Feeling lonely is linked to a increased disease risk, and now we know why thanks to a new study from the US.
Working with samples from humans and macaques, UCLA scientist Steven Cole and his colleagues have found that individuals who feel socially isolated, or - in the case of monkeys - display lower levels of social interactions and show the greatest fear of social engagement, have specific changes in the functioning of the immune system.
Specifically, the team found that a class of cells called monocytes were increased in the blood. These cells appeared to be in a state primed for activity and they responded less than normal to immune-dampening hormonal signals.
The levels of infection-fighting immune signals, including chemicals called interferons, were also much lower than in less socially isolated individuals.
The 141 human subjects considered in the analysis were part of a larger, long-term study called the Chicago Health, Ageing and Social Relations Study (CHASRS) that collected blood samples and measured social isolation scores anuually over roughly a five year period.
This data, however, shows only the association between perceived social isolation and changes to the fabric of the immune system.
To show that feelings of loneliness might cause these changes to occur, the team compared the immune responses of monkeys that were allowed to socialise regularly with a group of animals with which they were already well acquainted, and changing groups of animals that they had never seen previously and hence with whom they had no existing social relationships.
Animals in the latter group developed changes in their immune function similar to those seen in the lonely human subjects, and a poorer immune response to viral infection subsequently, leading the researchers to propose that social isolation causes these changes to occur.
Significantly, they also speculate in their paper, published this week in PNAS, that the ability of the abnormal immune cells seen in the blood of lonely individuals to access the brain might lead to what they dub "social sickness".
The abnormal cells produce spurious immune signals that alter brain function and promote greater social withdrawal, intensifying the likelihood of loneliness.
Overall, the study goes some way to explaining why social isolation is so strongly linked to mortality and greater rates of ill-health among the lonely...