- Cytokines are small secreted proteins released by cells which have a specific effect on the interactions and communications between cells.
- These are polypeptides secreted by leukocytes and other cells that act on hematopoietic cells which affected modulation of immune and inflammatory responses.
- Complex reactions involved in the immune response include multiple processes of cell maturation and differentiation maintained by the broad networks of intercellular communications.
- A great assemblage of highly variable regulatory molecules promotes closest interrelationships between immune cells. These mediators were named cytokines.
Cytokines possess a number of common features.
- There are low molecular weight secreted glycoproteins, usually of 15-25 kDa of molecular mass.
- Cytokine action is transient and usually of short-range. Unlike endocrine hormones, they normally display autocrine (self-directed effect) and paracrine action (affect the nearest cells).
- Cytokines are highly potent, acting at piko- or femtomolar concentrations.
- Every cytokine binds to its specific high-affinity cell surface receptor. This event transmits the signal into the cell thus regulating cellular mRNA transcription and protein synthesis.
- Cytokines often have multiple or pleiotropic effects, affecting a great variety of cell types; the action of different cytokines reveal considerable overlapping and redundancy. They mediate cell growth, inflammation, immunity, cell differentiation and cell reparation.
Cytokines are classified into the following sets.
- Interleukins maintain interrelationships among the immune cells as well as interlinks between immune and non-immune systems (central nervous system, endocrine, digestive, respiratory systems, etc.).
- Interferon family comprises various types of interferons with pleiotropic antiviral effects, immune modulation and complex cell regulatory actions.
- Tumor necrosis factor (TNF) group consists of 2 forms of cytokines: TNF-α and TNF-β (or lymphotoxin).
- The group of colony-stimulating factors comprises the set of related cytokines such as GM-CSF (granulocyte-macrophage-colony stimulating factor), G-CSF (granulocyte-colony stimulating factor) and some others.
- The group of growth factor cytokines comprises transforming growth factors (TGF), endothelial growth factor (EGF), platelet-derived growth factor (PDGF), etc.
- Chemokines comprise the numerous families of cytokines of various origins that regulate cellular directed movement along the chemokine concentration gradient. Chemokines can also be members of other cytokine groups, e.g. IL-8.