Phagocytosis is a method of endocytosis. It can define as the cellular mechanism where the vesicles form by the invagination of the plasma membrane and cause internalization of the extracellular contents into the cell. Internalization is a process where a plasma membrane undergoes successive invagination to form membrane-bound vesicles that later detach from the cell membrane and release into the cytoplasm. In phagocytosis, the internalized molecules are generally large particles (>0.5 mm in diameter).
It also refers as “Cell eating endocytosis” that occurs in a particular or specialized type of cell. It is a type of active transport mechanism that makes the use of ATP. The membrane-bound vesicles mediate the transportation of the extracellular biomolecules into the cell cytoplasm for further processing, degradation and recycling.
Definition of Phagocytosis
Phagocytosis is one of the cellular mechanisms that facilitate endocytosis and also refers as “Cell eating”. This process internalizes the large particles by the specific binding between the host cell receptors and the ligands present of the cell surface of the target organism. Phagocytosis was first reported by a scientist named Metchnikoff. Then the plasma membrane invaginates to form a large endocytic vesicle, refers as a phagosome. The diameter of the phagosome is greater than 250 nm.
The process of phagocytosis is carried out by the specialized cells refers to as “Phagocytes”. In mammalian cells, the process of phagocytosis is achieved by macrophages, neutrophils, dendritic cells and monocytes. It forms the second line of defence. A phagocytic cell comprises an intracellular compartment, i.e. lysosomes that contain microbicidal substances and hydrolytic enzymes like lipases, proteases, nucleases etc.
Process of Phagocytosis
The method of phagocytosis involves the following endocytic pathway:
Recognition of Pathogen
A macrophage comprises of discrete receptors that help in identification of target organism. There are pattern recognition receptors (PRRs) on the cell surface of phagocytes that recognize the pathogen. The phagocytic receptors can be non-opsonic or opsonic.
Non-opsonic receptors can recognize the molecular groups on the phagocytic targets, and most commonly includes CD169, CD33, and Dectin-1 receptor molecules etc. Opsonic receptors identify the host-derived opsonins that attach to the target cell and includes FcR, CR receptors etc.
The microbes also contain pathogen-associated molecular patterns (PAMPs). When the phagocyte recognizes the specific antigen, the cell receptors of phagocyte binds to the receptor molecules of the foreign body.
Example of binding between host and target cell receptors: Dectin-1 is a host cell receptor that specifically binds to the polysaccharides of the yeast cell.
Ingestion of Pathogen
After recognition of the target cell, the phagocytic receptors form a signalling cascade that eventually deforms the lipid content of the cell membrane and regulates the actin cytoskeleton. The deformation results in the extension of the bilayer cell membrane to capture the foreign particle. Thus, when a phagocyte encounters with a specific antigen, the plasma membrane invaginates to enclose the foreign particle and go through a series of sequential events to form a large sac-like structure.
Release of Phagosome
The binding of a host cell and target cell receptor molecules leads to a formation of the phagocytic cup as a result of remodelling of the actin cytoskeleton. The phagocytic cup will expand slowly to enclose the target molecule and finally closes its distal end to form a sac-like structure refers as “Phagosome”. Later, the vesicle pinches off into the cytoplasm and refers to as an intermediary phagosome.
Formation of Phagolysosome
A phagocyte comprises of small intracellular or endocytic compartments, i.e. lysosomes which tend to fuse with the large phagosome. The fusion between an intermediary phagosome and lysosome results into the formation of a large, mature, acidic and microbicidal vesicle refers as “Phagolysosome”. The granules of lysosome change the membrane and interior characteristics by the progressive acidification of the intermediary phagosome.
Inside the phagolysosome, microbial degradation occurs by the microbicidal activity of the lysosome. By the progressive acidification of the phagolysosome, the hydrolytic enzymes activate and causes microbial degradation. The degradation can be oxygen-dependent or oxygen-independent.
Oxygen-dependent degradation involves microbial killing via reactive oxygen species like hydrogen peroxide and makes the use of NADP. In contrast, oxygen-independent degradation involves hydrolytic enzymes that break down the complex microbial components into smaller and simpler fragments.
Functions of Phagocytosis
- It is an essential process for the nutrients uptake in the unicellular or prokaryotic organisms.
- In mammalian cells, the process of phagocytosis forms the first line of defense where the specialized immune cells like macrophages, neutrophils, monocytes etc. ingest the foreign particle.
- Phagocytosis is a cellular mechanism that not only limited to the microbial ingestion but also in the elimination of the apoptotic cells.
Phagocytosis is a complex cellular mechanism for the ingestion and egestion of microbial elements and apoptotic cells. The process of cell eating plays a significant role in tissue homeostasis and immune defense.