation Of The Target Cell

During this final stage, the lysosomal enzymes will be released, and they will quickly degrade the target which will take around an hour.

Comparing this to a pro-inflammatory response, efferocytosis is seen as a process that is immunologically silent.

Because of this it produces anti-inflammatory cytokines which inhibit any inflammatory signaling which is given out by the ingested cells.

The Steps Of Phagocytosis Of Bacteria Done By Professional Phagocytes

This process is different from phagocytosis of apoptotic cells (efferocytosis) because in that process both professional and non-professional phagocytes can be used.

For infection that is by pathogenic or invading microorganisms like bacteria as well as viruses, these can only be dealt with by professional phagocytes as they are the only phagocytes that respond to the immune response.

The process of phagocytosis of bacteria by professional phagocytes is different from the previous processes slightly and will be described below.

First Step – The Activation

The infection at a site in the body will stimulate delivery of neutrophils onto the site through the process of chemotaxis.

Here there will be attractants like tissue debris as well as bacteria product. These will activate the movement of a professional phagocytic cell to this infected cite. 

The neutrophils are able to reach the infected site so quickly because they are constantly present in blood circulation and because of this can quickly migrate through any vascular walls to get to the infection site.

Second Step – The Adherence Or Binding

Bacteria that have molecules or substances like C3b or IgG can be easily recognized by Phagocytes, and they can use their opsonin receptors that are present on the phagocytes’ plasma membrane to bind the molecules allowing the bacteria to get identified leading to its subsequent ingestion. 

This is where we notice the different type of surface receptors that different types of phagocytes possess.

This is where it is noticeable that the binding will only be successful if the surface molecules  on the target molecule are capable of binding and being engulfed.

Third Step – Ingestion Or Engulfment

After this the attachment and/or the binding has allowed the identification and then the ingestion of the target bacteria.

Similarly to other forms of phagocytosis, the cell membrane will invaginate which will surround the target bacteria and then engulf it. This is the bacteria getting engulfed by the phagocytes phagosome or vesicle.

Fourth Step – The Enzyme Action

After this, inside of the phagocyte and its vesicle, the invading bacteria will then come into direct contact with the aforementioned lysosomal granules which will proceed to create a phagolysosome (also called a digestive vacuole).

This is where the lysosomal activity will kill and destroy the target bacteria.

The Process Of Phagocytosis In A Protozoa

Protozoa is the categorization for unicellular organisms which are found in a variety of different habitats or environments.

By comparing them to the other cells mentioned in this guide we can recognize that the protozoa depend on the phagocytosis as it gives energy in which the process is utilized for a feeding mechanism.

Most Protozoa are seen as grazers, and they feed on other single celled organisms, this can include bacteria, or even other protozoa that are smaller than them.

Some protozoa also use a mouth-like opening which is used for feeding in comparison to other organisms that will simply take in food through their membrane.

Based on a variety of different studies, it is recognized that phagocytosis in species of protozoa like Amoeba Proteus will involve similar steps to a metazoan phagocyte.

Because of this it will involve the recognition of its prey, and then subsequent movement towards its prey, and then of course, its ingestion.

Because of this, similarly to the process of using phagocytes as neutrophils, there is also chemotaxis used when phagocytosis is used in protozoa.

The best recognized presence of phagocytosis used in protozoa is in prey like Tetrahymena. This displays that when the ciliates of prey come into contact with the target protozoa, they will stimulate formation of a single or even more pseudopodia.

This is where the prey is engulfed, or trapped, and then inevitably ingested.

It has been recognized that protozoa are shown to move between areas with low prey concentration (like bacteria) into an area with a higher concentration of prey.

This exemplifies that they are fully capable of identifying their prey through the process of chemotaxis, and then moving towards it.

After the prey has been trapped, and then ingested, the prey will stay in the vacuole as it is being the target of the appropriate present enzymes.

This is everything you need to know about phagocytosis summarized through its basic definitions and the different types of processes it does.

Cells which are capable of phagocytosis known as phagocytes work in a variety of ways and the roles they can carry out depend on their location, receptor types, and whether they are professional or non-professional.

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