Psychologist Steve Cole, who studies how social environments affect gene expression, says researchers have known for years that lonely people are at greater risk for heart attacks, metastatic cancer, Alzheimer’s and other ills. “But we haven’t understood why,” he said.

Then last year, Cole and his colleagues at the UCLA School of Medicine, along with collaborators at the University of California at Davis and the University of Chicago, uncovered complex immune system responses at work in lonely people. They found that social isolation turned up the activity of genes responsible for inflammation and turned down the activity of genes that produce antibodies to fight infection.

The abnormalities were discovered in monocytes, a type of white blood cell, produced in the bone marrow, that is dramatically changed in people who are socially isolated. Monocytes play a special immunological role and are one of the body’s first lines of defense against infection. However, immature monocytes cause inflammation and reduce antibody protection. And they are what proliferates in the blood of lonely people…

The pain of loneliness is like the pain of hunger — it’s a biological signal that something is wrong.