PHAGOCYTOSIS: STAGES AND FUNCTIONS - BIOLOGY - 2025

The phagocytosis is the process in which cells "capture" molecules and various substances from the surrounding medium due to the formation of invaginations of the plasma membrane, forming intracellular vesicles known as endosomes. Phagocytosis, together with pinocytosis and receptor-mediated endocytosis, add up to the three types of endocytosis

Pinocytosis has to do with the ingestion of fluid and small molecules, while receptor-mediated endocytosis involves the binding of specific molecules to membrane receptor proteins. Phagocytosis is considered a form of food, as it is related to the ingestion of large molecules, other cells, or “debris” from other cells.

Phagocytosis of a bacterium (Source: GrahamColm at English Wikipedia via Wikimedia Commons)

In multicellular organisms such as plants, animals and fungi, not all cells have the ability to engulf external elements, which means that there are some specialized cells for this purpose, which are known as “phagocytic cells”.

Phagocytic cells are distributed throughout all body tissues and perform different functions. Macrophages are a good example of phagocytic cells belonging to the immune system, whose function is to defend us from microorganisms that enter our body.

Phagocytosis / Photo recovered from phagositosis77.blogspot.com

The process of phagocytosis would not make sense in eukaryotic cells without the existence of a type of intracellular organelle called a lysosome, since it is there that the nutrients from the material that the cells phagocytose are “processed” or “digested”.

Phagocytosis is also known as "heterophagy" (ingestion of extracellular compounds), since it differs from "autophagy", which is the normal process that takes place in the lysosomes of virtually all eukaryotic cells.

Stages

Once the macrophage engulfs a virus (1-3), it breaks it into pieces with lysosome enzymes (4,5) which are then released from the cell as harmless waste (6). Photo retrieved from: askabiologist.asu.edu.

In higher eukaryotic organisms, the major phagocytic cells are derived from a common precursor originating in the bone marrow. These cells are known as "white blood cells" and are polymorphonuclear leukocytes (neutrophils), monocytes, and macrophages.

The phagocytosis process can be analyzed as a series of steps or sequential stages, which consist of (1) the recognition of the material that is phagocytosed, (2) in the formation of the phagosome, which is a kind of intracellular vesicle, and (3) in the formation of the phagolysosome, an event that ends with “digestion”.

Recognition stage

Phagocytosis is not a simple process. Among many other things, it involves the recognition of specific signals and the binding of particles or organisms to specific receptors located on the outer face of the plasma membrane of phagocytic cells.

This initial process can be considered as a kind of "neutralization", especially when it comes to phagocytosis mediated by certain cells of the immune system, which are responsible for the elimination of invading cells.

Thus, the surface of the plasma membrane of phagocytic cells (or of single-celled organisms that phagocytic) is endowed with a battery of receptors that are capable of recognizing specific molecules (ligands) found on the surface of invading cells or that are typical of food particles.

These receptors, which are generally integral membrane proteins with extracellular extensions, bind to their ligands, triggering a series of internal signaling events that send a message that translates as "there is food outside."

Phagosome formation stage

Once the cell that is engulfing a food particle or other “foreign” cell receives the message sent from the surface, an invagination occurs in the plasma membrane, which means that the cell “engulfs” the material to be phagocytosed, surrounding it with its own membrane..

At this stage it is observed how the membrane spreads over the other cell and this extension is sometimes known as a "pseudopod". When the ends of the pseudopod come together, enclosing the foreign element, an internal "vesicle" called a phagosome forms.

Phagolysosome formation and digestion stage

The phagosomes that contain the phagocytosed elements are intracellular vesicles covered by a membrane. These have the ability to fuse with other intracellular organelles: lysosomes.

The fusion between phagosomes and lysosomes gives rise to phagolysosomes, which correspond to the compound organelles where the "digestion" or "disintegration" of the phagocytosed compounds takes place (be they whole cells, parts of them or other extracellular molecules).

Since lysosomes are the organelles responsible for the degradation of deficient or waste intracellular material, they are endowed with different hydrolytic and proteolytic enzymes that give them the ability to disintegrate (into smaller fragments) the particles contained in the phagosomes with which they merge.

The material resulting from this phagolysosomal degradation can be definitively eliminated as waste material from phagocytic cells or it can be used as a “building block” for the synthesis of new intracellular compounds.

Features

Phagocytosis has many important functions in eukaryotic organisms. In protozoa and other unicellular beings, for example, this process is essential for nutrition, since most of the food is ingested in this way.

Phagocytosis in an amoeba (Source: Miklos via Wikimedia Commons)

In many multicellular organisms, on the other hand, phagocytosis is essential for specific and nonspecific defense, that is, for innate immunity and adaptive immunity.

It has primary functions in the "destruction" of invading pathogenic microorganisms such as bacteria, parasites, etc., and is also involved in the reestablishment of normal conditions in sites where infection or inflammation has occurred, that is, it is important for the wound repair.

Also in the immunological context, phagocytosis is essential for the processes of antigen presentation and activation of specific lymphocytes of the immune system (B cells and T cells), which participate in the defense of the body against foreign or foreign agents.

Phagocytosis is also involved in the elimination and "recycling" of cells in the body that go through apoptotic events, so that their components can be reused or directed to the formation of new intracellular molecules or organelles.

As a curious fact, macrophages in the human body are responsible for the daily ingestion of more than 100 million erythrocytes that wear out or malfunction in the bloodstream.

Cells of the immune system that perform phagocytosis

Cells of the immune system that perform phagocytosis can also use many mechanisms to destroy pathogens, such as:

Oxygen radicals

They are highly reactive molecules that react with proteins, lipids, and other biological molecules. During physiological stress, the amount of oxygen radicals in a cell can increase dramatically, causing oxidative stress, which can destroy cell structures.

Nitric oxide

It is a reactive substance, similar to oxygen radicals, that reacts with superoxide to create other molecules that damage various biological molecules.

Antimicrobial proteins

They are proteins that specifically damage or kill bacteria. Examples of antimicrobial proteins include proteases, which kill various bacteria by destroying essential proteins, and lysozyme, which attacks the cell walls of gram positive bacteria.

Antimicrobial peptides

Antimicrobial peptides are similar to antimicrobial proteins in that they attack and kill bacteria. Some antimicrobial peptides, such as defensins, attack bacterial cell membranes.

Binding proteins

Binding proteins are often important players in the innate immune system because they competitively bind to proteins or ions that would otherwise have been beneficial to bacteria or viral replication.

References

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