Myasthenia gravis (MG) is an autoimmune disorder in which the communication between nerves and muscles is disrupted. To understand MG and the rationale for different types of treatments, it is helpful to know something about the immune system and also something about the way in which nerves normally communicate with muscles.

The immune system is the body's natural defense again an infection with, for example, a bacteria or a virus. The normal functioning of the immune system depends on its ability to differentiate between self and something foreign (not part of the body, such as a bacteria or virus). The problem in an MG patient is that the immune system mistakenly targets the connection between nerve and muscle (what's known as the neuromuscular junction) as though it were foreign. This abnormal immune response is described as "auto-immune" because it recognizes "self" as though it were foreign and produces an immune response, known as an attack, against itself. The damage that results from the immune-mediated attack on the neuromuscular junction leads to impaired communication between nerve and muscle. The consequence is weakness, which is the most common symptom of MG.

Broadly speaking, there are two types of MG – what's known as ocular MG and what's known as generalized MG. MG is considered ocular when the symptoms are restricted to the eyes – symptoms may include drooping of the eyelids, double vision and weakness of eye closure. MG is considered generalized if there are symptoms indicative of weakness affecting muscles other than the eyes. Common symptoms include difficulty chewing and swallowing, slurring of speech, weakness of the jaw, difficulty holding the head up off the chest, weakness in the arms or legs, generalized fatigue and difficulty breathing. Generalized MG is thought to be potentially more serious than ocular MG and so more aggressive treatment is typically indicated and needed (see Available Treatments for MG).