What is Multiple Sclerosis?

Multiple sclerosis is a demyelinating disease of the central nervous system, which includes the brain and the spinal cord.

In other words, it is a idiopathic auto immune disorder and it primarily affects the white matter of brain. It is one disease almost exclusively of the central nervous system.

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 (BBB), 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. Once a T cell makes its way in it can get activated by something it encounters - in the case of multiple sclerosis, it is 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, 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 intereferon-gamma, and together dilate the blood vessels which allow more immune cells to get in, as well as directly cause damage to the oligodendrocytes. They cytokines also attract B-cells and macrophages as part of the inflammatory reaction. Those B-cells begin to make antibodies that mark the myelin sheath proteins, and then the macrophages use those antibody markers to engulf and destroy the oligodendrocytes. Without oligodendrocytes, there's no myelin to cover the neurons, and this leaves behind areas of scar tissue, also called plaques or sclera. In multiple sclerosis, these immune attacks typically happen in bouts. In other words, an autoimmune attack on the oligodendrocytes might happen, and then regulatory T cells will come in to inhibit or calm down the other immune cells, leading to a reduction in the inflammation. Early on in multiple sclerosis, the oligodendrocytes will heal and extend out new myelin to cover the neurons, which is a process called remyelination.

Unfortunately, though, over time as the oligodendrocytes die off the remyelination stops and the damage becomes irreversible with the loss of axons.