Chronic anxiety triggered by stress may be driven by an overcharged immune system, research suggests.
Scientists studying stressed mice found a link between the spleen, the immune system's "engine room", and the brain.
A surge of white blood cells was seen in the spleen that continued for almost a month after the initial stress.
The immune cells were thought to send messages to the brain, leading to a constant state of anxiety.
Lead researcher Daniel McKim, from Ohio State University in the US, said: "We found that immune cells in the spleen can contribute to chronic anxiety following psychological stress.
"Our findings emphasise the possibility that the immune system represents a novel therapeutic target for the treatment of mental health conditions."
The study involved subjecting mice to "social defeat" situations during which they repeatedly lost in confrontations with aggressive cage-mates.
The aim was to find ways to improve the well-being of people who suffer from chronic psychological stress and anxiety.
Co-author Dr Jonathan Godbout, also from Ohio State University, said: "Stress appears to prompt the release of stem cells from the bone marrow to the spleen, where they develop into white blood cells, or monocytes, and expand over time.
"Then the spleen becomes a reservoir of inflammatory cells."
The changes in the spleen acted "like a stress memory", he added.
Other work by the same scientists has indicated that mice exposed to chronic stress experience effects similar to post-traumatic stress disorder.
Studies have also shown that an immune system signalling molecule, interleukin-1, plays a critical role in the stress response of mice.
Dr Godbout added: "Maybe anxiety is a good thing for survival - it's beneficial evolutionarily - but the issue becomes what happens when that system is put into overdrive. That's when it gets problematic."
The research was presented at the Neuroscience 2016 meeting taking place in San Diego, US.