<P> Cytokines (cyto, from Greek "κύτος" kytos "cavity, cell" + kines, from Greek "κίνηση" kinisi "movement") are a broad and loose category of small proteins (~ 5--20 kDa) that are important in cell signaling . Their release has an effect on the behavior of cells around them . It can be said that cytokines are involved in autocrine signalling, paracrine signalling and endocrine signalling as immunomodulating agents . Their definite distinction from hormones is still part of ongoing research . Cytokines include chemokines, interferons, interleukins, lymphokines, and tumour necrosis factors but generally not hormones or growth factors (despite some overlap in the terminology). Cytokines are produced by a broad range of cells, including immune cells like macrophages, B lymphocytes, T lymphocytes and 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 . </P> <P> 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 . </P>

What are the main sources of immune system cytokines