Everything about Phagocytosis totally explained
Phagocytosis is the
cellular process of engulfing solid particles by the
cell membrane to form an internal
phagosome, or "food vacuole." The phagosome is usually delivered to the
lysosome, an
organelle involved in the breakdown of cellular components, which fuses with the phagosome. The contents are subsequently degraded and either released extracellularly via
exocytosis, or released intracellularly to undergo further processing.
Phagocytosis is involved in the acquisition of nutrients for some cells, and in the
immune system it's a major mechanism used to remove
pathogens and cell debris. Bacteria, dead tissue cells, and small mineral particles are all examples of objects that may be phagocytosed.
Phagocytosis is a specific form of
endocytosis involving the
vesicular internalization of solid particles, such as
bacteria, and is therefore distinct from other forms of endocytosis such as
pinocytosis, the vesicular internalization of various liquids.
Etymology
Phagocytosis is named from
Greek roots:
phago - to eat,
kytos - cell, and
-osis - a suffix indicating a process; thus it can be translated literally as "the cellular process of eating." It should be noted that the process is only homologous to eating at the level of single-celled organisms; in multicellular animals the process has been adapted to eliminate debris and pathogens, as opposed to taking in fuel for cellular processes, except in the case of the
Trichoplax.
Process
Phagocytosis is activated by attachment to
Pathogen-associated molecular patterns (PAMPS), which leads to
NF-κB activation.
Opsonins such as
C3b and
|antibodies can act as attachment sites and aid phagocytosis of pathogens.
Engulfment of material is facilitated by the actin-myosin contractile system. The phagosome of ingested material is then fused with the lysosome, leading to degradation
Degradation can be oxygen-dependent or oxygen-independent. Oxygen-dependent degradation depends on NADPH and the production of
reactive oxygen species. Hydrogen peroxide and myeloperoxidase activate a halogenating system which leads to the destruction of bacteria. Oxygen-independent degradation depends on the release of granules, containing proteolytic enzymes such as defensins, lysozyme and cationic proteins. Other antimicrobial peptides are present in these granules, including lactoferrin which sequesters iron to provide unfavourable growth conditions for bacteria.
Phagocytic cells
Many
protists are able to phagocytose particles; however, in animals phagocytosis is performed by specialized cells called
phagocytes, which are able to remove foreign bodies and thus fight infection. In humans and many other animals, phagocytes include
macrophages,
monocytes,
dendritic cells, and
granulocytes. The term
professional phagocytes can be used to describe both macrophages and neutrophils, as these cells are considered to have phagocytosis as their primary function.
A great body of evidence continues to mount showing that resident, neighbouring cells in a tissue will phagocytize their
apoptotic neighbours, thus maintaining tissue homeostasis. This clearance can, depending on the location, facilitate greater clearance than that achieved by resident macrophages.
Functions
Innate immunity
In humans, the most important facet of phagocytosis is its control of inflammation. Its most important function is the transport of cells into the blood. Depending on the phagocytosed particle, phagocytosis can induce inflammation or, as is the case with apoptotic cells, induce resolution of inflammation. Phagocytosis is also involved in immune tolerance, which prevents inflammation against normal components of the body.
Adaptive immunity
For the activation of the
adaptive immune response, phagocytosis is a necessary activation step. A
T helper cell must be presented foreign particles bound to the
major histocomptability complex class II (MHC II) receptor to become activated.
Nutrition
In many
protists, phagocytosis is used as a means of feeding, providing part or all of their nourishment. This is called phagotrophic nutrition, as distinguished from osmotrophic nutrition, which takes place by absorption. In some, such as
amoeba, phagocytosis takes place by surrounding the target object with
pseudopods, as in animal phagocytes. In other protozoa, for instance,
ciliates, there's a specialized groove or chamber in the cell where phagocytosis takes place, called the
cytostome or mouth. The resulting
phagosome may be merged with
lysosomes containing digestive
enzymes, forming a phagolysosome. The food particles will then be digested, and the released nutrients are diffused or transported into the
cytosol for use in other metabolic processes.
Resistance to phagocytosis
A substance or process which impedes or prevents the action of phagocytes is termed antiphagocytic (for example,
Yersinia pestis, the
bacterium causing the
plague). Additionally, some bacteria may need to be
opsonized before they're recognised as particles to be phagocytosed.
Certain intracellular pathogens, such the bacterial agents of
leprosy and
tuberculosis, are resistant to
lysosomal degradation once internalised via phagocytosis.
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