Cytokines are important protein mediators of immunity, inflammation, cell proliferation, differentiation, fibrosis, etc. (Oppenheim and Saklatvala, 1993). As these are the major biological processes underlying autoimmunity, it is not surprising that there is now convincing evidence that cytokines have an important role in the pathogenesis of autoimmunity (Brennan and Feldmann, 1996; Feldmann et al., 1996).

There has been much progress since we first highlighted the role of cytokines such as IFNγ in autoimmunity in the early 1980s (Bottazzo et al., 1983). The number of cytokines molecularly cloned has increased greatly, and the biochemical and structural basis of their actions are partly understood, as cytokine genes and that of their receptors have been cloned. Knowledge of cytokine signalling is rapidly expanding (see Chapter XIII). In medical terms, clear evidence of the importance of cytokines in autoimmunity is demonstrated by therapeutic advances. Thus it is possible to dramatically improve patients with rheumatoid arthritis and Crohn's disease by blocking TNFα, and a new target for therapy, TNFα, has thus been validated for both these diseases.