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Immunology => Immune System => Cytokines
Cytokines
Cytokines, an important group of proteins which mediate the communication between living cells. They can mediate either paracrine communication (between cells in close proximity within a tissue) or endocrine communication (between cells in different tissues, such as in the liver and the brain). Such communication is thought to be crucial to the synchronized functions of cells in both the tissues and the fluid-particularly the blood-compartments of the body.
By definition, only small quantities of these molecules are normally synthesized and released by cells at any one time, usually in a highly regulated fashion and in response to highly specific, local stimuli. Cytokines exert their actions on target cells by triggering highly specific receptors expressed on the surface of these cells. Receptors for some cytokines are also released by cells; these so-called "soluble" cytokine receptors are thought to be formed by the shedding of part of the cytokine receptor to the surface of cells. The function of these soluble cytokine receptors is unclear at present, although it has been suggested that they may block the actions of a given cytokine by binding to it and thus preventing it from binding to the surface-bound form of the same receptor on the target cells.
Cytokines were first recognized as the signals which passed between cells of the immune system to coordinate their response in wound healing and other immune reactions. At this time they were grouped into categories: lymphokines, monokines, interleukins, and growth factors. However, since we now know that these molecules perform a number of functions other than as immune cell signals, they have been grouped together under the generic title of "cytokines". The main groups of cytokines are the interleukins 1-15; interferons, alpha, beta, gamma; tumour necrosis factors, alpha and beta; colony stimulating factors G, M, G/M; transforming growth factors, alpha and beta; and fibroblast growth factors, acidic and basic.
In recent years it has been shown that the aberrant release of either various cytokines, or the expression of their (cell-surface or released) receptors, is involved in the chaotic growth and immune mechanisms which occur in such diseases as cancer, AIDS, psoriasis, sepsis, multiple sclerosis, and arthritis. Since many cytokines are "growth factors" for the cells of the immune system, many studies have focused on their therapeutic potential in boosting or controlling immune cell activity in such diseases. They are being used with increasing success in the treatment of various forms of malignant disease (particularly interleukin-2, tumour necrosis factor alpha, and the interferons; see Gene Therapy); chronic viral and parasitic infections; and some rare congenital immune deficiencies.
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