Soluble Mediators of Cellular Cooperation: The Cytokines
Cytokines are protein or glycoprotein factors secreted by cells of both the monocytoid (monokines) and lymphoid (lymphokines) lineages. These factors act on leukocytes and many other cell types; they influence cell division, differentiation, metabolism, motility, and expression of immune (and other) effector functions. Cytokines have evolved along with the immune system. Some are found only in highly adaptive vertebrate immune systems, while others have homologues in primitive creatures such as starfish. The cytokines are the “messengers” of the immune system, as are the hormones of the endocrine system. Indeed, these two systems are more closely related than previously thought (see below). As do endocrine hormones, cytokines exert their biological effects through highly specific receptors. One distinction between cytokines and endocrine hormones is the scope of their activity. While the target tissues of endocrine hormones are often far removed, cytokines act predominantly over very short distances from the point of secretion. Many cytokines exert a paracrine effect on neighboring cells, or even an autocrine effect on the very cell which secretes it. However, many cytokines also have inhibitory effects on intracellular signalling, and may exert negative feedback on their own synthesis, on the production of many other cytokines and immune effector molecules, and on cellular effector mechanisms.