IgG1 was the most common isotype for anti-PLP181−230 antibodies from MS patients; however, several patients had high levels of IgG2, IgG3, or IgG4 antibodies, suggesting that the microenvironment in which the anti-PLP antibody producing B cells mature and class-switch can vary from patient to patient. This evidence concerns the gene IGHG3 and myeloid sarcoma.