What causes MS?
MS is the result of damage to myelin – a protective sheath surrounding nerve fibres of the central nervous system. When myelin is damaged, this interferes with messages between the brain and other parts of the body. The cause of MS is not yet known, but researchers all over the world are meticulously putting the pieces of this complicated puzzle together.
The damage to myelin in MS may be due to an abnormal response of the body's immune system, which normally defends the body against invading organisms such as bacteria and viruses. Many of the characteristics of MS suggest an auto-immune disease, whereby the body attacks its own cells and tissues, which in the case of MS, is myelin. Researchers do not know what triggers the immune system to attack myelin, but it is thought to be a combination of genetic susceptibility and environmental factors.
Some genes are actually faulty and make defective proteins, but that is not thought to be the situation in MS: rather, the idea is that affected individuals happen to have slight variations, called polymorphisms, that are perfectly healthy genes but – by chance – fit together badly so that the normal workings of cells, especially those that make up the body’s immune system and the brain and spinal cord, are subtly altered.
Once this group of badly-fitted genes has come together by chance in the genome (a complete set of DNA) of one individual, it stands to reason that some or all of these factors may be shared within families through the normal mechanisms of inheritance.
Researchers have identified particular genes that make some people more susceptible togetting MS, in particular the HLA (also referred to as the “major histocompatibility complex” or MHC), as containing probably the most important susceptibility gene for MS. 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 the susceptibility, or risk, is just that; something else has to happen that releases the effect of these genes. A trigger, presumably something environmental, is what is thought to actually set off the disease process.
Read more in
MS in focus - genetics and hereditary aspects of MS
One theory is that a virus, possibly lying dormant in the body, may play a major role in the development of the disease and may disturb the immune system or indirectly instigate the auto-immune process. A great deal of research has taken place in trying to identify an MS virus. It is probable that there is no one MS virus, but that a common virus, such as measles or the Epstein-Barr virus (the common herpes virus), may act as a trigger for MS.
This trigger activates white blood cells (lymphocytes) in the bloodstream, which enter the brain by making the brain's defense mechanisms (i.e. the blood/brain barrier) vulnerable. Once inside the brain, these cells activate other elements of the immune system in such a way that they attack and destroy myelin.