Phagocytosis Definition, Steps, and Example

What is phagocytosis?

Phagocytosis refers to the process in which cells consume large particles (>0.5 micrometers) and vesicle-bound membrane vesicles known as phagosomes. These vesicles are then directed to the lysosomes where they will be subjected to enzymatic degrading. Optonization of bacteria can greatly enhance phagocytosis. Although phagocytosis may occur without the attachment of an IgG or complement fragment (C3), it can be greatly enhanced by attaching a specific IgG.

The primary receptors for complement opsonization are the CR1-CR3 complement receptors. The Polymorphonuclear Leukocytes, or PMNs, also have receptors for IgG fragment (FcgRs), which facilitate phagocytosis. FcgRII and FcgIII are the most commonly expressed Fcg receptors in circulating PMNs. Binding to these receptors triggers an oxidative burst. The phagocytic process is activated by the binding of complement and antibody receptors to the PMNs’ surface.

How does phagocytosis occurs?

To successfully phagocytize a substance, cells must complete certain steps. Let’s take an example of a macrophage, a type immune cell, phagocyting a virus. To make this easier to understand, let’s imagine that we are following it. However, there are many types of cells that perform phagocytosis.

1. Both the virus and the cell must come in contact. Sometimes, an immune cell accidentally encounters a virus in bloodstream. Sometimes cells can move through a process called “chemotaxis”. Chemotaxis is the movement of an organism/cell in response to chemical stimuli. Cytokines are small proteins that are used for cell signaling. Many immune system cells respond to them. Cytokines tell cells to move to the area where the virus (in this case, the particle) is located. This is common for infections that are specific to one area, such as a wound infected by bacteria.
2. The virus binds with the macrophage’s cell surface receptors. Different cell types have different receptors. Some receptors can be generalized, meaning they can identify self-produced molecules versus potential threats (and that’s it). Others are more specific, such as toll-like receptors and antibodies. Without successful binding to the cell surface receptors, the macrophage cannot initiate phagocytosis. Some viruses can have specific surface receptors that are only available to macrophages. In order to infect a host cell, viruses must access its cytoplasm and nucleus. To do this they use their surface receptors which interact with immune cells to enter the cell. Sometimes, a virus can be successfully destroyed by a host cell and the infection is stopped. Sometimes, the virus can trick the host cell into engulfing the virus and gaining access to the necessary resources to reproduce. The immune system recognizes the infected cells and destroys them. This stops viral replication.
3. The macrophage surrounds the virus and then engulfs it in the cell. Instead of moving large items across the plasma membrane which could permanently damage it, phagocytosis uses extensions from the cytoplasm (pseudopods), to surround the particle and seal it in a membrane. The virus and macrophage are bound to the cell’s surface. Once both pseudopods are visible, the virus will be enclosed. Remember that cells are flexible and fluid. The virus and macrophage are bound to the cell’s surface in our viral example. The virus is pulled inward by the cell, creating a pocket-like depression without causing damage to the plasma membrane. Remember that cells are flexible and fluid.
4. The virus is enclosed within a bubble-like structure called a “phagosome” within the cytoplasm. The pseudopodsIn’s extensions create a pocket. They extend their lips towards one another to close the gap. This creates a pocket, where the plasma membrane moves around the particle to encase it inside the cell.
5. The phagosome merges with a Lysosome to become a “phagolysosome”. Lysosomes can also be bubble-like structures similar to phagosomes and are responsible for removing waste from the cell. Lysis is a Greek word that means “to dissolve”, which makes it simple to recall the purpose of a lysosome. The phagosome would not be able to do any work with the contents of the container if it didn’t fuse with a Lysosome.
6. Phagolysosome reduces pH to degrade its contents. The pH of the phagolysosome’s internal environment is drastically decreased by a lysosome, or phagolysosome. The phagolysosome’s pH is reduced, making it more acidic. This is a very effective method of neutralizing or killing any organisms within the phagolysosome to prevent them from infecting their cells. Some viruses use the lower pH to escape thephagolysosome, and then start reproducing inside the cells. Influenza (flu virus) activates a conformational change that allows it to escape to the cytoplasm.
7. After the contents are neutralized, the residual body of the phagolysosome contains the waste products. The cell eventually expels the residual body.

