Multiple sclerosis - causes, symptoms, diagnosis, treatment, pathology

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.

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.