Interleukin-6 (IL-6), a 26 kDa glycoprotein composing of 184 amino acids, plays a pivotal role in inflammation, immune responses, hematopoiesis, and oncogenesis. While its typical circulating level in healthy adults is less than 5 pg/mL, they escalate rapidly during inflammation. IL-6 is expressed in diverse cell types, including adipocytes, fibroblasts, osteoblasts, neurons, and immune cells. Elevated IL-6 levels are associated with conditions like cardiovascular disease, insulin resistance, type 2 diabetes, asthma, rheumatoid arthritis, and advanced-stage cancer.
As a signaling molecule, IL-6 regulates cell growth and differentiation, particularly in immune responses and acute phase reactions. Secreted by various cell types, including T cells and macrophages, IL-6 exerts its actions through a heterodimeric receptor composed of IL-6R and glycoprotein 130 (gp130). This receptor complex transduces signals, contributing to immune responses, hematopoiesis, bone metabolism, and cancer progression.
IL-6 is not constitutively produced but is induced by factors like cytokines, lipopolysaccharide, or viral infections. Its broad tissue effects vary from inducing growth and inhibition to differentiation, depending on the target cells. Abnormal IL-6 production is implicated in various diseases, and measuring its levels in serum provides diagnostic insights. The multifaceted nature of IL-6 and its involvement in diverse physiological processes make it a central figure in immunology, with significant implications for both research and clinical applications.