Phagocytosis and the immune system

Phagocytosis plays a vital role in the immune system. Phagocytosis is performed by several types of immune system cells: macrophages and neutrophils as well as B lymphocytes and dendritic cell. By phagocyting foreign or pathogenic particles, cells of the immune systems can identify what they are fighting. The immune system can target specific particles that are circulating in the body by knowing the enemy.

The immune system also uses phagocytosis to infect and destroy pathogens, such as bacteria and viruses. The immune system reduces the speed at which the infection can spread by destroying infected cells. As we mentioned, the phagolysosome creates a acidic environment that can destroy or neutralize the contents. Other mechanisms can be used by the immune system cells to destroy pathogens within the phagolysosome.

• Oxygen Radicals: Oxygen radicals can be highly reactive molecules that react to proteins, lipids, and other biological molecules. The amount of oxygen radicals within a cell can rise dramatically during physiological stress. This can lead to oxidative stress which can cause cell structure destruction.
• Nitric Oxide: This reactive substance is similar to oxygen radicals. It reacts with superoxide and creates additional molecules that cause damage to various biological molecules.
• Antimicrobial Proteins: Antimicrobial Proteins are proteins that specifically kill or damage bacteria. Antimicrobial proteins include proteases that kill bacteria by destroying essential protein and lysozyme which targets the cell walls of Gram positive bacteria.
• Antimicrobial Peptides:  Antimicrobial peptides work in the same way as antimicrobial proteins to kill and attack bacteria. Defensins and other antimicrobial peptides attack the membranes of bacterial cells.
• Binding Proteins: Binding proteins play an important role in the innate immune response system. They competely bind to proteins and ions that could otherwise be beneficial to bacteria or viral replication. Lactoferrin is a binding protein that can be found in mucosal membranes. It binds ironions which are essential for the growth of bacteria.

The types of cells in the immune system that engulf microorganisms via phagocytosis

• Neutrophils: Neutrophils, which are plentiful in the blood, enter quickly into tissues and phagocytize pathogens during acute inflammation.
• Macrophages: Macrophages and monocytes are closely related. Chronic inflammation is more common with these longer-lived cells. They also release important inflammatory paracrine.
• Dendritic Cells: Phagocytosis is essential for the creation of an immune response, rather than directly attacking pathogens.
• B Lymphocytes: Sometimes, a small amount of phagocytosis is necessary for these cells to become cells that produce antibodies.

There are two types of killing mechanisms for PMNs. Both are oxygen-dependent, while the other is not. The production of superoxide radicals, and other reactive oxygen species is stimulated by phagocytosis. These powerful microbicidal agents include hydrogen peroxide, chloramines, and others. NADPH oxidase, which is located in the cell membranes of the PMNs, generates superoxide (known as the respiratory burst). Superoxide is unstable, and it quickly dissolves to hydrogen peroxide or other substances.

These are microbicidal. These reactions occur inside the phagolysosome, also known as the phagosome.

Steps in the phagocytosis of a bacterium

1. Pseudopodia is when Bacterium attaches to membrane evaginations.
2. When the bacterium is inhaled, it forms a phagosome.
3. A lysosome and Phagosome fuse.
4. The bacterium can be killed and then digested using lysosomal enzymes.
5. A cell releases digestive products.

Example: Macrophage phagocytizing a virus.

• Both the virus and the cell must come in contact.
• The virus binds to macrophage cell surface receptors.
• The virus is infected by the macrophage, which then engulfs it in a pocket.
• The virus invaginates and is enclosed within a bubble-like structure called a “phagosome” in the cytoplasm.
• The phagosome merges with a Lysosome to become a “phagolysosome”.
• Phagolysosome reduces pH to degrade its contents
• After the contents are neutralized, the residual body of the phagolysosome contains the waste products.
• The cell eventually expels the remaining body.

Recognition of Foreign particles for Phagocytosis

Professional phagocytes (neutrophils and macrophages), as well as other leukocytes, can detect microbial macromolecules through the presence of repetitive structural patterns. These unique microbial signatures can be called microbe-associated molecular pattern (MAMPs), but they can also be called pathogen-associated molecular pattern (PAMPs).

