Interleukin 6 (IL-6) is a cytokine that has a pro-inflammatory effect on the immune system. Cytokines are a broad and loose category of small proteins (\~5-20 kDa) that are important in cell signaling - they are released by cells and affect the behavior of other cells, and sometimes the releasing cell itself. IL-6 is an important mediator of fever and of the acute phase response. IL-6 is responsible for stimulating acute phase protein synthesis as well as the production of neutrophils in the bone marrow. The acute-phase response is the detectable change in acute phase proteins, a class of proteins whose plasma concentrations increase or decrease in response to inflammation. IL-6 is secreted by T cells and macrophages to stimulate the immune response during infection and after trauma, especially burns or other tissue damage leading to inflammation. Smooth muscle cells in the tunica media of many blood vessels also produce IL-6 as a pro-inflammatory cytokine. IL-6 is capable of crossing the blood-brain barrier and triggering production of Prostaglandin E2 in the hypothalamus, thereby changing the body's temperature set point. In muscle and fatty tissue, IL-6 stimulates energy mobilization that leads to increased body temperature. IL-6 can be secreted by macrophages in response to specific microbial molecules, referred to as pathogen-associated molecular patterns (PAMPs). IL-6 is also produced by adipocytes and is thought to be a reason why obese individuals have higher endogenous levels of CRP. IL-6 signals through a cell-surface type I cytokine receptor complex consisting of the ligand-binding IL-6Rα chain (CD126) and the signal-transducing component gp130 (also called CD130). CD130 is the common signal transducer for several cytokines but the expression of CD126 is restricted to certain tissues. As IL-6 interacts with its receptor, it triggers the gp130 and IL-6R proteins to form a complex, thus activating the receptor. These complexes bring together the intracellular regions of gp130 to initiate a signal transduction cascade through certain transcription factors.

Acute Myocardial Infarction (MI) occurs when myocardial ischemia, a diminished blood supply to the heart muscle, exceeds a critical threshold and overwhelms myocardial cellular repair mechanisms designed to maintain normal function. Ischemia at this critical threshold level for an extended period results in irreversible myocardial cell damage or death. A common clinical diagnostic classification scheme is based on electrocardiographic findings as a means of distinguishing between two types of acute MI, one that is marked by ST elevation (STEMI) and one that is not (NSTEMI). In acute MI IL-6 levels rapidly increase in response to ischemia and inflammation. In one study, plasma IL-6 levels were increased at all sampling points from admission to discharge in patients with acute MI as compared with IL-6 levels in controls. Cardiac catheterization did not influence plasma IL-6 levels. In another study, patients with acute MI demonstrated a peak in IL-6 levels on days 1 and 2 which then declined rapidly to lower, although not normalized, levels during hospitalization and at 6 and 12 weeks. It has also been demonstrated that elevated levels of IL-6 are associated with worse outcomes in acute MI. In one study elevated IL-6 levels at day 1 and day 30 were independent predictors of adverse events. In another study, on univariante analyses, baseline IL-6 was related to death but not recurrent non-fatal acute coronary syndromes. Another study demonstrated significant correlations between increased IL-6 levels and impaired left ventricle systolic and diastolic function supportive of a role of IL-6 in post-infarction cardiac damage. This same group also demonstrated that an increased level of IL-6 in acute MI was an independent predictor of left ventricle systolic and diastolic dysfunction 6 months after MI.