Chronic stress impacts the immune system by increasing white blood cell counts and worsening inflammation in atherosclerotic plaque in arteries, reports a study published this week in Nature Medicine. These findings help explain how stress increases the risk of heart attack in people.

Matthias Nahrendorf and his colleagues looked at how exposure to chronic stress in 29 medical residents working in a hospital’s intensive care unit affected their white blood counts after one week. They report that there were increased white blood cell counts.

Next, the authors looked at how chronic stress in healthy mice, induced by social or environmental factors, such as isolating the mice or tilting their cages, activated nerve fibers in their bone marrow, leading to stem cell proliferation and increased white blood cell production.

The increase in white blood cells seems to induce atherosclerotic plaque inflammation in the arteries of atherosclerosis-prone mice. Atherosclerotic plaques in stressed mice resembled the ‘vulnerable lesions’ found in humans that are prone to rupture and cause heart attacks.