Encyclopedia of Cancer (2015)
Cytokine is any of a variety of secreted polypeptides that control the development, differentiation, and proliferation of hematopoietic cells. The effects of cytokines on lymphocytes are usually mediated through membrane-bound cytokine receptors and are especially critical during immune responses.
A group of proteins mainly functioning as soluble signal transmitters in the immune system are primarily released by immune cells, but many other cell types can also release cytokines. Cytokines direct and mediate various functions of the adaptive and innate immunity and modulate functions of immune cells and other responsive cell types. Their local of systemic effects, elicited by binding to specific cell surface receptors, contribute mainly to innate and adaptive immune responses, so that their production is involved in a range of infectious, immunological, and inflammatory diseases. To this class belong interleukins, interferons, TNF-like molecules, etc. These molecular signals are similar to hormones and neurotransmitters and are used to allow one cell to communicate with another.