© 2014 Elsevier Inc. All rights reserved. Cytokines are small cell-signaling proteins that are secreted by numerous cell types. Cytokines have very prominent roles in regulating both the innate and adaptive immune systems, and in the process of inflammation. Cytokines function by acting on the cells that secrete them (autocrine) or on nearby cells (paracrine). Importantly, most cytokines function in their local microenvironment, rather than in a systemic manner. Cytokines bind to specific cell surface receptors, thereby activating intracellular signaling cascades that involve phosphorylation events that ultimately result in gene transcription. Because of their involvement in immune and inflammatory responses, cytokines hold promise as therapeutic targets.