Binding of swine IgM immunoglobulins to peripheral nerve myelin sheaths in electron microscopic immunocytochemistry

The IgM fraction of normal swine serum (NSS) was found to adhere to human peripheral nerve myelin sheaths from randomly selected neuropathies and control nerves in electron microscopic immunocytochemical assays. The reaction could be blocked by preincubation with undiluted fetal calf serum (FCS), whereas endoneurial background staining was already abolished by preincubation with 10% FCS. Only the IgM, but not the IgG or the IgA fraction of NSS adhered to peripheral nerve tissue. Such strong adherence of one of the components in an immunocytochemical assay may be a source of misinterpretation. Binding of swine IgM to human peripheral nerve myelin has not been described before and is of interest in the discussion about IgM binding to myelin-associated glycoprotein in IgM gammopathies.