Myelin-associated glycoprotein (MAG) is a component of myelin in the peripheral and central nervous systems. High-titre IgM antibodies against MAG are associated with sensory-motor demyelinating peripheral neuropathy. Sensory symptoms tend to occur early in the disease, with motor symptoms appearing later. Antibodies against MAG are usually associated with the presence of a monoclonal IgM protein. About 50% of patients with monoclonal IgM gammopathies and peripheral neuropathies have detectable MAG antibodies.
Patients with sensory neuropathy may have low MAG antibody titers, usually in combination with high sulfatide antibody titers. Moderate titers of MAG antibodies have also been reported in patients with multiple sclerosis and inflammatory neuropathies.
In general, antibodies recognizing MAG react with a carbohydrate also present in an acid glycolipid, sulfo-glucuronyl paragloboside(SGPG). Initially, SGPG was used as an antigen to test the presence of MAG antibodies. However, in a recent study of patients with sensory-motor demyelinating neuropathy, patients with antibodies recognizing the whole MAG protein but not the SGPG were found and patients with antibodies recognizing the SGPG but not the MAG protein were found (most patients however had antibodies that recognize both SGPG and MAG).
Based on these findings, some laboratories perform two separate ELISA procedures, one using SGPG as an antigen and one using MAG as an antigen, to maximize detection of MAG antibodies. Further, for increased specificity, samples that give a positive result by ELISA are automatically tested with a MAG Western blot antibody confirmatory antibody.
Autoimmune peripheral neuropathy can be acute or chronic and can result in either axonal degeneration or demyelinating. Self-antibodies can be the result of either monoclonal gammopathies or inflammatory disorders. In the latter case, the antibodies are polyclonal. Inflammatory polyneuropathies include Guillain-Barre syndrome (GBS), chronic inflammatory demyelinating polyneuropathy (CIDP), GALOP syndrome (gait disorder, autoantibodies), and multifocal motor neuropathy (MMN). Guillain-Barre syndrome is acute at onset, whereas CIDP is chronic and progressive.