A large class of glycoproteins similar to lymphokines but produced by non-lymphocytic cells such as normal macrophages, fibroblasts, keratinocytes and a variety of transformed cell lines. The most well known cytokines are the interferon (IFN) and interleukins (IL-1, IL-6). They were discovered through their role in regulating immunological and inflammatory and repair processes. However, they also regulate normal cell growth and differentiation.
Cytokines are analagous to hormones in their intercellular regulatory functions at very low concentrations, but are not produced by glands. [see Endocrinology]
Hence, some discovered by their action, rather than their chemical structure, have been named Growth_Factors, and yet these function throughout life, eg. to stimulate the endothelial cells to express 'selectins' on the internal surfaces of blood vessels, causing leukocytes to adhere, then be removed from circulation between endothelial cells.