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# IgA nephropathy and IgA vasculitis

- Lesion

The commonest glomerulonephritis in the world: abnormal IgA1 collects in the mesangium and causes recurrent bloody urine.

Updated Nov 12, 2026

## Key points

- Gross hematuria within a day or two of a respiratory or gut infection (synpharyngitic), young adults.
- IF: mesangial IgA, often with C3; serum complement is normal.
- LM: anywhere from normal to mesangial proliferation, segmental sclerosis or crescents.
- Pathogenesis in four hits: galactose-deficient IgA1, antibodies against it, complexes, mesangial deposition.
- IgA vasculitis: palpable purpura on legs and buttocks, abdominal pain, arthritis and nephritis.
- Many patients progress slowly to chronic kidney disease over two decades.

## What it is

In IgA nephropathy, IgA1 molecules lacking galactose in their hinge region trigger autoantibodies; the resulting complexes lodge in the mesangium and activate complement through the alternative and lectin pathways. The same deposits plus a small-vessel vasculitis of skin, gut and joints define IgA vasculitis (Henoch-Schönlein purpura), mostly in children. Secondary IgA deposits occur in celiac disease and cirrhosis, where IgA clearance is reduced.

## Structure

- Mesangial widening with more cells and matrix; some cases show segmental proliferation or sclerosis.
- Immunofluorescence shows dominant or codominant IgA in the mesangium.
- Electron microscopy shows electron-dense mesangial deposits.
- In IgA vasculitis the skin shows leukocytoclastic vasculitis with IgA in small vessels.

## Step by step

- Genetic and mucosal factors increase production of galactose-deficient IgA1.
- The exposed hinge region is recognized by IgG or IgA autoantibodies.
- Immune complexes form in the blood and are poorly cleared.
- They lodge in the mesangium and activate complement via the alternative and lectin pathways.
- Mesangial cells proliferate and release cytokines; hematuria follows, with proteinuria in more active disease.

## Hematuria after an infection: two diseases compared

| Feature | IgA nephropathy | Postinfectious GN |
| --- | --- | --- |
| Timing after infection | One or two days | One to several weeks |
| Complement | Normal | Low C3 |

## Clinical notes

- Painless episodes of brown urine in a young man after a cold; between episodes, microscopic hematuria persists.
- IgA vasculitis in a child: purpura, colicky pain (risk of intussusception), arthralgia and hematuria.

## Memory aid

IgA comes the same week as the cold; the streptococcal humps come weeks later.

## Test yourself

Think of the answer, then tap the question to see it.

## What is abnormal about the IgA in IgA nephropathy?

IgA1 molecules lack galactose in the hinge region, which makes them antigenic and prone to forming complexes.

## How does the timing of hematuria help separate IgA nephropathy from postinfectious GN?

IgA nephropathy follows the infection within a day or two; postinfectious GN appears after one to several weeks.

## Which two diseases cause secondary mesangial IgA deposits?

Celiac disease and cirrhosis (reduced hepatic clearance of IgA).

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## Elsewhere in DurgiPedia

- [Type III hypersensitivity (immune complexes)](https://durgical.com/en/immunology/type-iii-hypersensitivity/)
- [Mucosa-associated lymphoid tissue (tonsils, Peyer patches)](https://durgical.com/en/immunology/mucosa-associated-lymphoid-tissue/)

## Connections

## Related 5

- [Postinfectious glomerulonephritis](https://durgical.com/en/general-pathology/postinfectious-glomerulonephritis/)
- [Glomerular diseases: comparison table](https://durgical.com/en/general-pathology/glomerular-diseases-comparison-table/)
- [Hereditary glomerular disease: Alport, thin GBM, Fabry](https://durgical.com/en/general-pathology/hereditary-glomerular-disease-alport-thin-gbm-fabry/)
- [Immune-complex small-vessel vasculitis](https://durgical.com/en/general-pathology/immune-complex-small-vessel-vasculitis/)
- [Coeliac disease](https://durgical.com/en/general-pathology/coeliac-disease/)

## Related conditions 4

- [IgA nephropathy](https://durgical.com/en/conditions/iga-nephropathy/)
- [IgA vasculitis (Henoch–Schönlein purpura)](https://durgical.com/en/conditions/iga-vasculitis/)
- [Coeliac disease](https://durgical.com/en/conditions/coeliac-disease/)
- [Cirrhosis](https://durgical.com/en/conditions/cirrhosis/)

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Educational content on the mechanisms of disease; not a diagnostic or treatment guide and no substitute for a health professional's advice. No doses are given.

Sources: [IgA nephropathy: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/000466.htm) (MedlinePlus, US National Library of Medicine, 2025).

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Representative model, for educational purposes only.

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License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/general-pathology/iga-nephropathy-and-iga-vasculitis/