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During the last 30 years there has been a growing interest in cytokines as biological molecules able to regulate the most diverse functions in living org- isms, mainly at the level of cell-cell communication. Originally their definition was limited to the cells of the immune system (interleukins and lymphokines), but later that definition was extended to all cells, and their regulatory activity in such other processes as differentiation, apoptosis, angiogenesis, and wound he- ing has been now demonstrated. They comprise a group of small proteins (5-20 kDa) produced and released by cells in a tightly controlled fashion, active in the nano- or picomolar concentration range, and eliciting specific effects in nei- boring cells; therefore, their action is said to be autocrine, paracrine, or jux- crine. The latter property distinguishes them from hormones, which are produced by one tissue and are transported by the blood stream in order to act on a distant tissue. Chemokines are a subset of cytokines, but whether growth factors are included in the group is often a matter of discussion. The activity of several cytokines can be inhibited by other cytokines or by biological response modi- ers; therefore, the latter are sometimes called 'anti-cytokines. ' The biological response of a particular cell is usually the result of the sum of all interactions with cytokines present at a certain time and in a certain sequence in time-the 'cytokine network.
