Cytokines are a group of proteinaceous signalling compounds that, like hormones and neurotransmitters, are used extensively for inter-cell communication.

While hormones are secreted from specific organs to the blood and neurotransmitters are related to neural acitivity, the cytokines are a more diverse class of compounds in terms of origin and purpose. They are critical to the functioning of both innate and adaptive immune responses. Apart from their importance in the development and functioning of the immune system, cytokines play a major role in a variety of immunological, inflammatory and infectious diseases. They are also involved in several developmental processes during embryogenesis.

Cytokines are produced by a wide variety of cell types (both haemopoietic and non- haemopoietic) and can have effects on both nearby cells or throughout the organism, sometimes strongly dependent on the presence of other chemicals and cytokines. The cytokine family consists mainly of smaller water-soluble proteins and glycoproteins (proteins with an added sugar chain) with a mass of 8-30 kDa.

(source: Wikipedia)