IgA nephropathy is a common cause of glomerulonephritis. The presence of aberrantly galactosylated IgA1 and production of glycan-specific antibodies initiates mesangial deposition of immune complexes and subsequent tubulointerstitial injury. In this Review, Kar Neng Lai summarizes the latest findings in the pathogenesis of IgA nephropathy, including genetic factors and mesangial-derived mediators that lead to podocyte and tubulointerstitial injury via mesangio–podocytic–tubular crosstalk.