The Interleukin-6 (IL-6) test is a quantitative measurement of the cytokine IL-6 level in blood serum or plasma. IL-6 is one of the most important inflammatory mediators produced by immune system cells (such as macrophages and T cells) in response to infection, damaged tissue, or inflammatory diseases. It is a potent inducer of the acute inflammatory response and plays a role in immune regulation, hematopoiesis, and bone metabolism.