Because the immune system can respond to microorganisms other than pathogens, we use MAMP. MAMPs refer to specific areas within common microbial macromolecules such as lipopolysaccharide, peptidoglycan and fungal cell wall components. They also contain viral nucleic acid and other microbial structures. MAMPs don’t identify any microorganism, but they alert the host to the presence of a microbe and sound the alarm for potential infection.

Extracellular microbial invaders, such as most fungi, many bacteria and many bacteria, can be detected by cells, while intracellular pathogens, such as viruses, bacteria and a few fungal species, can be detected. The innate immune system’s cells recognize extracellular infectious agents by recognizing receptors on the surface of these agents, while intracellular pathogens can be detected in the cytosol by host receptors. Pattern recognition receptors (PRRs) are all receptors that recognize MAMPs regardless of their cellular location.

We have already introduced soluble PRRs. They include the acute-phase protein, the mannose-binding proteins (of the lectin supplement pathway), and the C-reactive protein. The membrane-bound PRRs can be found in phagocytic cell membranes, which allow them to degrade and ingest the MAMP source. These receptors are crucial in distinguishing self from non-self. Let’s take a closer look at them.

1. C-Type Lectin Receptors

C-type lectin regulators (CLRs), a large class of calciumdependent membrane-bound protein, have one or more domains that bind to one type of carbohydrate (lectin), found on MAMPs. Mannose, fucose and glucan carbohydrates are some ligands found on bacteria (Mycobacterium tuberculosis, Helicobacter Pylori), fungi, helminths, and some viruses. CLRs can also bind N-acetylglucosamine and capsular polysaccharides from Streptococcus pneumoniae, Klebsiella pneumoniae. CLRs are concentrated in macrophages, dendritic and dendritic cell populations. A signaling cascade that triggers cytokine gene transcription is initiated when a CLR located on the surface of a Phagocyte binds to a MAMP. The CLR cytoplasmic domain, also known as the immunoreceptor-tyrosine based activation motif (ITAM), is responsible for this. Binding can also lead to receptor-MAMP degradation and internalization.

2. Toll-Like Receptors

TLRs, a class of transmembrane receivers, are important for innate immunity. They are expressed by many cells, including macrophages and dermis cells. They can be found attached to plasma membranes as well as membranes of endosomes and lysosomes. These receptors detect MAMPs that have gotten into the host cell’s cytosol. TLRs contain an extracellular (or extraorganelle) domain that binds MAMPs and a cytoplasmic Toll/interleukin-1 (TIR) domain which activates downstream transcription. The binding of a MAMP and a TLR triggers the TIR to recruit a particular adapter protein, MyD88 or TRIF. This activates a signal transmission cascade that leads to activation of transcription factors necessary for the expression of genes encoding cytokines. For example, activation via the MyD88-dependent adaptationer activates the transcription factor NFkB which activates transcription pro-inflammatory cytokine gene genes. There are at most 10 distinct human TLRs, which recognize different but often overlapping MAMPS to initiate an immune response.

3. NOD-Like Receptors

Contrary to transmembranous TLRs nucleotide binding and oligomerizationdomain (NOD),-like receptors can only be found in the cytosol of the host cell. NOD-like receptors can sense intracellular MAMPs, such as viral RNA, bacterial toxins and bacteria, along with host molecules known as “damage-associated molecular pattern” (DAMPs). These include uric acid, heat-shock protein, and uric acid. DAMPs refer to a variety of molecules that are caused by cell injury or stress. Once one of these signals is detected, the oligomerization occurs of several NLRs. Next, cofactors and enzyme caspase-1 can be recruited to the oligomerized units. The NLR-caspase complex, also known as the inflammasome, triggers pro-inflammatory cytokines. This happens when caspase-1 cleaves, activating the cytokines IL-1, and IL-18. These cytokines are also made in response to signals from other PRRs such as TLRs. In other words, NLRs (inside host cell cells) and TLRs on the surface of host cells and some intracellular membranes work together to produce a strong inflammatory response.

