IL-6 (Interleukin-6) is a pleiotropic cytokine that acts in the acute phase reaction, inflammation, hematopoiesis, bone metabolism, and cancer progression. It contributes to chronic inflammation in obesity, inflammatory bowel disease, arthritis, sepsis, and atherosclerosis. IL-6 signals through a receptor complex of IL-6 R alpha and gp130. gp130 is also a component of the receptors for CLC, CNTF, CT-1, IL-11, IL-27, LIF, and OSM. Soluble forms of IL-6 R alpha are generated by both alternative splicing and proteolytic cleavage. In a mechanism known as trans-signaling, complexes of soluble IL-6 and IL-6 R alpha elicit responses from gp130-expressing cells that lack cell surface IL-6 R alpha.