Acetylcholine receptor (AChR) antibodies are autoantibodies produced by the immune system that mistakenly target proteins called acetylcholine receptors that are located on skeletal muscle fibers. This test detects and measures AChR antibodies in the blood. Acetylcholine receptors function as "docking stations" for acetylcholine, a chemical substance (neurotransmitter) that transmits messages between nerve cells. Muscle movement starts when an impulse is sent down a nerve to the nerve ending, where it stimulates the release of acetylcholine. Acetylcholine travels across a microscopic gap between the nerve ending and a muscle fiber at the "neuromuscular junction." When it reaches the muscle fiber, it binds to one of many acetylcholine receptors and activates it, initiating muscle contraction. AChR antibodies impede communication between nerves and skeletal muscles, inhibit muscle contraction, and cause rapid muscle fatigue by preventing activation of the acetylcholine receptors. They do this in three major ways: Binding antibodies attach to the receptors on nerve cells and may initiate an inflammatory reaction that destroys the receptors. Blocking antibodies may sit on the receptors, preventing acetylcholine from binding. Modulating antibodies may cross-link the receptors, causing them to be taken up into the muscle cell and removed from the neuromuscular junction. The end result of this interference is the development of myasthenia gravis (MG), a chronic autoimmune disorder associated with the presence of these antibodies and with their effects on muscle control. AChR antibodies may be detected in different ways to determine which mechanism may be the problem in a particular individual, and the antibodies may be referred to as "binding," "blocking," or "modulating." However, the technique that measures "binding" is the most commonly performed and, generally speaking, it is rare for the other two tests to be positive without the "binding" test being positive as well. These other approaches may be useful when a healthcare practitioner strongly suspects myasthenia gravis and the "binding" test is negative.