4. RIG-I-Like Receptors

Retinoic-acid-inducible gene I (RIG-I)-like receptors (RLRs) are yet another family of cytoplasmic receptors. These receptors were designed to sense viral DNA. The cytosol is home to RNA genomes that are replicated by viruses. This leads to the creation of dsRNA as well as triphosphate-capped, ssRNA. These forms of virus-specificRNA are recognized by RLRs and stimulate an antiviral response.

Intracellular Digestion of particles

Ingestion and digestion of foreign substances can be accomplished by two parallel processes: phagocytosis (Latin pseudo, false) and autophagy (Latin pseudo, foot). Extracellular particles are engulfed in pseudopodia (Latin pseudo; false; podium, foot), which form by the extension and contraction of the cell membrane. These pseudopodia surround the particle to create an internal phagosome. This can be receptor- or opsonin-mediated. When intracellular microorganisms or their products initiate autophagy

Intracellular membranes capture them through PRRs or they are coated with Ubiquitin, which is the molecule that tags proteins for recycling by proteasomes. Instead of targeting proteins, ubiquitin “labels” microorganisms to be captured by a Phaphophore (free-floating membrane in the cytosol). The phagophore surrounds the microorganism and creates the double-membrane self-phagosome. The autophagosome, on the other hand, is created from intracellular membranes.

After microorganisms are enclosed in a autophagosome/phagosome, the lysosomes fuse with them, creating an autophagosome and phagolysosome. Lysosomes contain a range of hydrolases, including lysozyme and phospholipase. Acidic vacuolar pH increases the activity of these enzymes. These enzymes work together to destroy the microorganisms entrapped in them. Additionally, toxic reactive oxygen substances (ROS), such as superoxide radical, hydrogen peroxide, H2O2, singlet oxygen (1O2) and hydroxyl radicals (*OH), are also produced. Myeloperoxidase is a heme-protein enzyme that catalyzes production of hypochlorous acids (bleach) in neutrophils. The recruitment of an NADPH-oxidase to phagosome membrane or autosome membrane is the first step in the generation of ROS. This enzyme uses molecular oxygen (molecular oxygen) to oxidize NADPH into NADP+ and create superoxide radical. The respiratory burst, which is the simultaneous release of ROS and O2 consumption, is also known as O2 or ROS.

Reactive nitrogen species (RNS) have been also formed by neutrophils, mast cells, and macrophages. These molecules include nitric dioxide (NO) as well as its oxidized forms, such as nitrite and nitrate. RNS can be very powerful cytotoxic agents. Nitric oxide is the most potent RNS. By complexing with iron in electron transportation proteins, it can block cell respiration. RNS is used by Macrophages to destroy a variety infectious agents and also kill tumor cells.

Neutrophil granules contain a variety of other microbicidal substances such as cationic peptides, the bactericidal permeabilityincreasing protein (BPI), and broad-spectrum antimicrobial peptides, including defensins. These substances are stored in compartments for extracellular secretion and delivery to phagocytic vessels. Some viruses, Gram-positive and Gram–negative bacteria, yeasts, molds, and others are all possible microbial targets.

What is Exocytosis?

The phagocyte can do one of two things once the microbial invaders are killed and broken down into small antigenic pieces. Exocytosis, step 5 in the fiure, is when the cell might expel the microbial particles. This is basically a reverse process of phagocytic. The phagolysosome combines with the cell membrane and releases extracellular microbial fragments. Neutrophils are devoted to phagocytosis, exocytosing their cargo until they exhaust themselves and die. Macrophages and dendritic cell become antigen-presenting. 

The phagolysosome passes microbial fragments to the endoplasmic retina. The peptides of the microbial fragments get combined with glycoproteins (major histocompatibility proteins (MHC)), destined for the cell membrane. This is done to insert a microbe-specific protein in the MHC. The peptide faces the outside when the MHC-peptide combination is attached to the macrophage plasma membrane or DC. This initiates the process of antigen presentation. It is crucial because it allows DCs and macrophages show or “presents” microbial antibodies to lymphocytes. This triggers the activation of adaptive immunity.