Interleukin 6 (IL-6), originally identified as a B cell differentiation factor, is a multifunctional cytokine, which regulates immune responses, hematopoiesis, acute phase responses, and inflammatory reactions. It induces the terminal maturation of activated B cells into antibody-secreting plasma cells and acts in synergy with IL-3 to support the proliferation of hematopoietic stem cells. IL-6 is produced by many cell types, such as monocytes, fibroblasts, endothelial cells, eosinophils, and T cells. Disturbed IL-6 production has been associated with pathological processes, including inflammatory autoimmune diseases and cancer.