Interleukin-6 (IL-6) is a protein produced by various cells. It may help modulate the immune response, making IL-6 testing useful as a marker of immune system activation. IL-6 can be elevated in inflammation, infections, autoimmune diseases, cardiovascular disease, and some cancers. This test measures the amount of IL-6 in your blood. IL-6 acts on various cells and tissues. It promotes the differentiation of B cells (white blood cells that produce antibodies), promotes cell proliferation in some cells, and inhibits proliferation in others. Stimulates the production of acute phase proteins. IL-6 also plays a role in regulating body temperature, bone maintenance, and brain function. It is primarily proinflammatory, but it also has anti-inflammatory properties.