Human Interleukin-6 (IL-6) is a multifunctional pro-inflammatory cytokine and a central member of the IL-6 cytokine family—including IL-6, IL-11, IL-27, IL-31, leukemia inhibitory factor (LIF), oncostatin M (OSM), ciliary neurotrophic factor (CNTF), cardiotrophin-1 (CT-1), and cardiotrophin-like cytokine factor 1 (CLCF1)—all of which signal through receptor complexes containing the shared gp130 subunit to activate JAK/STAT and MAPK pathways. In humans, IL-6 is produced by macrophages, monocytes, endothelial cells, fibroblasts, and other cell types in response to infection, tissue injury, and immune stimulation, where it regulates acute-phase protein synthesis (including C-reactive protein and fibrinogen), B-cell maturation, T-cell differentiation (particularly Th17 development), hematopoiesis, and fever. Dysregulated IL-6 signaling is implicated in a wide range of pathological conditions, including rheumatoid arthritis, inflammatory bowel disease, systemic lupus erythematosus, cytokine release syndrome, sepsis, cardiovascular disease, metabolic syndrome, and numerous cancers such as multiple myeloma and lymphoma. Elevated IL-6 levels are closely associated with disease severity and poor clinical outcomes in inflammatory and infectious diseases, making it a critical biomarker and therapeutic target, as evidenced by the clinical use of IL-6 and IL-6 receptor inhibitors. In biomedical research and clinical medicine, human IL-6 and its related family members are central to studies of immune regulation, chronic inflammation, oncology, and targeted immunotherapy development.