Cytokine

Cytokines are small protein molecules that regulate communication among immune system cells and between immune cells and those of other tissue types. These chemicals are actively secreted by immune cells as well as other cell types in response to external stimuli. Cytokines that are produced by immune cells form a subset known as lymphokines.

Effects

Cytokines mediate many important physiological functions including growth, development, wound healing and immune response. They may affect local tissues (paracrine effects) or more distant ones (endocrine effects) (Clark, 2005).

Cytokines act by binding to their cell-specific receptors. These receptors are located in the cell membrane, and each allows a distinct signal transduction cascade to start in the cell, that eventually will lead to biochemical and phenotypical changes in the target cell. Typically, receptors for cytokines are also tyrosine kinases.

Cytokines are said to be pleiotropic, meaning that the same cytokine can have different effects on a cell depending on the state of the cell. For instance, there are several known cytokines that have both stimulating and suppressing action on lymphocyte cells and immune response.

To make things even more complicated, cytokines often regulate the expression of other cytokines (either upwards or downwards), often triggering cascades of other cytokines. The cytokines in these cascades can interact with each other and the cells that produced them in complicated fashions to form cytokine networks. Cytokines often act together in ways that are synergistic or antagonistic.

Some studies have shown links between cytokine levels and clinical depression [1].

In the inflammatory response, white blood cells release cytokines, which also recruit more lymphocytes and allow them to pass through blood vessel walls into the inflamed area (Clark, 2005). Their release triggers stages in the wound-healing cascade. They are also implicated in brain tissue damage due to reperfusion injury in stroke (Clark, 2005).

The excessive production of cytokines is what leads to the death of individuals that are infected with the bird flu. The cytokines are produced by the immune system as a defensive maneuver (much like an allergic reaction), and too much causes the airways to become blocked. The young and healthy are more likely to succumb to the bird flu because the vigorous response of their immune systems leads to excessive levels of cytokines.

Types

There are three classes of cytokines. Hundreds of cytokines have been discovered, and the rate of discovery shows no sign of slowing. Some of the better known cytokines include: TNF-α, IL-1, and IL-6.

Most cytokines are not growth factors.

Characteristics

Cytokines weigh from 8 to 30 kDa and are active in picomolar concentrations (Clark, 2005).

History

The first lymphokines were identified in the mid-1960's, with the best known being migration inhibition factor (MIF), simultaneously discovered by John David and Barry Bloom. The first cytokines were identified in cultures of kidney cells, by Stanley Cohen and his colleagues, in 1974.

See also

• Adipokines
• Apoptosis
• Chemokines
• Cytokine secretion assay
• Cytokine storm
• ELISPOT
• Interleukins
• Interferon
• Granulocyte-colony stimulating factor
• Signal transduction
• Tumor necrosis factor

Links

• Cytokine Tutorial
• Cell Interactions: Cytokines
• Reperfusion Injury in Stroke.
• Cytokines Online Pathfinder Encyclopaedia