Depression, even mild cases, can affect the immune system in older adults, according to the results of a study published in the Archives of General Psychiatry (October 2003). During the study, individuals who reported even a few symptoms of depression had higher levels of interleukin-6 (IL-6), an immune symptom protein that promotes inflammation. The research team compared 47 people who were present or past caregivers for a spouse with dementia with 72 similarly aged people who had never been a caregiver.

    The participants answered questions about symptoms of depression, and the researchers measured levels of IL-6 in blood samples taken before and after the participants had a flu shot. The study found that people who had symptoms of depression, but who were not clinically depressed, had higher levels of IL-6 before and after a flu shot, compared with people with fewer symptoms. Furthermore, 2 weeks after a flu shot, IL-6 levels in people with more depressive symptoms had increased, whereas people with few symptoms of depression did not have a big increase in IL-6 levels.