Scientists say they may have unravelled how chronic stress leads to heart attack and stroke: triggering overproduction of disease-fighting white blood cells that can be harmful in excess.
Surplus cells clump together on the inner walls of arteries, restricting blood flow and encouraging the formation of clots that block circulation or break off and travel to another part of the body.
White blood cells "are important to fight infection and healing, but if you have too many of them, or they are in the wrong place, they can be harmful", said Matthias Nahrendorf of the Harvard Medical School in Boston, co-author of the study.
Doctors have long known that chronic stress leads to cardiovascular disease, but have not understood the mechanism.
To find the link, Nahrendorf and a team studied 29 doctors working in a hospital intensive care unit.
Their work environment is considered a model for chronic stress exposure.
Comparing blood samples taken during work hours and off duty, as well as the results of stress questionnaires, the researchers found a link between stress and the immune system.
In particular, they noticed stress activated bone marrow stem cells, which in turn triggered overproduction of white blood cells.
White blood cells can turn against their host, with devastating consequences for people with diseases like atherosclerosis - a thickening of artery walls caused by a plaque build-up.
The study then moved on to mice, which were exposed to the rodent equivalent of stress through techniques like crowding and cage tilting.
They found that excess white blood cells produced as a result of stress accumulated inside arteries and boosted plaque growth.