what is phygocytosis?

Answer 1

Phagocytosis is mediated by macrophages and polymorphonuclear leucocytes.
Phagocytosis involves the ingestion and digestion of the following:

microorganisms
insoluble particles
damaged or dead host cells
cell debris
activated clotting factors

There are several stages of phagocytosis:

1. Chemotaxis
This is the movement of cells up a gradient of chemotactic factors. It may be directly induced by a substance such as C5a, produced as a result of complement activation. It can also be indirectly induced as a consequence of release of preformed mediators within mast cells by the action of C3a or C5a e.g. eosinophil chemotactic factor, or neutrophil chemotactic factor. Leukotrienes, produced by the metabolism of mast cell arachidonic acid, are also chemotactic.

2. Adherence
This works reasonably well for whole bacteria or viruses, but less so for proteins or encapsulated bacteria. In order to deal more effectively with encapsulateed bacteria, antibodies directed against the capsule enable the phagocytic cells to ingest the organisms, using their Fc receptors

3. Pseudopodium formation
This is the protrusion of membranes to flow round the "prey".

4. Phagosome formation
Fusion of the pseudopodium with a membrane enclosing the "prey" leads to the formation of a structure termed a phagosome.

5. Phago-lysosome formation
The phagosome moves deeper into the cell, and fuses with a lysosome, forming a phago-lysosome. These contain hydrogen peroxide, active oxygen species (free radicals), peroxidase, lysozyme and hydrolytic enzymes. This is known as the oxidative burst, and leads to digestion of the phagolysosomal contents, after which they are eliminated by exocytosis. Some peptides however, undergo a very important separate process at this stage. Instead of being eliminated, they attach to a host molecule called MHC class II and end up being expressed on the surface of the cell within a groove on the MHC molecule (antigen presentation).

The speed of phagocytosis can be increased markedly by bringing into action two attachment devices present on the surface of phagocytic cells:


Fc receptor: which binds the Fc portion of antibody molecules, chiefly IgG. The IgG will have attached the organism via its Fab site.
Complement receptor: the third component of complement (C3) also binds to organisms and then attaches to the complement receptor.

Several types of cells in the immune system engulf microorganisms via phagocytosis.

Neutrophils. Neutrophils are abundant in the blood, quickly enter tissues, and phagocytize pathogens in acute inflammation.
Macrophages. Macrophages are closely related to monocytes in the blood. These longer-lived cells predominate in chronic inflammation. They also release some important inflammatory paracrines.
(Dendritic Cells and B Lymphocytes. Phagocytosis in these cells is important for the elaboration of a specific immune response rather than for directly destroying the pathogens. Much more later.)
Phagocytosis begins with the neutrophil or macrophage flowing around the pathogen and engulfing it so that it winds up enclosed in a phagosome (phagocytic vesicle). But this is only the first step, because the more challenging task of destroying the microorganisms remains. Indeed, some pathogens have special, effective mechanisms for frustrating this destruction step.

The next step is the fusion of lysosomes with the phagosome. The result is called a phagolysosome. Lysosome are derived from the Golgi apparatus, much like secretion vesicles, but their contents are focused on destroying microorganisms.

The following are important factors that help destroy microorganisms within a phagolysosome:

Hydrogen Ion Transport. Transporters for hydrogen ions acidify the phagolysosome, which kills various microorganisms.
Oxygen Radicals. A complex of proteins called NADPH oxidase in the membrane of a phagolysosome generates oxygen radicals in the phagosome. These highly reactive molecules react with proteins, lipids and other biological molecules. See the next webpage for details.
Nitric Oxide. Nitric oxide synthase synthesizes nitric oxide, a reactive molecule that damages various biological molecules. (But nitric oxide is also, remarkably enough, an important regulatory molecule elsewhere. More on this later this quarter.)
Anti-Microbial Proteins. Important here are lysozyme, an enzyme that attacks the cell walls of certain (gram positive) bacteria, and special proteases which break down protein when in an acid environment.
Anti-Microbial Peptides. Defensins and certain other peptides attack bacterial cell membranes. Similar molecules are found throughout much of the animal kingdom.
Binding Proteins. Lactoferrin binds iron ions, which are necessary for growth of bacteria. Another protein binds vitamin B12.
In addition to destroying the microorganism, macrophages also release paracrines that alert other parts of the immune system that an infection is present. (Two important examples are IL-1 and TNF-alpha.) Among other things, these paracrines promote inflammation.



Identification of Pathogen
Neutrophils and macrophages have some ability on their own to recognize microorganisms and begin phagocytosis. This is because such organisms have molecules much different than those found in a human. But phagocytosis is far more effective if microorganisms are labelled as such by special molecules that bind to their surface. (Any molecule that binds to a microorganism and thereby speeds phagocytosis is called an opsonin). Most important here are antibodies (such as IgG), which specifically identify molecules at the surface of specific microorganisms. With this attached to the surface of the microorganisms, phagocytosis is much more effective and rapid. More on this later.



Difficult Pathogens
But, as mentioned above, sometimes phagocytes have a difficult time with certain pathogens. Tuberculosis is an important example. A macrophage can usually engulf the tuberculosis bacterium, but then apparently the bacterium has a means for preventing the lysosomes from fusing with the phagosome. If the macrophage is not "activated" by paracrines from a specific immune response, the bacteria may remain alive for long periods within the macrophage. In this circumstance, other macrophages surround and wall off the infected macrophages, forming a type of chronic inflammation called a granuloma. Leprosy is another bacterium that is difficult for macrophages to destroy. Again, more on this later.

