Studies of military personnel suggest that immune function can influence the development of PTSD. Soldiers whose blood contains high levels of the inflammatory protein CRP before they are deployed2, or who have a genetic mutation that makes CRP more active3, are more likely to develop the disorder. 

To directly test whether altering the immune system affects fear and anxiety, Lowry and colleagues injected mice with a common bacterium, Mycobacterium vaccae, three times over three weeks to modulate their immune systems. The scientists then placed these mice, and a control group of unimmunized mice, in cages with larger, more aggressive animals.

Mice that had received the injections were more ‘proactive’ in dealing with the aggressor, Lowry says, rather than simply surrendering, as most mice do. And the guts of the immunized mice remained healthy, whereas the animals in the control group developed inflamed colons and their gut bacteria shifted to favour species associated with stress.