Cytokines are Chemical Messengers, a diverse and potent group of proteins and peptides that are signalling compounds, produced by immune cells to communicate with one another. They act via cell-surface cytokine receptors, and are the chief communication signals of T cells.

Cytokines include interleukins, growth factors, and interferons.
are naturally occurring cytokines that may boost the immune system's ability to recognize cancer as a foreign invader.
When cytokines attract specific cell types to an area, they are called chemokines. These are released at the site of injury or infection and call other immune cells to the region to help repair damage and defend against infection.

Cytokines secreted by lymphocytes, (both T-cells and B-cells) are called lymphokines.
Cytokines secreted by phagocytes (monocytes and macrophages) are called monokines
Cytokines with chemotactic activities ( able to stimulate chemotaxis- the movement of cells through chemical stimulation )are called chemokines.
Cytokines made by one leukocyte and acting on other leukocytes are called interleukins, serving as a messenger between white cells, (or leukocytes.)
Cytokines may act on the cells that secrete them (autocrine action), on nearby cells (paracrine action), or in some instances on distant cells (endocrine action).
Cytokines are often produced in a cascade, as one cytokine stimulates its target cells to make additional cytokines.
Cytokines can also act synergistically (two or more cytokines acting together) or antagonistically (cytokines causing opposing activities).
Cytokines, binding to specific receptors on target cells, recruit many other cells and substances to the field of action.
Cytokines encourage cell growth, promote cell activation, direct cellular traffic, and destroy target cells -- including cancer cells.
The cytokine family consists mainly of smaller water-soluble proteins and glycoproteins (proteins with an added sugar chain.) They act like hormones and neurotransmitters, but whereas hormones are released from specific organs into the blood, and neurotransmitters are produced by neurons, cytokines are released by many types of cells.
Due to their central role in the immune system, cytokines are involved in a variety of immunological, inflammatory, and infectious diseases. However, not all their functions are limited to the immune system, as they are also involved in several developmental processes during embryogenesis. (The development and growth of an embryo, especially the period from the second week through the eighth week following conception)
When the immune system is fighting pathogens, cytokines signal immune cells such as T-cells, and macrophages to travel to the site of infection. In addition, cytokines activate those cells, stimulating them to produce more cytokines.

Cytokines have been recently divided into two groups according to the population of cells whose functions they promote:
ie:- The proliferation and functioning of helper T-cells, type 1 and type 2.
A key focus of interest has been that the second category (type2) of cytokines, tend to inhibit the effects of those in the other. This tendency is under intensive study for its possible role in the pathogenesis of autoimmune disease.(ie;in prevention of exaggeration of pro-inflammatory immune responses.