Anthrax is an example of a bacterium surrounded by a capsule that make phagocytosis difficult. Anthrax spores from the lungs or a cut in the skin make their way first to lymph nodes, where they change to their "vegetative form" and begin dividing. But because they are difficult to destroy, they quickly become quite numerous and accumulate in the blood, causing septicemia. Not only do the bacteria release toxins, but also, as is the case in certain other dangerous infections, macrophages respond to the crisis by releasing enough IL-1 and TNF-alpha to cause inflammation throughout the body. Indeed, this hyperinflammation by itself can quickly be fatal. It can include, for example, widespread formation of small blood clots, which is termed disseminated intravascular coagulation.

Answer 2

Once a white cell has left the blood vessel and migrated to the enemy, the next job is to EAT the microbe. This human macrophage, like its cousin the neutrophil, is a professional "phagocyte" or eating cell (phago = "eating", cyte = "cell"). The macrophage is using its internal cytoskeleton to envelop cells of the fungus Candida albicans. View this sequence in a 510K time-lapse movie.

But eating the organisms is not enough. To insure that the organisms not grow and divide within the macrophage, the white cell must kill the organisms by some means such as the OXIDATIVE BURST.



Phagocytosis clips in a larger, longer, silent format may be downloaded for classroom use as part of the "Immune System Collection" or as a single clip showing a neutrophil ingesting E. coli.
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Phagocytosis is mediated by macrophages and polymorphonuclear leucocytes.
Phagocytosis involves the ingestion and digestion of the following:

microorganisms
insoluble particles
damaged or dead host cells
cell debris
activated clotting factors

There are several stages of phagocytosis:
1. Chemotaxis
This is the movement of cells up a gradient of chemotactic factors. It may be directly induced by a substance such as C5a, produced as a result of complement activation. It can also be indirectly induced as a consequence of release of preformed mediators within mast cells by the action of C3a or C5a e.g. eosinophil chemotactic factor, or neutrophil chemotactic factor. Leukotrienes, produced by the metabolism of mast cell arachidonic acid, are also chemotactic.
2. Adherence
This works reasonably well for whole bacteria or viruses, but less so for proteins or encapsulated bacteria. In order to deal more effectively with encapsulateed bacteria, antibodies directed against the capsule enable the phagocytic cells to ingest the organisms, using their Fc receptors (see below).
3. Pseudopodium formation
This is the protrusion of membranes to flow round the "prey".
4. Phagosome formation
Fusion of the pseudopodium with a membrane enclosing the "prey" leads to the formation of a structure termed a phagosome.
5. Phago-lysosome formation
The phagosome moves deeper into the cell, and fuses with a lysosome, forming a phago-lysosome. These contain hydrogen peroxide, active oxygen species (free radicals), peroxidase, lysozyme and hydrolytic enzymes. This is known as the oxidative burst, and leads to digestion of the phagolysosomal contents, after which they are eliminated by exocytosis. Some peptides however, undergo a very important separate process at this stage. Instead of being eliminated, they attach to a host molecule called MHC class II and end up being expressed on the surface of the cell within a groove on the MHC molecule (antigen presentation).
The speed of phagocytosis can be increased markedly by bringing into action two attachment devices present on the surface of phagocytic cells:


Fc receptor: which binds the Fc portion of antibody molecules, chiefly IgG. The IgG will have attached the organism via its Fab site.
Complement receptor: the third component of complement (C3) also binds to organisms and then attaches to the complement receptor.

Answer 3

It is Phagocytosis and more can be seen from the source.
Phagocytosis (literally "cell-eating") is a form of endocytosis wherein large particles are enveloped by the cell membrane of a (usually larger) cell and internalized to form a phagosome, or "food vacuole."

In animals, phagocytosis is performed by specialized cells called phagocytes, which serve to remove foreign bodies and thus fight infection. In vertebrates, these include larger macrophages and smaller granulocytes, types of blood cells. Bacteria, dead tissue cells, and small mineral particles are all examples of objects that may be phagocytosed. Virulent bacteria may need to be coated in antibodies before they can be consumed. Certain pathogenic bacteria, such as those of leprosy and tuberculosis, once internalized through phagocytosis, are resistant to killing by the phagocytes that have ingested them. Anything that impedes or prevents the action of phagocyctes is termed antiphagocytic.

VR

Answer 4

Phygocytosis (Greek -phagos, “one that eats”; kytos, “cell”), process of ingestion of matter by cells known, in this context, as phagocytes. Single-celled life forms that bodily engulf and ingest foreign matter—whether other cells, bacteria, or nonliving material—display phygocytosis. In multicellular organisms the process is relegated to specialized cells, generally for the purpose of defending the organism as a whole from potentially harmful invaders.

Answer 5

It is the process by which the cell engulfs the food or foreign material for example WBC in our body kills the bacteria or germs by phagocytosis and digests it. I guess i solved your problem!!!

Answer 6

u mean phagocytosis...Phagocytosis (literally "cell-eating") is a form of endocytosis wherein large particles are enveloped by the cell membrane of a (usually larger) cell and internalized to form a phagosome, or "food vacuole."

Answer 7

phagocytosis means engulfing of food particle as a whole by a cell like amoeba which engulfs its food and wbc which engulf foreign particle in the body

Answer 8

phagocytosis is the part of imnue system...this is performed by tthe macrophages, monocytes etc...these cells undergo the cell eating mechanism...

Answer 9

it is the destruction by phagocytes.