Depression is a whole-body disorder. There's accumulating evidence that the illness has deleterious effects on the heart, the brain, the bones and metabolism. Now comes proof that it undermines the immune system as people age.

Ohio State University researchers have found an exaggerated inflammatory response to an immune challenge among 47 people feeling stressed and suffering subclinical depression. Following a single flu shot, their bodies overproduced the immune system component interleukin-6, a marker of long-term inflammation.

Chronic inflammation is linked to cardiovascular disease, osteoporosis, arthritis, diabetes and periodontal disease among others.

The study subjects, all caregivers to spouses with Alzheimer's disease, had modest levels of depression—they felt blue and had sleep problems—but did not have the full-blown disorder.

"A person's mental health really does matter," says psychologist Janice Kiecolt-Glaser, Ph.D., a coauthor of the study. "The body responds differently—even to everyday challenges—depending on whether a person is depressed or not. This study shows there are long-term changes taking place in your immune system."