Genetic & environmental factors
Some genes are actually faulty and make defective proteins, but that is not thought to be the situation in multiple sclerosis. People living with the disease happen to have slight variations, called polymorphisms, that are perfectly healthy genes but unfortunately they fit together badly. This means the way the cell works is subtly altered, especially the nerves that make up the body’s immune system and the brain and spinal cord.
Once this group of badly-fitted genes has come together by chance in the genome (a complete set of DNA) of one person, it stands to reason that some or all of these factors may be shared within families through inheritance.
Researchers have identified particular genes that make some people more susceptible to getting multiple sclerosis. The HLA (also referred to as the “Major Histocompatibility Complex” or MHC) contains what is probably the most important susceptibility gene for multiple sclerosis. HLA proteins are found on the surface of all body cells. They act as a signal to the immune system to confirm that the cell is part of the body and should not be attacked.
These are perfectly healthy structures and something else has to happen to release the effect of these genes. A trigger, presumably something environmental, is what is thought to actually set off the disease process.
One theory is that a virus (possibly lying dormant in the body) may play a major role in the development of the disease. It may disturb the immune system or indirectly instigate the auto-immune process.
A great deal of research has taken place in trying to identify a multiple sclerosis virus. It seems likely there is no one multiple sclerosis virus, but a common virus like measles or the Epstein-Barr virus (common herpes virus), may act as a trigger for multiple sclerosis.
This trigger activates white blood cells (lymphocytes) in the bloodstream, which enter the brain by making the brain's defence mechanisms vulnerable. Once inside the brain, these cells activate other elements of the immune system in such a way that they attack and destroy myelin.