Smooth muscle antibodies, also called anti-smooth muscle antibodies (ASMA), are abnormal proteins produced by the immune system. Healthy people typically do not have these antibodies present in their blood. They are produced in association with a number of diseases, but are most closely linked to a condition called autoimmune hepatitis. Checking a person’s blood for the presence of these antibodies can be useful in diagnosing what underlying diseases he or she might have.
Antibodies are proteins produced by the white blood cells. They bind to foreign or pathologic substances within the body so that the immune system can attack and destroy them. In certain people, the immune system becomes dysregulated and starts to attack native components of the body; the antibodies produced as a result of this pathologic process are called autoantibodies. Smooth muscle antibodies are considered to be autoantibodies because they attach to the body’s own smooth muscle, a type of tissue prevalent in the blood vessels, the respiratory tract, the gastrointestinal tract, and the eyes.
Smooth muscle antibodies are often present in association with autoimmune hepatitis, which is a condition that causes inflammation and dysfunction of the liver. Measuring the concentration of these antibodies in the blood can help to confirm the diagnosis of autoimmune hepatitis if their level reaches a certain threshold. Unfortunately, monitoring the levels of these antibodies is not helpful in determining whether treatments for autoimmune hepatitis are working. In other words, a decreasing antibody level does not necessarily mean that the disease is improving.
Other diseases can also result in low levels of smooth muscle antibody production. Patients with a disease called primary biliary cirrhosis, a different autoimmune disease affecting the function of the liver, can sometimes have low levels of smooth muscle antibodies in their blood. Infectious mononucleosis, a condition commonly referred to as mono, can also occasionally cause patients to have positive tests for the presence of these antibodies.
The most important clinical use of checking for smooth muscle antibodies is for diagnostic purposes. For example, checking this level is useful in patients with unexplained abnormalities in laboratory tests measuring liver function. A high level of these antibodies points to a diagnosis of autoimmune hepatitis as the cause of the abnormal liver tests. Differentiating lupus from autoimmune hepatitis is another common use for these antibodies. Lupus is an autoimmune disease that affects many systems of the body including the liver, but patients with lupus should not have smooth muscle antibodies.