Over the last decade, research has suggested that cells communicate with each other through discrete chemical molecules called cytokines. Cytokines have been found to be involved in a wide variety of diseases, including cancer, allergy, infection and inflammation. They also play a part in the development of blood vessels, embryo formation and differentiation. This is an introductory guide to the genetics, chemistry and biology of cytokines. It describes how they regulate cell growth and moderate immunological resonse. It covers various types of cytokines, including interferon and interleukin-2.