US researchers say they've uncovered the reason why you're more likely to catch a cold when you're stressed.
The researchers, led by Sheldon Cohen a professor of psychology from Carnegie Mellon University suggest that long-term stress may affect the body's ability to regulate inflammation.
The immune system is partly regulated by cortisol, a hormone that binds to white blood cells and suppresses inflammation.
But Cohen and colleagues suggest that white blood cells become less sensitive to high levels of cortisol — such as those triggered in response to long-term stress. This phenomenon, known as glucocorticoid receptor resistance (GCR), impairs the hormone's ability to regulate the white blood cells.
"Without sufficient glucocorticoid regulation, the duration and/or intensity of the inflammatory response increases, heightening risk for acute exacerbations," they write in the Proceedings of the National Academy of Science .
"Because inflammation plays a role in progression of multiple diseases, this model not only provides an explanation for the increased risk of upper respiratory infections under stress, but might provide a more general explanation for why prolonged stress would play a role in other inflammatory diseases as well."
Their theory is based on the reanalysis of two studies they conducted in the late 1990s.
In the first study, the researchers screened 276 healthy adults for stressful life events and other general indicators of health such as age and body mass index. They also took blood samples to measure levels of cortisol, and the presence of antibodies to two strains of cold virus.
They then quarantined the study participants for 24 hours before infecting them with two strains of cold virus. The participants were quarantined for five days during which time they were monitored for signs of infection.
The researchers found that people who had been exposed to a recent major life stress event had an impaired ability to regulate inflammation.
"For those people who were experiencing a long-term threatening stressful event, cortisol had no influence on the numbers of white blood cells. That is, in stressed people cortisol was unable to signal the cells what to do," says Cohen.
In turn, people with GCR were also more likely to develop colds.
The second study of 79 people found that people who were less able to regulate their immune systems produced more cytokines, chemical messengers that promote inflammation.
"The immune system's ability to regulate inflammation predicts who will develop a cold, but more importantly it provides an explanation of how stress can promote disease," says Cohen.
Professor Andrew Lloyd from the Infection and Immunity Group at the University of New South Wales says the study is a good insight into the "relatively difficult science of psychoimmunology."
"That field has got lots of studies in animals, but very few in humans in natural settings."
"[The researchers] have lined up all the dots so to speak in thoroughly exploring that sequential relationship to see if … stress is associated with a change in [the glucocortocoid response pathway]."
But while being "an important step forward" in studying the impact of stress on the human immune system, Lloyd urges caution about the findings.
"We need a really good prospective study in humans to really test the hypothesis they're pointing to," says Lloyd.
"That is to set up a new study, characterise individuals who have or have not had a recent life stress event, measure their glucocorticoid response, then watch them in a truly natural setting when they do get a cold and see what the pattern to the response to the cold is."
Lloyd, who has researched the role of genetic susceptibility to inflammatory diseases such as glandular fever, suspects the glucocorticoid response may be part of a larger genetic puzzle.
"There are genes that determine whether you or me will respond vigorously with these immunological hormone production or not so vigorously," he says.
"There's quite a lot of plausibility that some of those genes also relate to how we respond to a stress like a major life event. It might be what we call a higher order underpinning the whole phenomenon."