Severe muscle weakness caused by myasthenia gravis — a highly debilitating autoimmune disorder — can be prevented or reversed by blocking a key step in the immune response that brings on the disease, researchers have found. Myasthenia gravis, which affects about 120,000 Americans, is caused when the immune system produces antibodies that attack and damage acetylcholine receptors, which are mechanisms that play a key role in transmitting the electrical impulses that cause muscles to move and contract.