Cytokines are usually extracellular signalling proteins, usually less than 80 kD in size, and many are glycosylated. They are produced by many different cell types that are involved in cell-to-cell interactions acting through specific receptors on the surface of target cells. Cytokines usually have an effect on closely adjacent cells and therefore function in a predominantly paracrine fashion, although they may also act at a distance (endocrine) and may have effects on the cell of origin (autocrine). Cytokines may be regarded as a mechanism for cell-cell communication, and within this group may be included growth factors and cytokines with primarily chemoattractant properties (chemokines). They act on target cells to cause a wide array of cellular functions including activation, proliferation, chemotaxis, immunomodulation, release of other cytokines or mediators, growth and cell differentiation, and apoptosis. Cytokines were originally characterised (and named) according to some aspect of their functional activity that was initially discovered, but the cloning of the genes for these cytokines has now provided a better insight into their classification and grouping. It is apparent that there is a wide pleiotropy and element of redundancy in the cytokine family in that each cytokine has many overlapping functions, with each function potentially mediated by more than one cytokine.

The effect of an individual cytokine in the context of disease may not be easy to predict because it may be influenced by other cytokines released simultaneously from the same cell or from target cells following activation by the cytokine. The effects of cytokines are mediated by binding to cell surface high affinity receptors usually present in low numbers. The number of these receptors can be upregulated with cell activation, and there the effect of a cytokine may depend on the modulation of its receptors. Cytokines themselves may induce the expression of receptors which may …