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Scientists have found new evidence that the body's immune system extends to the brain. The finding challenges longstanding beliefs about how the brain protects itself from diseases. And as NPR's Jon Hamilton reports, it may change our understanding of disorders ranging from multiple sclerosis to Alzheimer's.
JON HAMILTON, BYLINE: In the body, cells that fight disease often travel through a network called the lymphatic system. Its vessels extend all over the place, sort of like blood vessels, except these vessels carry a clear fluid called lymph. But for centuries, scientists thought the brain simply wasn't connected to this system. Daniel Reich is a neurologist and radiologist at the National Institutes of Health.
DANIEL REICH: The brain is thought to be what is called immune privileged. It has a different immune system from the rest of the body.
HAMILTON: Except no one could figure out exactly how the brain's immune system worked. So Reich was intrigued when he heard a talk by a scientist from the University of Virginia a couple of years ago.
REICH: And he showed very clearly in this talk that there are real lymph vessels in the head, which I had learned in medical school didn't exist.
HAMILTON: Reich says that suggested the lymphatic system was interacting directly with the brain. But the evidence was in mice.
REICH: I immediately thought, well, he's showing this in mice. Do they exist in people, as well?
HAMILTON: To find out, Reich and a team of scientists used MRI to study the brains of several people. The team injected a special dye into the bloodstream, then watched to see where it went. They focused on a membrane that surrounds the brain and nervous system. And as expected, they saw some of the dye leak out of the blood vessels in this membrane. But then, Reich says, the leaking dye was collected by different vessels. That's how the lymph system works. It collects and carries away things that don't belong there.
REICH: That gave us some evidence that there are vessels here that are behaving different from blood vessels. But we weren't sure that they were lymphatic vessels.
HAMILTON: To be certain, Reich's team spent several years perfecting a technique that revealed the lymphatic vessels in brains taken from human cadavers. This allowed them to confirm the presence of these vessels near the surface of the brain. And it strongly suggested that the lymphatic system is connected to the brain. Reich says the finding could help explain multiple sclerosis, a disease in the brain and nervous system that seems to be triggered by the immune system.
REICH: How the immune system interacts with the brain is fundamental to how multiple sclerosis develops and how we treat multiple sclerosis.
HAMILTON: The new research published in the online journal eLife also has implications for diseases like Parkinson's and Alzheimer's. These diseases occur as toxic waste products accumulate in the brain. Michael Weiner is a professor of radiology at the University of California, San Francisco.
MICHAEL WEINER: The discovery of a lymphatic system in the brain raises the possibility that a disorder of the lymphatic system is somehow involved in the causation of Alzheimer's disease.
HAMILTON: Weiner says the discovery also could explain why people with sleep disorders seem to be more likely to develop Alzheimer's. A few years ago, researchers found evidence that the brain flushes out waste products during sleep. But it wasn't clear how those waste products drain out of the head. Weiner says the new research suggests the answer may be the lymphatic system.
WEINER: So it may be that the reason why sleep and Alzheimer's disease are related is because of the effects of sleep on the lymphatic system.
HAMILTON: A system once thought to have nothing to do with the brain. Jon Hamilton, NPR News.
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