Interleukin (IL)-6 cDNA was originally cloned as a terminal B cell differentiation factor into antibody-producing plasma cells. This revealed that it is a multifunctional cytokine that acts on a variety of cells. From the clinical viewpoint, it is especially important that IL-6 acts on hepatocytes to induce acute-phase reactants, including C-reactive protein, serum amyloid A protein, and fibrinogen, and to decrease serum albumin levels. Very recently, this cytokine has been found to enhance the synthesis of a peptide called hepcidin in the liver which regulates iron recycling, resulting in anemia due to hypofferemia. It has also been shown that IL-6 is responsible for various clinical symptoms, including the appearance of autoantibodies, fatigue, anemia, anorexia, fever, and increases in the erythrocyte sedimentation rate, all of which develop in patients with various chronic autoimmune inflammatory diseases. In practice, blocking the IL-6 signaling pathway with a recombinant humanized anti-IL-6 receptor antibody, tocilizumab (TCZ), has dramatically improved all the signs and symptoms of these patients. A study in mice demonstrated that IL-6 promotes the development of a new type of T-helper cells called Th17 cells that impact the pathogenesis of autoimmune diseases. This suggests that TCZ is not only an antiinflammatory agent but also might affect basic autoimmunity. In this review, recent advances in the immunobiology of interleukin-6 related to immune-mediated diseases are discussed.