The brain is kind of like an exclusive nightclub; if you’re not on the list, the bouncer can, and will, turn you away. That bouncer’s name is the blood-brain barrier (affectionately known as the BBB), and its job is to keep the riffraff out of the club, er, brain.
The BBB is literally a security system for your brain that’s made up of a network of specially designed blood vessels. All the other blood vessels in your body are lined with endothelial cells that are constantly swapping different molecules — nutrients, waste products, etc.— into and out of the vessel walls. These vessels are continuously circulating cells and delivering them like packages to their destinations. For instance, when you cut yourself, a surge of immune cells hitch a ride in your blood stream, and exit through the wall of the blood vessel into the injured tissue where they start the repair process. But the blood vessels that make up the BBB are a bit different.
The endothelial cells that line the BBB’s vessel walls are squeezed tightly together to create an extremely selective barrier. Because of this, only basic things like oxygen, carbon dioxide, and glucose are able to migrate back and forth between the tissues of the brain and the rest of the body. This is to ensure that toxins, bacteria, and other harmful substances, including immune cells, are kept out of the central nervous system. However, as with any other part of the body, the BBB does not always function as it was meant to.
In the case of MS, the super-tight junctions between the endothelial cells that line the blood-brain barrier’s vessels loosen up and become leaky. In other words, the bouncer is not doing its job and molecules that are not on the list, including white blood cells (WBCs), are sneaking by.
In the rest of your body, the WBCs of your immune system work around the clock to keep you healthy. They are always on patrol, looking for cells that don’t belong and destroying them. They determine whether a cell is a friend or foe by “scanning” its antigens, a substance found on the surface of every cell that acts like a photo ID. When a WBC detects a cell that doesn’t belong, it engulfs it and initiates the immune response. This process serves to destroy the foreign invader and to build up immunity should it attack again. Unfortunately in autoimmune diseases such as multiple sclerosis, the immune system mistakes your own, healthy cells for foreigners and attacks them.
Scientists are still not sure why or how the blood-brain barrier becomes leaky, or exactly what role it plays in the development and progression of MS. The theory is that an infection can weaken our defenses and some cells, including WBCs, may be able to slip past the door of the BBB while the bouncer is under the weather. What we think happens after that is the white blood cells see myelin (the fatty coating around your nerve cells, or neurons) for the first time and pick a fight.
Since a white blood cell wouldn’t ordinarily be mucking around in the central nervous system, it sees a completely unfamiliar neighborhood and assumes that it’s been dropped into enemy territory. So they then do what they are trained to do: attack foreigners. In this case they go after the cells that make myelin (oligodendrocytes) and the myelin itself. This results in inflammation and the lesions, or scars, that are the hallmark of MS. “Multiple sclerosis” literally means “many scars.”
When an MRI with contrast dye is done during one of these attacks, or relapses, we can see bright white spots. These bright spots are called “enhancing lesions,” and they light up because the dye is able to leak through the damaged BBB and migrate to the area that’s under attack. Enhancing lesions tell us there is active inflammation occurring. Once the relapse is over and the BBB has tightened back up, the contrast is no longer able to leak into the central nervous system. If we see an enhancing lesion on an MRI we know that we are witnessing a live battle going on in the brain. But if we see lesions that do not light up (non-enhancing lesion) we know that we are looking at the casualty of a prior attack.
We still have a lot to learn about the blood-brain barrier and its exact role in MS. Current research is looking into what specifically causes the blood-brain-barrier to become leaky, and how we can prevent this from occurring. If we can fortify the central nervous system’s defenses, we may just have a shot at halting MS progression altogether. Until then, the most effective method we have of slowing progression is to use disease-modifying drugs that alter or suppress the immune system and hopefully keep it from attacking the central nervous system.
Article by: Stephanie Buxhoeveden MSCN, FNP-BC Stephanie is a nurse practitioner who was diagnosed with MS at age 25. Shortly after being diagnosed she realized she could use her experiences as a patient to make a difference in the lives of others, so she became a multiple sclerosis certified nurse. Stephanie completed her master’s in nursing at Rutgers University, and now specializes in the care of people with MS and other neurological diseases.
Her blog, www.justkeepsmyelin.com, offers a unique perspective on MS from both a healthcare provider’s point of view, and through the eyes of a person living with the disease every day. Her mission is to bring compassion, humor, and a deeper understanding of MS to anyone who reads it. She also writes for MultipleSclerosis.net, MSFocus Magazine, serves as a District Activist Leader for the National MS Society and is on the membership committee of iConquerMS.bbb, blood vessels, blood-brain barrier, brain, MS