Phagocytosis: Definition, Explanation, Mechanism and Stages

Phagocytosis is the process where a living cell engulfs other cells or particles. To be simplified, it is commonly understood as the ‘cell eating’ process. The cell which carries out phagocytosis is known as a phagocyte. In the case of amoeba, phagocytosis is used for the feeding process. As an action mechanism, phagocytosis is used for the protection of living cells against harmful viruses, particles, foreign organisms, and cellular debris.

• Phagocytosis is one of the 3 endocytosis.
• Endocytosis is defined as a cellular process in which substances are brought into the cell.
• Depending on the phagocyte intake, fluid, particles, cells, etc its types are assigned.
• Phagocytosis is adapted by the immune system of the human body.
• Neutrophils and Macrophages are the main two types of phagocytes in white blood cells (WBCs).
• Other types include monocytes, tissue dendritic cells, and mast cells. The phagocyte uses its pseudopodia to engulf substances.

Read More: What are Tissues

Key Takeaways: Phagocytosis, cell-eating process, amoeba, phagocyte, endocytosis, cellular debris, pseudopodia.

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What is Phagocytosis?

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Phagocytosis is defined as the cellular process for ingesting and eliminating particles larger than 0.5 μm in diameter, that includes microorganisms, foreign substances, and apoptotic cells. Phagocytosis exists in many types of cells, and it is, as a result, an essential process for tissue homeostasis.

• Phagocytes may present as a free-living one-celled organism, such as an amoeba, or one of the body cells, such as a white blood cell.
• Phagocytosis is a means of feeding in some forms of animal life, such as amoebas and sponges.
• In higher animals, it is chiefly a defensive reaction against infection or invasion of the body by external substances.

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Phagocytosis Mechanism

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Phagocytosis happens in two scenarios, for feeding (in organisms like algae, amoeba, and other unicellular organisms) and the second is for defense against harmful foreign bodies.

• It is also used in tissue homeostasis.
• It plays an important role in maintaining the immune system.
• Phagocytosis helps to identify the pathogens for other cells.
• This helps the overall immune system to target a particular pathogen throughout the body.
• Phagocytosis destroys the infected cells, digests the pathogens, thus limiting the spread of infection throughout the body.

Mechanism of phagocytosis involves following steps

The cell, cell debris, or external particle inside the phagosome is subjected to different kinds of treatment leading to their degradation. After the event of engulfing, the phagosome travels towards the centromere of the phagocyte and then fuses with the lysosomes.

The new vesicle formed is called phagolysosome.

The hydrolytic enzymes in lysosomes carry out the hydrolysis of the external particles and cause their degradation. After the digestion of the contents, the released nutrients are transported to the cytoplasm for use in different metabolic processes.

The nutrients are utilized by the phagocyte In the case of protists in which the phagocytosis mechanism is a mode of nutritional gathering. But in cases of white blood cells, for instance, macrophages and neutrophils, in which the phagocytosis process acts as a defense mechanism, the digested particles are used to produce antibodies against specific parts of the foreign cell.

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Stages of Phagocytosis

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Different stages of phagocytosis is as follows.

Particle Adherence

It is important for the target bacteria to be in contact with the phagocytes to begin the process. In response to the suspected chemical stimulus, phagocytes travel towards it through ‘chemotaxis’. Sometimes the phagocytes move in response to a cytokine signal indicating the infected area in the body (eg- bacteria in a wounded area). There are receptors on the surface of phagocytes. A thin film of a protein component known as opsonins is present around the bacteria. The receptors in the phagocytes get attached to this thin film of the bacteria followed by the next stage.

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Particle Engulfment

After being attached to the bacteria, the phagocyte extends its cytoplasm in the form of pseudopodia around the target bacteria. It starts engulfing it into the membrane. When the phagocyte completely takes the bacteria inside itself, it forms a circular enclosed membrane around the bacteria, this region is now known as the phagosome. It slowly takes the bacteria towards the centromere as it fuses with the lysosome and becomes a phagolysosome. The lysosome helps in breaking the contents down thus it is important for the phagosome to fuse with the lysosome. Diverse types of phagocytes take different amounts of time to completely engulf bacteria/particles of varied sizes. The final stage of digestion begins hereafter.

Particle Digestion

Lysosomes contain hydraulic enzymes which perform hydrolysis making the environment acidic enough to kill the bacteria. The lowered pH thus created neutralizes the bacteria inside the phagolysosome. After digestion, the residuals are expelled through a residual body out of the cell. Different types of phagocytes will have different digestion habits. In general, the immune phagocyte like a macrophage expels the undigested fragments into the tissues. This demands the immune system to produce more phagocytes.

The phagocytes used for feeding as in the case of amoeba have the nutrients absorbed into the cell during digestion. These nutrients are then used in various metabolic activities of the cell.

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Important Definitions

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• Cytoplasm- it is a thick solution that fills each cell and is enclosed by the cell membrane.
• Pseudopodia- temporary/ false legs used for feeding or movement.
• Plasma membrane- it is a microscopic semipermeable membrane of lipids and proteins forming the external boundary of the cytoplasm of a cell and regulates the passage of molecules in and out of the cytoplasm.
• Lysosome- membrane-enclosed organelles that contain an array of enzymes capable of breaking down all types of biological polymers.
• Centromere- it is a cellular structure involved in the process of cell division.
• Enzymes- a produced by a living organism that acts as a catalyst in a biological reaction.
• Phagocyte- it is a type of cell that can engulf bacteria and other small cells within the body.
• Phagosome- a vacuole in the cytoplasm in a cell, containing a phagocytosed particle within a part of the cell membrane.
• Phagolysosome- it is a cytoplasmic body formed by the fusion of lysosome and a phagosome.
• Homeostasis- self-regulating process by which biological systems maintain stability while adjusting to changing external conditions.

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Things to Remember

• Phagocytosis is a process of engulfing any cells or particles.
• It is also called as cellular eating.
• Phagolysosomes generate an acidic environment for neutralizing or destroying the content.
• Phagolysosomes can also produce oxygen radicals, nitric oxide, antimicrobial proteins, antimicrobial peptides, and binding proteins to destroy pathogens captured inside them.
• Unicellular organisms cannot initiate phagocytosis using their cell membranes thus they use hair-like structures called ‘cilia’.
• Phagocytosis is specifically for engulfing particles having a larger diameter than 0.5µm.
• There are selective cells that are professional in performing phagocytosis with full efficiency. They are able to ingest micro-organisms. The un-professional phagocytes are equally important as they maintain homeostasis by eliminating dead cells. Professional phagocytes can identify foreign chemicals easily.

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