Myasthenia gravis is known as an autoimmune disease, because it is caused by the body's own immune system.
If you have myasthenia gravis, your body's immune system produces antibodies that block or damage muscle receptor cells, reducing the number available on the muscle fibres (see box, right). When this happens, the normal communication between your nerves and muscles is affected, your muscles do not contract well, and they become easily tired and weak.
The reason why some people's immune systems make antibodies that act against muscle receptor cells is not fully understood. It is believed the thymus gland - part of your immune system located in your upper chest - may be linked to the production of the antibodies, and about 10% of those with myasthenia gravis have a benign (non-cancerous) tumour of the thymus gland.
It is known that myasthenia gravis can sometimes be triggered by particular viruses or medicines. It is also thought that some people's genes make them more likely to develop autoimmune diseases.
