A recent study explored this question by conducting experiments in mice and exposing their intestines to several types of immune challenges, such as parasites, chemical gut irritants, and Salmonella infection. The findings are published in Nature Neuroscience.

They saw that upon encountering irritants, CD4⁺ T cells, white blood cells responsible for coordinating the battle against invaders, become activated and adapt specifically to fight that exact type of threat they are facing.

These highly specific T cells then find their way to the dura mater through a specialized chemical homing pathway. Once there, they remain for the long term, building immune memory around the region's blood vessels to help fight future infections and inflammation.

Elusive gut-brain link

The gut is a major source of microbes that can enter the bloodstream via fenestrated endothelium, a type of capillary structure that contains small pores. At the same time, immune cells in the dura mater stand ready to defend the brain's borders against these foreign entities, with inflammatory signals that can influence brain activity.

Scientists have known for a while that gut inflammation can be linked to brain-related symptoms and conditions. For instance, people who have inflammatory bowel disease (IBD) are known to experience higher rates of depression and anxiety.