The immune system chemical in question is interleukin-6, or IL-6. Immune cells release IL-6 and other so-called cytokines when the body is battling the flu or an infection, says University of Pittsburgh researcher Anna Marsland. Those cytokines tell the brain to slow down, and that helps the body heal. But as people enter midlife, they start to develop chronic disorders such as heart disease that keep cytokines elevated all the time. Marsland's research suggests that a 24/7 elevation in these cytokines can lead to sluggish performance on thinking and memory tests.