Withdraw cash <a href=" http://www.kade.ie/Table/Resources/ "></a> The immune system's role might also fit into the "second-hit" idea of mental illness, where two or more factors, such as genetics, immune challenges and, say, a hit to the head, combine to cause brain problems. That may explain why traumatic brain injuries often lead to depression, says Jonathan Godbout, a neuroscientist and associate professor at the Institute for Behavioral Medicine Research at The Ohio State University Wexner Medical Center.