The defense of the organism against pathogens involves complex processes in which many different types of cells take part. Their functions are coordinated by polypeptide mediators collectively named cytokines. The intense research devoted to these molecules in the past has yielded an insight into their molecular characteristics, mode of action, physiological function, and their deleterious role in numerous diseases. Yet therapeutic application of that knowledge--either controlled administration of cytokines or neutralization of their negative effects in certain pathological processes--despite initial achievements has remained a major challenge.