The effects of chronic stress on cardiovascular health may be a side effect of having an immune system that can fight infection, U.S. researchers said. Lead author Paul Marvar, a postdoctoral fellow at Emory University School of Medicine, and colleagues subjected mice to psychological stress by confining them in a small space for 1 hour and then putting them in cages where other mice had already left their scents. Two hours of stress per day, for a week, results in a short-term rise in systolic blood pressure in normal mice, Marvar said. However, mice that were genetically engineered to lack T cells -- helpful for fighting infections -- did not display an increase in blood pressure under the same regimen. Introducing T cells into mice that lacked them made their blood pressure sensitive to stress again. Dr. David Harrison, who moved from Emory to Vanderbilt University last year, Marvar and colleagues had previously shown T cells are needed for the increase in blood pressure coming from high dietary salt or the hormone angiotensin, which regulates blood pressure. Several studies in animals have suggested medications now used to control blood pressure, such as angiotensin receptor blockers or ACE inhibitors, might also be helpful in the reduction of stress and anxiety, Marvar said. \"Further understanding the mechanisms underlying these observations and determining whether they may benefit people with anxiety disorders, for example post-traumatic stress disorder, is a current goal of my research,\" Marvar said. The findings were published in the journal Biological Psychiatry.