Pre-existing differences in the sensitivity of a key part of each individual's immune system to stress confers a greater risk of developing stress-related depression or anxiety, the findings showed.

"Our data suggests that pre-existing individual differences in the peripheral immune system predict and promote stress susceptibility," said lead author Georgia Hodes from the Icahn School of Medicine at Mount Sinai in US.

Under normal conditions when the immune system perceives a threat such as an invading virus, inflammatory proteins called interleukins are released by white blood cells as an adaptive mechanism to limit injury or infection.

But the researchers found that interleukin 6 (IL-6) levels were higher in mice that were more susceptible to stress than in stress-resilient mice.

They also found the levels of leucocytes (white blood cells that release IL-6) were higher in stress susceptible mice before stress exposure.

"Additionally, we found that when mice were given bone marrow transplants of stem cells that produce leucocytes lacking IL-6 or when injected with antibodies that block IL-6 prior to stress exposure, the development of social avoidance was reduced," Hodes added.

The findings demonstrated that the emotional response to stress can be generated or blocked in the periphery.

Evidence in the current study is the first to suggest that interleukin 6 response prior to social stress exposure can predict individual differences in vulnerability to a subsequent social stressor.

The study appeared in the journal Proceedings of the National Academy of Sciences.

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