In a US cohort, reduced C3 (20%), C4 (19%), or both (11%) were present in 103 IgG4‐RD patients and were associated with a more inflammatory phenotype, higher serum IgG4 levels, and IgG4‐related kidney involvement.32 Recently, a small study reported increased IgG4 G0 N‐glycans and IgG4 fucosylated N‐glycans in patients with IgG4‐RD (n = 12) compared with HCs (n = 8), with IgG4 non‐fucosylated N‐glycans decreased in those with hypocomplementaemia (n = 7).33 In line with this, we found a negative correlation between C3 and C4 levels and both IgG4 fucosylation, and IgG1/2/3 hybrid structures. This evidence concerns the gene C4A and immunoglobulin G4-related sclerosing disease.