The term "cytokine" encompasses a wide range of low-weight molecular proteins that play different roles in regulating various aspects of the immune response such as its duration and intensity.
Examples of cytokines include the interleukins, the interferons, the mesenchymal growth factors, the chemokine family, the tumour necrosis factor family and the adipokines. However, there is no unified cytokine classification system and cytokines are identified in various ways which include:
By way of the numeric order of discovery. Currently 35 interleukins have been identified and are numbered 1 to 35 according to the order of their discovery.
Depending on their functional activity. Tumor necrosis factor (TNF), for example, causes apoptosis while granulocyte colony-stimulating factor (GCSF) stimulates the bone marrow to produce and release white blood cells into the blood.
According to their kinetic or functional role in inflammation which may be proinflammatory or anti-inflammatory, early or late, innate or adaptive, for example.
Depending on the type of cell the cytokine originated from. The cytokine may be described as a monokine if the primary cell of origin is a monocyte for example, or a lymphokine if it was a lymphocyte.
More recently, classification has been based on structural similarities observed between related cytokines. The chemokines can be divided into superfamilies that share similar sequences and show homology in their receptor systems, although they do not share similar functions. These members of the super family also contain regulatory cell membrane receptor-ligand pairs, indicating that similar structural motifs are employed in varying immune functions. The TNF receptor superfamily includes cytokines such as lymphotoxins, TNF- α and cellular ligands such as FasL (CD95) and CD40L. The latter mediates B cell and T cell activation while FasL promotes apoptosis or programmed cell death.