(Science: cytokine, protein) a variety of naturally occuring polypeptides that are members of the family of cytokines which affect functions of specific cell types and are found in small quantities. They are secreted regulatory proteins produced by lymphocytes, monocytes and various other cell types and are released by cells in response to antigenic and non-antigenic stimuli.
Interleukins are of the larger class of T-cell products, lymphokines which are now more frequently considered as cytokines. The interleukins, of which there are 12 identified to date, modulate inflammation and immunity by regulating growth, mobility and differentiation of lymphoid and other cells. Included among the cytokines are cachectin and lymphotoxin which are now known as tumour necrosis factor-alpha and tumour necrosis factor-beta, respectively.
interleukin-1 inhibitors are the first well-described proteins involved in the feedback regulation of interleukin activities. The origin and functions of the two most novel cytokines, interleukin-11 and interleukin-12 (also referred to as natural killer cell stimulatory factor), have only recently begun to be understood.