Loneliness is more than a feeling; it can actually make you ill, according to a recent study.

Researchers at the University of Chicago found that for older adults, perceived social isolation is a major health risk that can increase the risk of premature death by 14 percent. Their study also showed that loneliness leads to fight-or-flight stress signaling, which can ultimately affect the production of white blood cells.

For the study, researchers examined loneliness in both humans and rhesus macaques, a highly social primate species. They looked at the gene expression in leukocytes, cells of the immune system that are involved in protecting the body against bacteria and viruses.

Next, the team investigated the cellular processes linking social experience to CTRA gene expression in rhesus macaque monkeys at the California National Primate Research Center, which had been behaviorally classified as high in perceived social isolation. Like the lonely humans, the "lonely like" monkeys showed higher CTRA activity. They also showed higher levels of the fight-or-flight neurotransmitter, norepinephrine.

Previous studies have found that norepinephrine can stimulate blood stem cells in bone marrow to make more of a particular kind of immune cell--an immature monocyte that shows high levels of inflammatory gene expression and low levels of antiviral gene expression. Both lonely humans and "lonely like" monkeys showed higher levels of monocytes in their blood.

Finally, the researchers determined that this monocyte-related CTRA shift had real consequences for health. In a monkey model of viral infection, the impaired antiviral gene expression in "lonely like" monkeys allowed simian immunodeficiency virus (the monkey version of HIV) to grow faster in both blood and brain.

The team plans to continue research on how loneliness leads to poor health outcomes and how these effects can be prevented in older adults.

The findings are detailed in the journal Proceedings of the National Academy of Sciences.