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What is IgA nephropathy can occur in - and explain in detail?
Mesangial proliferative glomerulonephritis -

It is characterized by the proliferation of mesangial cells
It is classified according to the predominant type of immunoglobulin present in the glomerulus.

(a) IgG in the mesangium -

Increased no. of mesangial cells in the glomeruli
Immunoflorescence shows the presence of IgG and C3 in the mesangium.

(b) IgA in the mesangium (IgA nephropathy or Berger's disease)

It is characterized by focal segmental or diffuse proliferation of mesangium.
Immunoflorescence shows the presence of IgA deposits in the mesangium C3 is also present.