Immune reactive C3d on the surface of myelin sheaths in neuropathy.

Immunofluorescence studies of sural nerve demonstrated immune reactive C3d and IgM on the surface of myelin sheaths in seven patients with neuropathy and an anti-myelin-associated glycoprotein (MAG) IgM M-protein. Similar deposits of C3d and sometimes IgM were found in four of six patients with acute or chronic inflammatory demyelinating polyneuropathy and in three of six patients with vasculitic neuropathy (including one with acquired immunodeficiency syndrome (AIDS)). C3d was not found in 80 patients with other peripheral nerve disorders except for two with metachromatic leukodystrophy. None of the C3d deposits contained immune reactive C3c implying substantial degradation of C3b. C3d is a sensitive index of complement activation in nerve and may be useful in classification of neuropathies.