Cytokines are a broad and loose category of small proteins that are important in cell signaling. They are released by cells and affect the behavior of other cells, and sometimes the releasing cell itself.
Cytokines include chemokines, interferons, interleukins, lymphokines, tumour necrosis factor but not generally not hormones or growth factors.
Cytokines are produced by broad range of cells, including immune cells like macrophages, B lymphocytes and T lymphocytes, mast cells, as well as endothelial cells, fibroblasts, and various stromal cells; a given cytokine may be produced by more than one type of cell.
They act through receptors, and are especially important in the immune system; cytokines modulate the balance between humoral and cell-based immune responses, and they regulate the maturation, growth, and responsiveness of particular cell populations. Some cytokines enhance or inhibit the action of other cytokines in complex ways.