The opsonins are the immune system molecules that bind to the antigen and the immune cells known as phagocytes, facilitating phagocytosis. Some examples of phagocytic cells that can participate in this process are macrophages.
Once a pathogen overcomes the anatomical and physiological barriers of the host, it is possible to cause infection and disease. Therefore, the immune system reacts to this invasion by detecting the foreign body through sensors and attacking it with an elaborate response mechanism.
Action of opsonins. By Graham Colm, from Wikimedia Commons While phagocytes do not require opsonins to allow them to recognize and envelop their targets, they operate much more efficiently in their presence. This mechanism of binding opsonins to foreign pathogens and acting as a tag is called opsonization. Without this mechanism, the recognition and destruction of invading agents would be inefficient.
Features
Opsonins coat the particles to be phagocytosed by interacting with antigens. In this way, phagocytic cells such as macrophages and dendritic cells, which express receptors for opsonins, bind to opsonized pathogens through these receptors and finally phagocytose them.
Thus, the opsonins act as a kind of bridge between the phagocyte and the particle to be phagocytosed.
Opsonins are responsible for counteracting the repellent force between negative cell walls and promote absorption of the pathogen by the macrophage.
Without the action of opsonins, the negatively charged cell walls of the pathogen and phagocyte repel each other, so the foreign agent can circumvent their destruction and continue to replicate within the host.
So, opsonization is an antimicrobial strategy to slow down and eliminate the spread of a disease.
Types
There are several types of opsonins, including mannose-binding lectin, immunoglobulins of the IgG isotype, and components of the complement system such as C3b, iC3b, or C4b.
Mannose-binding lectin is produced in the liver and released into the blood. It has the ability to bind to repetitions of sugars present in microorganisms, favoring their destruction by activating the complement system through the association of serine proteases.
IgG is the only immunoglobulin isotype that has the ability to cross the placenta, due to its small size. There are 4 subisotypes, which have specific functions.
C3b, is the main component formed after the breakdown of the C3 protein of the complement system.
iC3b is formed when complement factor I cleaves the C3b protein.
Finally, C4b is the product of the proteolysis of C1q, which is a complex of proteins that, upon the formation of antigen-antibody complexes, are activated following a sequence.
Importantly, opsonization of a pathogen can occur through antibodies or the complement system.
Antibodies
Antibodies are part of the adaptive immune system, which are produced by plasma cells in response to a certain antigen. An antibody has a complex structure that confers specificity to certain antigens.
At the end of the heavy and light chains, antibodies have variable regions (antigen-binding sites), which allow the antibody to fit like "a key in a lock." Once the antigen-binding sites are occupied, the stem region of the antibody binds to the receptor on phagocytes.
In this way, the pathogen becomes engulfed by the phagosome and is destroyed by the lysosomes.
Furthermore, the antigen-antibody complex can also activate the complement system. Immunoglobulin M (IgM), for example, is very efficient in activating complement.
IgG antibodies are also capable of binding to immune effector cells through their constant domain, triggering a release of lysis products from the immune effector cell.
Complement system
The complement system, for its part, has more than 30 proteins that enhance the ability of antibodies and phagocytic cells to fight invading organisms.
Complement proteins, identified with the letter "C" for complement, are made up of 9 proteins (C1 to C9), which are inactive when they circulate throughout the human body. However, when a pathogen is detected, the proteases cleave the inactive precursors and activate them.
However, the body's response to the presence of a pathogen or foreign body can be carried out through three routes: the classical, the alternative and the lectin route.
More than 3o proteins work together to complement the action of antibodies in destroying pathogens. By Perhelion, from Wikimedia Commons. Regardless of the activation pathway, all three converge at a single point where the membrane attack complex (MAC) is formed.
MAC is made up of a complex of complement proteins, which are associated with the outer part of the plasmatic membrane of pathogenic bacteria and form a kind of pore. The ultimate goal of pore formation is to cause lysis of the microorganism.
Receivers
Once C3b has been generated, by any of the pathways of the complement system, it binds to multiple sites on the cell surface of the pathogen and then adds to receptors expressed on the surface of the macrophage or neutrophil.
Four types of receptors that recognize C3b fragments are expressed on leukocytes: CR1, CR2, CR3 and CR4. The deficiency in these receptors makes the person more susceptible to suffer from continuous infections.
C4b, like C3b, can bind to the CR1 receptor. While iC3b joins the CR2.
Among the Fc receptors, the FcℽR stand out, which recognize different subisotypes of IgG.
Binding of the opsonized particle to cell surface phagocyte receptors (Fc receptors), triggers the formation of pseudopods that surround the foreign particle in a zipper-like manner through receptor-opsonin interactions.
When the pseudopods meet, they fuse to form a vacuole or phagosome, which then binds with the lysosome in the phagocyte, which discharges a battery of enzymes and toxic antibacterial oxygen species, initiating digestion of the foreign particle to eliminate it.
References
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