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Chemokines are signalling proteins that are produced by immune system cells and promote cell migration. Because it is derived from chemotaxis, or movement in response to a chemical stimulus, and cytokines, which are tiny proteins implicated in cell signalling, their name is suggestive of their function.
In other words, chemokines are cytokines that stimulate mobility. Chemokines have important roles in immunological reactions as secondary pro-inflammatory mediators triggered by primary pro-inflammatory mediators, as well as in homeostasis, where they control cell mobility to maintain immune system health and normal functioning.
Chemokine receptors are primarily found on the surface of white blood cells, which is not surprising (cells predominantly involved in immunological reactions). Chemokines have been identified in all vertebrates, as well as certain unicellular species like viruses and bacteria, but none have been found in non-vertebrates.
Chemokines are small proteins that range in size from 8 to 10 kDa. There are several varieties, but they are all quite similar. The majority of them have four cysteine amino acids in common, two of which are utilised to categorise all chemokines into four kinds.
The categorization of chemokines into the CC, CXC, C, and CX3C kinds is based on two amino acids close to the N-terminus. CXC chemokines contain one amino acid between the two cysteine residues; C chemokines have just two cysteines instead of four, one of which is at the N-terminus; and CX3C chemokines have three amino acids between the two cysteines. These four kinds of chemokines have the following structures:
Chemokines are proteins that cause cells to move around. Not only that, but chemokines play two important roles in the immune system: they are involved in immunological responses as well as immune system homeostasis.
Chemokines and Immunological Reactions
Chemokines play a pro-inflammatory immunological role when some immune system cells release them and other cells recognise them. Chemokines are chemoattractants, meaning they attract other cells to the place where they are released.
When a pathogenic agent needs to be fought off in order for the organism to be healthy, these cells produce chemokines. In this way, the presence of foreign agents, bacteria, or viruses triggers the production of chemokines, allowing additional immune cells, such as white blood cells, to reach the invasion site.
The receptors on the cell surface of cells that detect chemokines are activated when chemokines attach to them, resulting in an intracellular signalling cascade that leads to movement. G protein-coupled receptors (GPCRs) are the receptors that chemokines bind to.
So far, nineteen different types have been discovered, and they are classified into four groups, similar to chemokines, based on which chemokine binds to them: CCR, CXCR, CR, and CX3CR (R for receptor). When chemokines activate GPCRs, the phospholipase C (PLC) signal transduction pathway is triggered.
Chemotaxis, or cell movement in response to chemical signals, results in the migration of these cells to the infection site and the release of poisonous chemicals to kill the pathogens. Immune cells designated leukocytes (white blood cells) such as monocytes, macrophages, and T-lymphocytes are attracted to the infection site.
Chemokines and Homeostasis
Chemokines play a role in homeostasis by allowing cells to migrate by chemotaxis. However, in this case, chemokines are not produced in response to any pathogen.
Homeostasis is the control of all variables in the body in order to maintain the balance required for good health. When blood glucose levels are too high, for example, the body reacts to bring them back to healthy levels, or to maintain homeostasis.
In the case of homeostatic chemokines, their function comprises immune system monitoring, which includes activities like the movement of leukocytes—basal leukocytes—across the immune system in pursuit of pathogens and reacting to them by generating an immunological response.
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