IgA is a glycoprotein with multiple O-glycans. Under-O-glycosylation of the hinge in IgA in patients with IgA nephropathy (IgAN) is reported. The development of IgAN is frequently preceded by episodes of upper respiratory tract infections such as tonsillitis. Therefore, the tonsils may be related to the pathogenesis of IgAN. However, the mechanism of underglycosylation in tonsillar IgA has not yet been fully elucidated. Since O-glycans in IgA are produced by glycosyltransferases, we hypothesized that dysregulation of the enzymes is associated with underglycosylation.