A study of rhesus monkeys has revealed the stress of low social status can be damaging to the immune system of the animals.
Researchers believe the findings may help explain why people with poor and deprived backgrounds have higher rates of heart disease and diabetes, both of which are linked to inflammation.
“Social adversity gets under the skin,” says Noah Snyder-Mackler, of Duke University in the US, who co-led the investigation. “If we can help people improve their social standing and reduce some of these hierarchies, we may be able to improve people’s health and wellbeing.”
In the US, life expectancy between rich and poor differs by more than a decade.
American health inequality is often attributed to the availability of medical care and lifestyle habits such as smoking, exercise and diet.
But the new results underline the important role played by stress.
The team studied female rhesus monkeys at the Yerkes National Primate Research Centre at Emory University.
First, 45 unrelated females that had never met were sorted into social groups and observed as they formed a social pecking order. Monkeys introduced to their groups earlier tended to be ranked higher than the “last in”.
To find out how rank affected health, scientists measured the activity of 9,000 genes in the animals’ immune cells.
Activity in more than 1,600 genes differed between lower and higher-ranking monkeys, which was especially true within “natural killer cells” – white blood cells that are a first line of defence against infection. The cells were more active in higher-ranking monkeys, giving them better protection against viruses.
Turning the tables
Lower-rankers produced a stronger immune response against bacteria, but this also fuelled potentially harmful inflammation. When their immune cells were exposed to a bacterial toxin in test tubes, they went into overdrive.
“A strong inflammatory response can be life-saving in the face of infectious agents,” says co-author Luis Barreiro, of the University of Montreal in Canada. “But the same self-defence mechanism, the ones that make infected tissue swollen and red, can also cause damage if not properly controlled.”
When the monkeys were re-sorted so that low-rank animals were moved up the social ladder, it had a striking effect on their immune systems.
As the animals improved their social status and enjoyed the benefits, such as more grooming, their immune cells became less likely to trigger inflammation.
“This suggests the health effects of status aren’t permanent, at least in adulthood,” says Jenny Tung, another member of the Duke team.