Interleukin 6 production in the body, illustration. Interleukins are signal molecules and proteins that are secreted by the body and play a key role in the immune system. Here, interleukin 6 (IL-6) is being secreted from muscle cells (left) and circulating in the blood (centre) to reach the brain (upper right), the liver (centre right) and adipose (fat) tissue (lower right). IL-6 is a pro-inflammatory cytokine and is also an anti-inflammatory myokine (produced from muscle tissue). IL-6 can cross the blood-brain barrier where it helps influence the body's temperature regulating mechanism. IL-6 plays a role in liver damage and regeneration. In fatty tissue, IL-6 stimulates energy production. For this illustration without labels, see image C047/4520.