Actin (Smooth Muscle) Antibody (IgG) - Actin is the major antigen to which smooth muscle antibodies react in autoimmune hepatitis.

F-Actin IGG antibodies are found in 52-85% of patients with Autoimmune Hepatitis (AIH) or chronic active hepatitis and in 22% of patients with Primary Biliary Cirrhosis (PBC). Anti-Actin antibodies have been reported in 3-18% of sera from normal healthy controls.

What is AIH?

Autoimmune hepatitis (AIH) is caused by chronic inflammation within the liver, resulting in damage to the hepatocytes (=chief functional cells of the liver).

Symptoms and diagnosis?

Initially, patients with AIH may be clinically asymptomatic, usually identified only through an incidental finding of abnormal liver function tests.

At a more advanced stage, patients may manifest with symptoms such as jaundice, pruritus, or ascites, which are secondary to the more extensive liver damage.

As implied by the name, AIH has many characteristics of an autoimmune disease, including:

- female predominance,

- hypergammaglobulinemia,

- association with specific HLA alleles,

- responsiveness to immunosuppression,

- and the presence of autoantibodies.

Because the clinical symptoms of AIH are nonspecific, being found in a variety of liver diseases (drug/alcohol-associated hepatitis, viral hepatitis, primary sclerosing cholangitis, etc), the diagnosis can be challenging. A set of diagnostic criteria for AIH has been published, and includes the presence of various autoantibodies, elevated total IgG, evidence of hepatitis on liver histology, and absence of viral markers.

The combination of autoantibody serology, specifically anti-SMAs and anti-F-actin antibodies with liver histology, and thorough clinical evaluation are useful in the evaluation of patients with suspected autoimmune hepatitis.

What are autoantibodies associated with AIH?

There are several autoantibodies associated with AIH, although the most common is anti-smooth muscle antibody (anti-SMA). SMAs are generally identified by indirect immunofluorescence using a smooth muscle substrate. The antigen specificity of anti-SMAs in the context of AIH has been identified as filamentous-actin (F-actin).