Opsonization is a process where pathogens are marked for destruction by the immune system. This can happen through phagocytosis or antibody-dependent cellular cytotoxicity. Opsonins, such as IgG and IgA antibodies, and proteins in the complement system, help immune cells bind to pathogens for destruction. Defects in this process can lead to increased susceptibility to infections.
Opsonization is a process in which pathogens are coated with a substance called opsonin, which marks the pathogen for destruction by the immune system. Once a pathogen has been tagged, it is killed via one of two mechanisms: it can be ingested and killed by an immune cell, or killed directly without ingestion.
The process of killing and ingesting a pathogen is called phagocytosis. Cells called phagocytes ingest pathogens and then kill them by exposing them to toxic chemicals. Chemicals are stored in small membrane-bounded parcels inside cells, and these parcels are triggered to open when a phagocyte ingests a pathogen.
Opsonization also leads to the death of pathogens through a second mechanism called antibody-dependent cellular cytotoxicity, in which immune cells directly kill pathogens without ingesting them. In this process, antibodies mark pathogens and then activate immune cells called granulocytes. These cells then release toxic chemicals into the environment around the pathogens to kill them. In addition to killing them, this process also causes tissue damage through inflammation.
This method of destruction is necessary because both immune cells and pathogenic cells are negatively charged. This means that it is difficult for a cell to get close enough to a pathogen to initiate direct ingestion or killing. The evolution of opsonins solves this problem because they have receptors that recognize and bind to protein molecules on immune cells. Therefore, when a pathogen has been coated with opsonins, the receptors can bind to immune cells, bringing the cells close enough to the pathogens to allow for direct ingestion or killing.
There are several substances that can act as opsonins and all of these are active proteins in the immune system. There are two types of antibodies called IgG and IgA. IgG is active in blood and tissue, and IgA is active in mucosal surfaces such as the airways, urogenital system, and intestines. Several proteins that act in the complement system are also opsonins. The complement system is a cascade of reactions between a number of different proteins, the end result of which is the opsonization of pathogens, as well as direct killing via the formation of a protein complex that punctures the cell walls of bacteria.
Several types of inherited genetic disorders can cause defects in this process. For example, people with diseases that cause deficiencies in the complement system are more susceptible to infections, particularly bacterial infections. Diseases affecting B lymphocytes, the cells that produce antibodies, also lead to increased susceptibility to infections. People with these immune deficiencies have varying levels of risk of serious and even fatal infections with pathogens that would not cause disease in healthy people.
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