The term "cytokines" refers to a group of mostly soluble molecules proteinaceous signalling compounds that mediate intercellular communication. Their physiological effects are manifested in nano- to picomolar concentrations, and they function close to the secretion site. They consist of interleukins, chemokines, and other molecules (e.g. growth factors).

Cytokines serve as chemical signals between all the different immune cells in the bloodstream, and between nerve cells and immune cells. Basically all cells that are involved in an immune response in some way or other (or the ones derived from blood cell precursors) will react to cytokines which in this case function as immune modulators and growth factors.

Cytokines have been variously named as lymphokines, chemokines and interleukins, based on their presumed function, cell of secretion or target of action. Cytokines display considerable redundancy and pleotropism though, so such distinctions (with some notable exceptions) are now obsolete.

Cytokines can be detected using the ELISA technique. Individual cytokine-secreting cells can be enumerated using the ELISPOT method.