Cytokines

Definition

Cytokines are low-molecular-weight proteins or glycoproteins of usually less than 30–40 kDa. They are mainly released from leukocytes, to a lesser degree from other cell types, in response to exogenous stimuli. At picomolecular concentrations, they are high-affinity ligands for specific membrane receptors on other cells (occasionally their own cells) and alter or regulate their function.

Characteristics

Originally discovered as biologically active factors in the supernatants of in vitro cultured leukocytes, they were defined and received their name because of their specific activities on other leukocytes. Usually they are soluble components (some also exists in membrane-bound forms) that do not transfer antigen specificity, as soluble antibodies do, but efficiently alter cell functions. Preferentially they exert paracrine activities (i.e., modulating functions of neighboring cells), but some also exhibit autocrine (action on producing cell itself) and endocrine (action at...