Cytokines are soluble mediators of intercellular communication. They contribute to a chemical signalling language that regulates development, tissue repair, haemopoiesis, inflammation and the immune response. Potent cytokine polypepides have pleiotropic activities and functional redundancy. They act in a complex network where one cytokine can influence the production of, and response to, many other cytokines. In the past five years, this bewildering array of more than 100 effector molecules and associated cell surface receptors has been simplified by study of cytokine and cytokine receptor...