in a patient with a sensory and motor peripheral polyneuropathy, macroglobulinemia, and amyloidosis, bone marrow cells reacted specifically in immunoflourescence with antibodies against light chain and μ heavy chain. Characterization of the paraproteins, however, disclosed two types of molecules, high molecular weight (HMW)-lgM and low molecular weight (LMW)-lgM, that could be separated easily by molecular sieve techniques. The two proteins were immunochemically identical and contained only light chains and μ heavy chains. The presence of LMW-IgM in the serum explained why a hyperviscosity state was not encountered. Correlated histochemical and ultrastructural studies revealed classical amyloid fibrils in Congo red positive areas of nerve biopsy, and amyloid was detected in blood vessel walls in fresh-frozen muscle.
Bigner DD, Olson WH, McFarlin DE. Peripheral Polyneuropathy, High and Low Molecular Weight IgM, and Amyloidosis. Arch Neurol. 1971;24(4):365–373. doi:10.1001/archneur.1971.00480340097011