Illustration of a neuromuscular junction of a person suffering from Myasthenia Gravis. Myasthenia gravis occurs when the immune system makes antibodies that damage or block many of the muscle's acetylcholine (ACh) receptors on the surface of muscle cells. This prohibits ACh from binding to the damaged receptors and acting on the muscle, which reduces muscle contractions, leading to weakness and fatigue. This can result in fewer receptors than normal and damage to the postsynaptic membrane.