The major findings of this study were that lower serum levels of IgG2, and total IgG (which can be partially explained by lower IgG2 levels) were detected in drug–naïve people recently diagnosed with CIS compared with HC, and that there was an association between higher serum IgG3 levels at CIS diagnosis and earlier conversion to MS. Here, IGHG3 is linked to in situ carcinoma.