Sera from patients with chronic active hepatitis contain antibodies to smooth muscle antigens that are detectable by IIF and bind smooth muscle of all organs. The major antigen of the smooth muscle is actin. The antibodies belong mainly to the IgG class, but they can also be found in the IgM class. The test is performed on unfixed cryostat sections of rodent stomach as substrate. Smooth muscle antibodies arc found in 40-70% of patients with active chronic hepatitis, with lower titers found in 50% of patients with primary biliary cirrhosis, and 28% of patients with cryptogenic cirrhosis. These antibodies are also found at low titer in patients with acute viral hepatitis, infectious mononucleosis, asthma, yellow fever and malignant tumors (carcinomas of the ovary, malignant melanoma). They have been found in less than 2% of the normal population.
The presence of antibodies to cardiac striated muscle in patients with cardiomyopathy may help to distinguish autoimmune myocarditis from other forms of myocarditis (e.g. viral). This finding, in turn, may influence therapy. These antibodies are most readily detected by IIF with sections of rat heart as tissue substrate and bind to fibrillary antigens as well as to sarcolemma or subsarcolemma antigens. The most severe cases of myocarditis or dilated cardiomyopathy have been associated with high titers of IgG antibodies specific for cardiac and not skeletal striated muscle. Many normal individuals may have low titers of antistriated muscle antibodies, but they are generally of the IgM class and react with both skeletal and cardiac striated muscle.
Autoantibodies to endomysial membranes, the outside lining of smooth muscle bundles, are found in patients with dermatitis herpetiformis and with celiac disease, and are primarily IgA. The major test is IIF using the lower part of monkey esophagus as tissue substrate. It appears to be a more specific and sensitive marker of celiac disease than testing for reticulin autoantibodies. In a few patients with celiac disease that have an IgA immunodeficiency, the antibody to endomysial antigens were found to be IgG. Isotype-specific anti-immunoglobulins conjugated to FITC are used as specific reagents to determine the relevant autoantibodies.
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