Cytokines (/ˈsaɪtəkaɪn/) are a broad and loose category of small proteins (~5–25 kDa) important in cell signaling. Cytokines are produced by a broad range of cells, including immune cells, as well as endothelial cells, fibroblasts, and various types of connective tissue cells. A single cytokine may be produced by more than one type of cell.
Cytokines are usually too large to cross cell membranes and enter cells. They typically function by interacting with specific cytokine receptors on the surface of target cells. Cytokines include chemokines, interferons, interleukins, lymphokines, and tumour necrosis factors, but generally not hormones or growth factors (despite some overlap in the terminology).