IL-6 is a cytokine that can be used as a marker of immune system activation. Released from macrophages, levels may be raised with inflammation, infection, autoimmune disease, cardiovascular disease, stroke, diabetes and some cancers. High levels may also be seen after recent exercise (see below)
As part of the NFkB inflammatory signalling pathway, IL-6 is rapidly and transiently released in response to infection or injury and stimulates the synthesis of acute phase proteins and the production of neutrophils. In addition, it downregulates regulatory T cells and activates the growth of B cells.
In contrast, it also has an anti-inflammatory role when released from muscle cells during exercise. The resulting signalling cascade is different from the inflammatory macrophage pathway.