Recent research may shed new light on what, exactly, is going on the the brains—and bodies—of people with Alzheimer's disease. It's all about how the brain interacts with the lymphatic system. As blood vessels carry blood around the body, lymphatic vessels carry a fluid called lymph. Lymph is recycled blood plasma without the cells, so it's colorless, and the lymphatic system returns it to the heart to be recombined into blood. In addition to playing a vital role in the body's immune response, the lymphatic system carries waste products of metabolic processing away from the organs.
However, there are no lymph glands in the brain, despite the amount of metabolic processing the brain does. This is important because the proteins that are waste products from this processing have an even more dramatic impact on functioning in the brain than in other parts of the body. What researchers have found, however, is that there is a separate system, distinct from the network of lymph vessels, for getting these waste products out of the brain.
Rather than lymph, this system, called the glymphatic system after the neural cells known as glia, uses the cerebrospinal fluid that the brain is surrounded by in a similar way. This fluid gets around the brain using channels formed by star-shaped glia called astrocytes. The astrocytes serve a function in the brain analogous to the function of lymph vessels in the rest of the body, with cerebrospinal fluid. Like lymph, this cerebrospinal fluid takes in waste products as it flows, gathering them up, removing them from the brain, and depositing them in the lymph glands to be processed like any other waste.
Sometimes, however, there are lapses in this system. That is believed to be what's behind neurodegenerative diseases such as Alzheimer’s. The beta amyloids that form the plaques that cause Alzheimer's disease are waste products that are supposed to be swept away, but in some people, the glymphatic system gets less efficient as they age. That means that the beta amyloids are allowed to build up, doing damage. This waste removal mechanism may also be what is malfunctioning in the case of Parkinson's disease and other degenerative conditions.