Although first reported in 1816, the fact that the brain is surrounded by lymphatic vessels, which connect the brain and the immune system, was only rediscovered in 2015.
Lymphatic vessels are part of the body's circulatory system. In most of the body they run alongside blood vessels. They transport lymph, a colorless fluid containing immune cells and waste, to the lymph nodes. Blood vessels deliver white blood cells to an organ and the lymphatic system removes the cells and recirculates them through the body. The process helps the immune system detect whether an organ is under attack from bacteria or viruses or has been injured.
Since then, brain scans have indicated that our brains drain some waste out through lymphatic vessels, and could act as a pipeline between the brain and the immune system.
More recent research suggests the vessels are vital to the brain's ability to cleanse itself. When a compound was used to improve the flow of waste from the brain to the lymph nodes in the neck of aged mice, their ability to learn and remember improved dramatically.
Moreover, obstructing the vessels in mice worsened the accumulation of harmful amyloid plaques in the brain.
(2018). Functional aspects of meningeal lymphatics in ageing and Alzheimer’s disease.
Nature. 560(7717), 185 - 191.
Absinta, Ha et al. Human and nonhuman primate meninges harbor lymphatic vessels that can be visualized noninvasively by MRI, October 3, 2017, eLife: 10.7554/eLife.29738