Multiple sclerosis is a demyelinating disease of the central nervous system, which includes the brain and the spinal cord.
Myelin is the protective sheath that surrounds the axons of neurons, allowing them to quickly send electrical impulses.
This myelin is produced by oligodendrocytes, which are a group of cells that support neurons.
In multiple sclerosis, demyelination happens when the immune system inappropriately attacks and destroys the myelin, which makes communication between neurons break down, ultimately leading to all sorts of sensory, motor, and cognitive problems.
Now, the brain, including the neurons in the brain, is protected by things in the blood by the blood brain barrier, which only lets certain molecules and cells through from the blood.
For immune cells like T and B cells that means having the right ligand or surface molecule to get through the blood brain barrier, this is kind of like having the a VIP pass to get into an exclusive club.
Once a T cell makes its way in it can get activated by something it encounters - in the case of multiple sclerosis, it’s activated by myelin.
Once the T-cell gets activated, it changes the blood brain barrier cells to express more receptors, and this allows immune cells to more easily bind and get in, it’s kind of like bribing the bouncer to let in a lot of people.
Now, multiple sclerosis is a type IV hypersensitivity reaction, or cell-mediated hypersensitivity. And this means that those myelin specific T-cells release cytokines like IL-1, IL-6, TNF-alpha, and interferon-gamma, and together dilate the blood vessels which allows more immune cells to get in, as well as directly cause damage to the oligodendrocytes.