Immunoglobulin A (
IgA) is the most abundant isotype of
antibodies, provides a first line of defense at mucosal surfaces against pathogens, and thereby contributes to mucosal homeostasis.
IgA is generally considered as a non-inflammatory antibody because of its main function, neutralizing pathogenic virus or bacteria. Meanwhile,
IgA can induce
IgA-mediated diseases, such as
IgA nephropathy (IgAN) and
IgA vasculitis. IgAN is characterized by the deposition of
IgA and
complement C3, often with
IgG and/or
IgM, in the glomerular mesangial region, followed by mesangial cell proliferation and excessive synthesis of extracellular matrix in glomeruli. Almost half a century has passed since the first report of patients with IgAN; it remains debatable about the mechanism how
IgA antibodies selectively bind to mesangial region-a hallmark of IgAN-and cause glomerular
injuries in IgAN. Previous
lectin- and mass-spectrometry-based analysis have revealed that IgAN patients showed elevated serum level of undergalactosylated
IgA1 in O-linked
glycans of its hinge region, called
galactose-deficient IgA1 (Gd-IgA1). Thereafter, numerous studies have confirmed that the glomerular
IgA from IgAN patients are enriched with Gd-
IgA1; thus, the first hit of the current pathogenesis of IgAN has been considered to increase circulating levels of
Gd-IgA1. Recent studies, however, demonstrated that this aberrant glycosylation alone is not sufficient to disease onset and progression, suggesting that several additional factors are required for the selective deposition of
IgA in the mesangial region and induce
nephritis. Herein, we discuss the current understanding of the characteristics of pathogenic
IgA and its mechanism of inducing
inflammation in IgAN.