Phagocytosis [from phagein, eat + cyto, cell + -sis, process] allows the cell to incorporate viruses, bacteria, whole cells and their debris, and any other kind of particulate. Many protozoa and some single-celled eukaryotes, such as amoebas, make phagocytosis their fundamental strategy for obtaining the nutrients they need. In animals, including humans, there are some cells specialized in phagocytosis, capable as such of englobing and digesting bacteria and other foreign particles. These protagonists of the immune system (white blood cells) take the generic name of phagocytes and are represented by the so-called macrophages (derived from monocytes) and microphages (neutrophils leukocytes). In addition to ingesting and destroying invading microorganisms, these scavengers also engulf dead, abnormal or severely damaged cells, insoluble particles and clots.
Generally speaking, neutrophil granulocytes are particularly active in the defense of the organism from pyogenic bacteria, while macrophages are more effective in responding to infection by intracellular microorganisms. Alongside these cells, for which phagocytosis is a prominent function, there are also the so-called facultative phagocytes (fibroblasts, mast cells, endotheliocytes, etc.) for which the process is completely marginal.
The process of phagocytosis
The process of phagocytosis (the "sporadic" act of eating) allows the "intake of particles with a diameter greater than 1-2 μm. A similar cellular activity, called pinocytosis (the act of drinking), allows the" intake of droplets of liquid, together with the solutes and the various molecules they contain Finally, in "endocytosis (the" routine "act of eating), the incorporated molecules have intermediate dimensions.
Unlike pinocytosis, phagocytosis is a selective process that requires the cell to consume energy, hence ATP.
Broken down into various steps, phagocytosis consists of the following phases:
A) Recognition and attachment of particles to the phagocyte surface
B) Ingestion (endocytosis) of the particle itself
C) Killing and degradation of the ingested microbe or embedded particle
Recognition is the initial stage of phagocytosis and is made possible by specific membrane receptors. A direct recognition is distinguished, in which the phagocyte has specific receptors for the particle to be incorporated, and an indirect one. In the latter case, although not possessing specific receptors, the phagocyte recognizes the foreign cell by means of signal molecules, called antibodies, which other actors of the immune system apply to the foreign cell (opsonization) in order to make it more "palatable" The antibodies, or immunoglobulins, then act as ligands for specific membrane receptors of the phagocyte (see figure).
The ingestion process is mediated by the contractile proteins of the cytoskeleton, which allow the cell involved in phagocytosis to envelop the bacterium with its cell membrane, up to including it in a newly formed vacuole surrounded by a membrane (phagosome). As soon as this vesicle closes and is internalized, it merges with lysosomes, cellular organelles responsible for the degradation and digestion and / or destruction of foreign molecules. The so-called phagolysosome is thus formed, inside which (we are talking about the phagocytes of the immune system) the "killer" mechanisms responsible for the destruction of the pathogen involved are activated. In this sense, oxidizing radicals and other substances (lysosomal enzymes) capable of inactivating and destroying the pathogen are involved.
As anticipated, phagocytosis is a process very similar to endocytosis, an alternative mechanism by which large molecules or particles can enter cells. In this case, however, the cell membrane is not forced to envelop these substances (however smaller than those englobed by phagocytosis), but generates a hollow with the formation of much smaller vesicles. Therefore, there is an invagination of the membrane rather than the ejection of the same. Both endocytosis and phagocytosis are receptor mediated, while pinocytosis is a nonspecific process.