From Greek "κύτος" (kytos: "cavity, cell") and "κίνησις" (kinēsis: "movement")kytos, meaning cavity or cell, and kinesis, meaning movement, cytokines are small proteins (~5–20 kDa) involved in cell signaling. They are produced by many types of cells such as macrophages, B-cells, T-cells, mast cells, endothelial cells, fibroblasts, and various stromal cells, and a given cytokine may be produced by more than one type of cell.
They interact with cells via membrane receptors, and are especially important in the immune system and the regulation of basic cellular processes like cell maturation, cell growth, and responsiveness of particular cell populations.