In most higher animal cells, phagocytosis is a means of protection rather than a means of feeding. Higher animals have some specialized phagocytic cells, including macrophages and neutrophils. They ingest and destroy infected bacteria, viruses, and damaged cells, aging red blood cells by phagocytosis. Phagocytosis is one of the oldest and most basic defense mechanisms in organisms. It is not specific for the subject it is to destroy, and is called non-specific immunity in immunology. The phagocytosis of neutrophils and monocytes is very strong. Although eosinophils are very migratory, their phagocytic ability is weak. Leukocytes can ooze out of the blood vessels through the endothelial gap of the capillaries and migrate in the interstitial space.
Figure 1. Overview of phagocytosis.
Phagocytosis is also known as swallowing. Refers to the activity of cells ingesting solid particles from their surroundings. It is generally believed that the mechanism by which this effect occurs is essentially the same as the pinocytosis of the ingested liquid. When the solid matter is adsorbed on the cell membrane, the membrane protrudes or sinks, and once the cell membranes on both sides are fused, the solid matter surrounded by the membrane is enclosed in the cells. This is called a phagosome (phagocytic follicle), but soon it is combined with a lysosome to form a digestive vesicle. The encapsulated solid substance is decomposed by the action of a degrading enzyme in the lysosome, becoming a low molecule and thus absorbed. Within the cytoplasm. The feeding pattern seen in protozoa such as root-footed species is the original form of phagocytosis, but the same cellular activities are also commonly distributed in posterior animals, which have various biological significance. The phagocytosis of vertebrate white blood cells has the function of defending the body and removing waste. The phagocytosis of white blood cells (mainly neutrophils) to bacteria invading the body can have an important defense against bacterial infections compared to the formation of antibodies. Macrophages such as mononuclear white blood cells and tissue spheres also prey on parasitic protozoa, pigment particles, tissue cell debris, and aging deformed blood cells. In the metamorphosis of insects and amphibians, the same phagocytosis can help absorb larval tissue. Osteoblasts also belong to such phagocytic cells. In addition, various cells of the reticuloendothelial system also have the same function, acting as fixation phagocytic cells to restore the useless red blood cells and bile pigments.
Process of phagocytosis
The function of phagocytosis involves the directional migration, recognition, phagocytosis and killing of phagocytic cells.
(1) Contact between phagocytic cells and pathogenic organisms: phagocytic cells accumulate and migrate to the invasion site of pathogenic organisms under the action of chemokines (cytokines, activated complement fragments, certain bacterial bacterial components and products), phagocytic cells and Pathogenic organisms meet and bind to their pathogens and their corresponding ligands through their surface receptors (mannose receptors, lipopolysaccharide receptors). It can also be contacted with the corresponding pathogenic organism through the LGF receptor and the C3b receptor.
(2) Swallowing pathogens: phagocytic cells for large granular foreign bodies such as bacteria, the pseudopods are enclosed and taken up into the cells to form a phagosome surrounded by a layer of cell membranes, that is, phagocytosis. For small substances such as viruses, the phagocytic membrane at the attachment site is invaginated to form a swallowing vesicle, that is, a pinocytosis
(3) Killing pathogenic organisms: After phagosome formation, lysosomes in phagocytic cells approach phagosomes and fuse with them to form phagolysosomes, lysozyme, lactoferrin, defensin, and medulla in lysosomes. Peroxidase can kill pathogenic organisms, and proteases, polysaccharide enzymes, nucleases, lipases, etc. decompose them, and finally discharge the indigestible residue out of the phagocytic cells.
The granules in neutrophils are lysosomes containing a variety of hydrolases that can digest the pathogens or other foreign bodies they ingest. Usually a white blood cell treats 5 to 25 bacteria and it dies by itself. The dead white blood cell group and bacterial decomposition products constitute pus. Monocytes are produced by bone marrow and live in the blood for only 3 to 4 days, that is, into tissues such as liver, spleen, lung, and lymph, which are transformed into macrophages. After becoming a macrophage, the volume is increased, the lysosome is increased, and the ability to phagocytosis and digestion is also enhanced. However, its phagocytosis is mainly caused by pathogenic substances such as viruses, malaria parasites and bacteria that enter the cells. Macrophages are also involved in activating the specific immune function of lymphocytes. In addition, it also has the function of identifying and killing tumor cells and eliminating aging and damaged cells.
Figure 2. 3D rendering of a neutrophil.
In immune system
Phagocytosis is one of the main mechanisms of innate immune defense. It is one of the earliest processes in response to infection and one of the starting branches of an adaptive immune response. Although most cells have phagocytic function, some cell types will be part of their primary function. These are called "professional phagocytic cells." Phagocytosis is ancient in evolution and even in invertebrates.
Figure 3. The engulfing of a pathogen by a phagocyte.
Consequences of phagocytosis
The consequences of phagocytosis vary with the type, virulence, and immune status of the phagocytic pathogens swallowed by the phagocytic cells.
(1) Complete phagocytosis: refers to the fact that phagocytic cells can both swallow and kill and digest pathogens. For example, after being phagocytized, pyogenic cocci is usually dead for 5 to 10 minutes, and digested and decomposed for 30 to 60 minutes.
(2) Incomplete phagocytosis: refers to phagocytic cells that can only be swallowed by pathogens but cannot be killed and digested. Incomplete phagocytosis allows the engulfed pathogen to be protected in phagocytic cells from non-specific antimicrobial molecules, specific antibodies or antibiotics in the body fluids. Some pathogens can even multiply in phagocytic cells, causing phagocytic cells to die, or spreading phagocytic cells through the lymph or blood to other parts of the body, causing widespread infection.
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