Phagocytosis [from phagein, eat + cyto, cell + - sis, process] allows the cell to absorb viruses, bacteria, whole cells and their debris, and any other kind of particulate. Many protozoa and some monocellular eukaryotes, such as amoebas, make phagocytosis their fundamental strategy to get the nutrients they need. In animals, including humans, there are some cells specialized in phagocytosis, capable as such of incorporating 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 micro-cells (neutrophil leukocytes). In addition to ingesting and destroying invading microorganisms, these scavengers also engulf dead, abnormal or severely damaged cells, insoluble particles and clots.
The process of phagocytosis
The phagocytosis process ("sporadic" act of eating) allows the intake of particles with a diameter greater than 1-2 μm. A similar cellular activity, called pinocytosis (act of drinking), allows the intake of liquid droplets, together with the solutes and the various molecules they contain. Finally, in endocytosis ("rutinary" act of eating), the incorporated molecules have intermediate dimensions.
Unlike pinocytosis, phagocytosis is a selective process that requires the consumption of energy, and therefore of ATP, by the cell.
Decomposed in various steps, the phagocytosis consists of the following phases:
A) Recognition and attachment of particles to the surface of the phagocyte
B) Ingestion (endocytosis) of the particle itself
C) Killing and degradation of the ingested microbe or incorporated particle
Recognition is the initial phase of phagocytosis and is made possible by specific membrane receptors. A direct recognition is distinguished, in which the phagocyte possesses specific receptors for the particle to be incorporated, and an indirect one. In the latter case, although it does not have specific receptors, the phagocyte recognizes the foreign cell using signal molecules, called antibodies, which other actors of the immune system apply to the foreign cell (opsonization) so as to make it more "palatable" . The antibodies, or immunoglobulins, therefore act as ligands for specific phagocyte membrane receptors (see figure).
The ingestion process is mediated by the cytoskeletal contractile proteins, which allow the cell involved in the phagocytosis to wrap the bacterium with its cell membrane, up to including it in a neo-formation vacuole surrounded by membrane (phagosome). As soon as this vesicle closes and is internalized, it fuses with the lysosomes, cellular organelles responsible for the degradation and digestion and / or destruction of foreign molecules. Thus the so-called phagolysosome is formed, in which (we are talking about the phagocytes of the immune system) the "killer" mechanisms activated to destroy the pathogen involved are activated. In this sense, oxidizing radicals and other substances (lysosomal enzymes) are able to inactivate and destroy the pathogen.
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 pushed to envelop these substances (however smaller than those incorporated by phagocytosis), but generates a hollow with formation of much smaller vesicles. There is therefore a membrane invagination instead of its extroflexion. Both endocytosis and phagocytosis are receptor mediated, whereas pinocytosis is a non-specific process.
