How does stress weaken the immune system?

Grayson, Sheldon Cohen, Ph.D., professor of psychology at the Carnegie Mellon University, gave 400 people a questionnaire designed to quantify the amount of stress they were under. He then exposed them to nose drops containing cold viruses. Ninety percent of the stressed subjects caught a cold. They had elevated levels of corticotrophin releasing factor, which interferes with the immune system.