In MS, the immune system (which normally works to protect the body from bacteria and viruses) begins to misguidedly attack myelin, the protective sheath surrounding the nerve fibres in the brain and spinal cord. The theory behind bone marrow stem cell transplantation is that by destroying a person's existing immune system and allowing it to re-develop, the new immune system may not go on to attack myelin.

The treatment involves the following stages:

1. A chemical is given which forces stem cells (a type of cell which can grow and form a new immune system) from the person's bone marrow (where stem cells are normally found) into their blood stream.

2. In a process similar to kidney dialysis, blood is taken from the person and the stem cells removed from it for later use.

3. The person is given a course of chemotherapy which destroys the immune system. (Cells of the immune system are fast growing and chemotherapy destroys fast growing cells. This is why it is used to treat cancer. It is also why you lose hair during cancer treatment.)

4. The stem cells collected earlier are put back into the person's blood stream, where they should grow to form a new immune system.

This treatment is still very experimental in MS and the number of cases in which it has been tried is too low to draw any, firm conclusions as to its effectiveness. Clinical trials are currently underway in the USA, and a large European trial is in the planning stages. However, due to its severe side effects, this treatment would only be considered for people with very severe, rapidly progressive MS.