The blood-brain barrier (BBB) was highlighted in a session at the #MSParis2017 conference where researchers presented their recent findings on the relationship between the BBB and MS. The BBB is a network of cells (called endothelial cells) that line all the blood vessels in the brain. These cells are so tightly connected that they prevent harmful substances such as bacteria and toxins from entering the brain and spinal cord (collectively called the central nervous system or CNS). In MS, the BBB is compromised and becomes leaky, allowing white blood cells from the immune system to pass through the barrier and enter the CNS. The white blood cells then attack myelin (the coating around the nerve cells in the CNS) which results in inflammation and lesions. Many research teams are examining what causes the BBB to weaken and ways to prevent this from happening.