Cytokine

Cytokines are small secreted protein messengers that are involved in a range of physiological processes, including development and wound healing. They have a particularly pivotal role in the immune system. Similar to hormones, cytokines affect the behavior of their target cells—causing effects such as growth, proliferation, and migration. Yet, unlike hormones, cytokines are produced by a variety of cells, rather than specialized glands.

The roles of different cytokines are not completely understood, and they are the subject of intense research efforts. As a result of the important role of cytokines in the immune response, these molecules may have medical applications that have only begun to be explored.

[Page 469]When a pathogen breaches the skin and establishes a focus of infection, the white blood cells that initially detect this infection become activated and release cytokines. These cytokines act on nearby blood vessels and attract other white blood cells to attack the infection, thus leading to an inflammatory response. The cytokines that are released in response to an infectious agent determine the course of the immune response—for instance, whether it will be a cell-mediated or antibody-mediated response. Because different kinds of immune responses are required to effectively combat various disease agents, a proper cytokine profile is crucial in ensuring an appropriate response to a pathogen or tumor.

A cytokine can act on any cells that have a receptor for it—this can be the same cell that secretes it, nearby cells, or even distant cells throughout the body. In most cases, cytokine receptors are found on white blood cells, although some cytokine receptors are found on other cell types, such as blood cell precursors and the endothelial cells lining blood vessels.

Interestingly, a single cytokine can have different effects on various cell types. Cytokine networks can be extremely complex because various cytokines can have similar effects, and each of these chemical messengers can promote or interfere with the actions of other cytokines. This cross-talk and redundancy causes it to be difficult to discern the biological roles of individual cytokines.