What Causes Myasthenia Gravis?

Myasthenia gravis is just one of many autoimmune diseases, which include arthritis and type 1 diabetes. Normally, the immune system produces antibodies that recognise foreign things that enter the body, such as bacteria and viruses. This leads to them being destroyed and cleared from the body.

In the case of an autoimmune condition, the body’s immune system produces antibodies against things in the body that aren’t foreign. In myasthenia gravis it is the structure at the junction of the nerves and the muscles (the neuromuscular junction) that is attacked.

About 85 percent of patients with myasthenia gravis produce antibodies against a protein called the ‘acetylcholine receptor’ (AChR). This is found at the neuromuscular junction and acts as a receiver for the chemical signal ‘acetylcholine’ (ACh) that is released from the nerve to tell a muscle to contract. The antibodies bind to the acetylcholine receptors on the surface of the muscle and greatly reduce their ability to receive the chemical signal. As a result the patient experiences muscle weakness which becomes worse as they repeatedly try to use the same muscle.

The Neuromuscular Junction

Many people with myasthenia gravis who don’t have antibodies to the AChR have antibodies to a protein called ‘muscle-specific kinase’ (MuSK). This protein helps organize ACh receptors on the muscle cell surface. Research is ongoing to find out what antibody is responsible in the approximately 10 percent of patients who don’t have antibodies to AChR or MuSK. Recently antibodies to a protein called ‘LRP4’ were found to be the cause for some of these patients.

Scientists don’t know what triggers most autoimmune conditions, but they have a few theories. One possibility is that certain viral or bacterial proteins mimic ‘self-proteins’ in the body (such as AChR), stimulating the immune system to accidentally attack it.

There is also evidence that the thymus gland plays a role in myasthenia gravis. About 15 percent of people with myasthenia gravis have a thymic tumour, called a thymoma, and another 65 percent have an over active thymus, a condition called thymic hyperplasia. When the thymus doesn’t work properly, the immune system might lose some of its ability to distinguish self from non-self, making it more likely to attack the body’s own cells.

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