Cytokines can be described as small messenger and signaling molecules released by cells that have a specific effect on the interactions between cells, on the communication between cells and on the behavior of cells.
The term ‘cytokine’ encompasses a large and diverse family of messenger and signaling molecules produced throughout the body by cells of diverse embryological origin. There is no general agreement as to which molecules should be termed cytokine, and part of this difficulty in distinguishing cytokines from other molecules, is that some of the immunomodulating effects of cytokines are systemic, meaning that they act on the whole body system rather than on a localized system or organ.
Cytokines were officially recognized in 1979 and this discovery created a revolution in immunology and medicine in general. Since then, an enormous amount of research has been devoted to cytokines.
Cytokines are critical to the development and functioning of both the innate and adaptive immune response, they are often secreted by immune cells that have encountered a pathogen, thereby activating and recruiting further immune cells to increase the system’s response to the pathogen.
Although pivotal in triggering and regulating the immune response, they are not limited to the immune system alone, in fact many cytokines are now known to be produced by cells other than immune cells and they can have effects on non-immune cells as well.
Cytokines are also involved in several developmental processes during embryo genesis. The complex network of cytokines balances pro-inflammatory and anti-inflammatory effects, and an imbalance between pro- and anti-inflammatory cytokines or the uncontrolled production of cytokines can result in chronic inflammatory disease, allergies or auto-immune disease.
Broadly speaking, cytokines can include different types of molecules like ‘monokines’ produced by mononuclear phagocytic cells, ‘chemokines’ produced by many kinds of leukocytes and other cell types, ‘lymphokines’ produced by activated lymphocytes (especially T helper cells), ‘interleukins’ that act as mediators between leukocytes, ‘peptides’ (cell signaling molecules), ‘growth factors’ which promote cell growth and ‘interferons’ (INF) which respond to infected cells and cancer cells.
We could also divide cytokines according to their biological role such as growth factors which promote cell growth proliferation and differentiation, interleukins and lymphokines which are capable of creating a communicating network within the immune system, and chemokines and lymphokines which are mainly involved in inflammation.