Antibodies against myelin are observed in neuropathies associated with IgM monoclonal gammopathy. This gammopathy is usually benign but rarely can be malignant (Waldenström's disease). The concentration of immunoglobulin IgM is usually low (< 5 g/L) and the monoclonal band in electrophoresis requires a lot of care to identify.
Anatomically, demyelination of the peripheral nerves as a result of Schwann cell metabolism disorders results in a reduced rate of nerve impulse conduction. Demyelination is directly attributable to the action of monoclonal IgM antibodies against myelin antigens. There are several target antigens:
- MAG (Myelin Associated Glycoprotein), although quantitatively small (0.1% of peripheral nervous system myelin proteins), it is the target for more than 75% of myelin antibodies. It is a 100 kDa glycoprotein belonging to the immunoglobulin family and plays a key role in attaching the myelin sheath to the cell's axon.
- P0 is a substance that makes up 60% of myelin proteins and is a very weak immunogen.
- SGPG (sulfo-glucuronyl-paragloboside) and SGLPG (sulpho-glucuronyl-lactosyl paragloboside) are nerve membrane substances that are specific to the adult peripheral nervous system.
Of the myelin antibodies:
- 70% are anti-MAG and anti-SGPG/SGLPG and recognize the same epitope: 3-glucuronyl sulfate, which is found in NK cells and is called HNK1. HNK1 is present in MAG and a few other myelin glycoproteins and in SGPG. MAG is more strongly bound.
- 20% is binding in SGPG/SGLPG only.
- 10% is MAG specific.
Antibodies against myelin are useful in the diagnosis of sensory-motor neuropathies (polyneuritis) that have a polymorphic clinical expression with predominantly sensory symptoms.
The beginning and the development are chronic and progressive. Symptoms of symmetric nerve sensation are observed. These diseases particularly affect men over the age of 60, starting with lower extremity hallucinations accompanied by hand tremors and ataxia.
Treatment of these neuropathies is relatively difficult with chloraminophenε being the best drug although it is effective in up to 30% of cases. Plasma coagulation and high-dose immunoglobulins and interferon are also used.
See also: MAG Antibodies