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The exact cause of myasthenia gravis is not known. One theory is that the condition may be triggered by a virus or other infection that has a similar structure to a part of the acetylcholine receptor (found in the neuromuscular junction and needed for normal function). The antibodies that the immune system produces to fight the virus then also mistakenly attack the receptors.

What we do know is that certain antibodies have been found in the blood of people with myasthenia gravis that are built to attack acetylcholine receptors.

Sometimes women with myasthenia gravis who give birth transmit these particular antibodies to their babies, causing muscle weakness in the infant. This is called neonatal myasthenia and occurs in about 12% of babies whose mother has myasthenia gravis. It is different from congenital myasthenia because the infant gets better in a few weeks as the antibody blood levels go down.

Some people with myasthenia gravis do not have antibodies to acetylcholine receptors. Many of these people, though, have antibodies to an enzyme involved in acetylcholine receptor function.

The other abnormality that often shows up in people with myasthenia gravis is an overactive, overlarge, or otherwise malfunctioning thymus. The thymus is a gland located where the neck joins the chest. It is important for the normal development of the immune system. Normally, the thymus is larger in childhood than in adulthood and becomes inactive around puberty. In people with myasthenia gravis, however, it often keeps going into adulthood.

Some people with myasthenia gravis turn out to have a thymoma (a tumour in the thymus). This is a type of cancer, but very few people die of thymomas (see "Treatment and Prevention").

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