Cytokines are a highly diverse group of intracellular messages. Many cDNAs that encode the cytokines have been cloned in 1980's and the structure of the molecules have been determined. The main function of the cytokines is to amplify the immune and inflammatory responses, keeping them under control at the same time. They orchestrate the response and maintain a proper balance among the various cell types. In this chapter, we summarized the immunoregulation by cytokine network, focussing on the control of antibody production and immunoglobulin class switch by various cytokines produced by helper T cell subsets.