Cytokines (Greek ''cyto-'', cell; and ''-kinos'', movement) are any of a number of substances that are secreted by specific cells of the immune system which carry signals locally between cells, and thus have an effect on other cells. They are a category of signaling molecules that are used extensively in cellular communication. They are proteins, peptides, or glycoproteins. The term cytokine encompasses a large and diverse family of polypeptide regulators that are produced widely throughout the body by cells of diverse embryological origin.
Basically, the term "cytokine" has been used to refer to the immunomodulating agents (interleukins, interferons, etc.). Conflicting data exists about what is termed a cytokine and what is termed a hormone. Anatomic and structural distinctions between cytokines and classic hormones are fading as we learn more about each. Classic protein hormones circulate in nanomolar (10) concentrations that usually vary by less than one order of magnitude. In contrast, some cytokines (such as IL-6) circulate in picomolar (10) concentrations that can increase up to 1,000-fold during trauma or infection. The widespread distribution of cellular sources for cytokines may be a feature that differentiates them from hormones. Virtually all nucleated cells, but especially endo/epithelial cells and resident macrophages (many near the interface with the external environment) are potent producers of IL-1, IL-6, and TNF-α. In contrast, classic hormones, such as insulin, are secreted from discrete glands (e.g., the pancreas). As of 2008, the current terminology refers to cytokines as immunomodulating agents. However, more research is needed in this area of defining cytokines and hormones.
The action of cytokines may be autocrine or paracrine, but not endocrine. The reason for them not being endocrine signals is that the signal must be released in the general region of the pathogen-infected cells, so other immune molecules which follow the signal will arrive at that site (where this signal is released). Cytokines are critical to the development and functioning of both the innate and adaptive immune response, although they are not limited to the immune system. They are often secreted by immune cells that have encountered a pathogen, thereby activating and recruiting further immune cells to increase the system's response to the pathogen. Cytokines are also involved in several developmental processes during embryogenesis.
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