What Is the Four-Hit Model of IgA Nephropathy? Explained

The “four-hit model” is a leading framework that helps explain the biology of how IgA nephropathy develops. It is not a diagnostic test, but rather a useful step-by-step description of the biological process that can lead to kidney injury ^[1][2].

It is important to know that this disease is complex. Genetic, mucosal, and environmental factors also influence who develops it, and not everyone with the first “hit” will go on to develop kidney disease ^[3].

The Four Hits Explained

1. Hit 1: Altered IgA Production. The immune system produces an antibody called Immunoglobulin A1 (IgA1) to help fight infections. In people with IgA nephropathy, the body produces an increased proportion of IgA1 molecules that are missing certain sugar molecules (galactose) in a specific area. This is called galactose-deficient IgA1 ^[4][1].
2. Hit 2: Autoantibodies Target the Altered IgA. Some people’s immune systems produce autoantibodies—antibodies that mistakenly react to the person’s own tissues or molecules. In this case, these autoantibodies recognize the altered sugar pattern on the galactose-deficient IgA1 and bind to it ^[4][1].
3. Hit 3: Immune Complexes Form. When the autoantibodies bind to the galactose-deficient IgA1 in the bloodstream, they link together to form clusters called circulating immune complexes ^[4][1].
4. Hit 4: Deposition in the Kidneys. Some of these circulating immune complexes travel to the kidneys and deposit (build up) in the mesangium. The mesangium is the supportive tissue located between the delicate capillary loops of the kidney’s filtering units (glomeruli) ^[4][1].

How the Four Hits Can Lead to Kidney Damage

Once the immune complexes accumulate in the mesangium, they can trigger a cascade of events that injure the kidneys:

• Complement Activation and Inflammation: The deposited complexes can activate the complement system, a part of the immune system that promotes inflammation. This can cause the cells in the mesangium to become irritated, multiply, and release inflammatory chemicals ^[5][6].
• Hematuria (Blood in the Urine): Inflammation can damage the glomerular filtration barrier. When this barrier is damaged, red blood cells can leak into the urine. This is why many patients experience visible or microscopic blood in their urine, especially during active inflammation ^[7][8].
• Proteinuria (Protein in the Urine): Inflammation can also injure podocytes (specialized filtering cells) and alter the pressure in the filter, allowing essential proteins to leak into the urine ^[9][10]. Persistent protein in the urine is a central marker of ongoing kidney damage and a key predictor of long-term risk ^[11][12].
• Scarring (Sclerosis and Fibrosis): Over time, chronic immune-mediated injury can lead to permanent damage. This includes scarring within the filtering units (segmental glomerulosclerosis) and in the surrounding kidney tissue (tubulointerstitial fibrosis), which progressively reduces the kidneys’ ability to filter waste ^[10][13].

What This Means for Your Diagnosis and Care

While the four-hit model explains the disease process, doctors do not routinely test your blood or urine for these four hits to make decisions about your care. The model cannot by itself predict your individual outcome ^[1][2].

Instead, a diagnosis of IgA nephropathy is usually confirmed with a kidney biopsy when clinically appropriate. The biopsy allows doctors to look directly at the IgA deposits and assess the extent of active injury and permanent scarring (often summarized as an Oxford MEST-C score) ^[14][2].

To estimate your risk and guide treatment, your care team will monitor your kidney function (eGFR), blood pressure, and the amount of protein in your urine (usually measured as a protein-to-creatinine or albumin-to-creatinine ratio).

Safety Note: Contact your clinical team if you notice new visible blood in your urine, severe swelling, a substantial drop in urine output, or shortness of breath. While dark urine can happen during a respiratory infection, it is always best to report new symptoms to your doctor.