The brain has lymph vessels running around it. (Image: Popular Science/Public domain)

One of the the most important systems in the human body is the lymph system, through which immune cells travel to places where they’re needed to fight pathogens. But one crucial part of the body, scientists thought, was cut off from the lymph system: no connection had ever been found between lymph vessels and the brain.

Now, though, scientists at the University of Virginia have discovered lymph vessels in the brain, along with evidence that these pathways could allow the immune system to control behavior and personality, ScienceAlert reports.

The vessels are in the meninges, the layer of connective tissue that surrounds the brain. In the past, scientists have examined this tissue by slicing it up and putting those slices under a microscope. But the UVA lab developed a technique that allowed the scientists there to look at the whole of a mouse’s meninges at once. They saw, under the microscope, trails of immune cells that looked a lot like vessels—and tests confirmed that, in fact, they were.

This was a new detail of mammalian anatomy, and the team found evidence that these vessels were present in the human meninges, as well. The connection between the lymph system and the brain, they think, could help explain the workings of diseases like Alzheimer’s. In a second experiment, the scientists explored how changes in the immune system might affect the brain: they turned off one immune molecule in a mouse, and watched what happened in its nervous system.

They found that it made some parts of the brain “hyperactive,” which changed how the mice in the experiment behaved. They shied away from others, becoming less social. These results have only been seen in mice, but what they indicate, says Jonathan Kipnis, the scientist leading the research, is that “some of our behavior traits might have evolved because of our immune response to pathogens.”

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