Brain, like every other tissue, is connected to the peripheral immune system through meningeal lymphatic vessels. This link was recently discovered.

"I really did not believe there are structures in the body that we are not aware of. I thought the body was mapped. I thought that these discoveries ended somewhere around the middle of the last century. But apparently they have not," said lead researcher Jonathon Kipnis, director of centre for brain immunology and glia at University of Virginia.
"We believe that for every neurological disease that has an immune component to it, these vessels may play a major role. [It’s] hard to imagine that these vessels would not be involved in a [neurological] disease with an immune component," he added.
The discovery that may lead to a complete revision of biology textbooks was made possible by the work of Antoine Louveau, a postdoctoral fellow in Kipnis’ lab.
The vessels were detected after Louveau developed a method to mount a mouse’s meninges -- the membranes covering the brain on a single slide so that they could be examined as a whole.
After noticing vessel-like patterns in the distribution of immune cells on his slides, he tested for brain’s elusive lymphatic vessels and there they were. The impossible existed.
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The unexpected presence of the lymphatic vessels raises a tremendous number of questions that now need answers, both about the workings of the brain and the diseases that plague it. For example, take Alzheimer’s disease. In Alzheimer’s, there are accumulations of big protein chunks in the brain.
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There is an enormous array of other neurological diseases, from autism to multiple sclerosis, that must be reconsidered in light of the presence of something science insisted did not exist.
The study appeared in the journal Nature.
Source-IANS