What is Acetylcholine Receptor (AChR) Antibody?

Also Known As: AChR Antibody, Muscle nicotinic Acetylcholine Receptor (AChR) Binding Antibody, Myasthenia Gravis Antibodies, Acetylcholine Receptor Binding Antibody, Acetylcholine Receptor Blocking Antibody, Acetylcholine Receptor Modulating Antibody,

Formal Name: Myasthenia Gravis (MG) Evaluation, serum

Acetylcholine receptor (AChR) antibodies are autoantibodies produced by the immune system that mistakenly target proteins called acetylcholine receptors that are located on muscles that you can consciously or voluntarily control (known as skeletal muscle fibers). This test detects and measures AChR antibodies in the blood.

Muscle movement starts when an impulse is sent down a nerve to the nerve ending, where it stimulates the release of acetylcholine, a chemical substance (neurotransmitter) that transmits messages between specific types of cells. Acetylcholine travels across the very small gap between the nerve ending and muscle fiber (this gap is called the “neuromuscular junction”). When acetylcholine reaches the muscle fiber, it binds to one of many acetylcholine receptors or “docking stations” 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 in 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, 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 two tests may be useful when a healthcare practitioner strongly suspects myasthenia gravis and the “binding” test is negative.

How is the test used?

An acetylcholine receptor (AChR) antibody test is used to help diagnose myasthenia gravis (MG) and to distinguish it from other conditions that may cause similar symptoms, such as chronic muscle fatigue and weakness.

Three types of AChR antibodies may be tested:

• AChR binding antibodies
• AChR blocking antibodies
• AChR modulating antibodies

The test that measures binding antibodies is most commonly used because it is generally rare for the other two tests to be positive without the binding antibody test being positive as well. These other two tests may be used when a healthcare practitioner strongly suspects myasthenia gravis and the binding antibody test is negative.

One or more of these AChR antibody tests may be ordered as part of a panel of tests that may also include a striated muscle antibody test to help establish a diagnosis. If AChR antibody test results are normal but a healthcare practitioner strongly suspects myasthenia gravis, an anti-MuSK (muscle-specific tyrosine kinase) antibody test may also be ordered.

People with MG often have an enlarged thymus gland and may have thymomas (typically benign tumors of the thymus). Located under the breastbone, the thymus is an active part of the immune system during childhood but normally becomes less active during the teen years. If a thymoma is detected, such as during a chest computed tomography (CT) scan done for a different reason, then an AChR antibody test may sometimes be used to determine whether the person has developed these antibodies.