IgA nephropathy and blood in the urine after infection
IgA nephropathy is inflammation of the kidney's filters (glomerulonephritis) caused by deposition of IgA, an antibody, in the mesangium, the supporting tissue between the capillary loops.
Remember
In IgA nephropathy, visible haematuria (blood you can see in the urine) usually starts 1 to 2 days after a sore throat or similar illness, and serum C3 and C4 (complement proteins) are typically normal. Post-streptococcal glomerulonephritis comes weeks later, typically with a low C3.
Memory aid
picture the mesangium as the scaffold between the capillary loops: IgA settles on the scaffold, not on the filter wall
Use the scaffold picture to recall the site of the IgA deposits.
IgA nephropathy is also called Berger disease. The mesangium is the supporting tissue between the capillary loops of each glomerulus. IgA immune complexes (clumps of antibody bound to other molecules) are deposited there. The deposits activate the mesangial cells and set off local inflammation, which damages the nearby capillary wall, so red blood cells leak into the urine.
IgA is the antibody that normally protects mucosal surfaces (the moist linings of the nose, throat and gut). That is why a mucosal infection, typically an upper respiratory tract infection, can trigger an episode. Visible haematuria (blood you can see in the urine) usually starts 1 to 2 days after the infection. This is called synpharyngitic haematuria, meaning blood in the urine that comes with or just after a sore throat. It may recur, and it may also be invisible, found only on testing.
On a biopsy, immunofluorescence (a stain that lights up antibody deposits) shows granular IgA in the mesangium. Blood tests usually show normal C3 and C4, which are complement (a group of immune proteins). Serum IgA may be raised.
Post-streptococcal glomerulonephritis mimics IgA nephropathy and follows a streptococcal infection, such as a streptococcal sore throat. It appears weeks after the infection, typically with a low C3. Timing and complement help to tell the two apart.
Key facts
- Site
- IgA immune complexes are deposited in the mesangium, between the capillary loops of the glomerulus.
- Timing
- visible haematuria usually starts 1 to 2 days after a mucosal infection.
- Complement
- serum C3 and C4 are typically normal.
- Biopsy
- immunofluorescence shows granular IgA in the mesangium.
- Serum IgA
- it may be raised.
- Mimic
- post-streptococcal glomerulonephritis follows the infection by weeks, typically with a low C3.
Exam traps
- A markedly low C3 argues against IgA nephropathy, because it is the classic finding in post-streptococcal glomerulonephritis, which appears later after the infection.
- Immune complex glomerulonephritis after a streptococcal infection describes post-streptococcal glomerulonephritis, which typically arises after a delay and with a low complement level, unlike IgA nephropathy.
- Ischaemic damage from chronic hypertension does not explain IgA nephropathy, because it causes hypertensive nephrosclerosis, a scarring process that is not immune-mediated.
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