IgA1 immune complexes (IgA1-ICs) containing galactose-deficient IgA1 (GdIgA1) play a critical role in the formation of renal immune deposits in IgA nephropathy (IgAN). Previously, we found that patients with IgA myeloma (IgAMM) without renal IgA deposition exhibit unexpectedly elevated plasma levels of GdIgA1 and poly-IgA complexes. Despite these insights, the pathogenic O-glycosylation profile of IgA and the molecular composition of circulating IgA1-ICs in IgAN remain incompletely characterized.