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What is C3 glomerulopathy?

C3 glomerulopathy is a rare kidney disease affecting 2 to 3 out of every 1 million people. An estimated 50 percent of these patients may develop kidney failure if they don’t receive effective treatment.

C3 or “complement component 3” is a protein in the blood that’s important to your immune system. Glomerulopathy refers to damage in your glomeruli. Glomeruli are groups of tiny blood vessels in the kidneys that help filter blood and make urine. With C3G, this C3 related part of your child’s immune system becomes overactive and causes inflammation in the glomeruli (kidney filters).

Before 2013, the terms membranoproliferative glomerulonephritis (MPGN) type I, II, or III and mesangioproliferative glomerulonephritis were used. However, we now classify this group of conditions as C3G.

Symptoms & Causes

What are the symptoms of C3G?

People with C3G may experience:

  • Blood in urine (hematuria)
  • Protein in urine (proteinuria)
  • Edema or swelling, either localized in one area or generalized (this can present as weight gain)
  • High blood pressure
  • Kidney failure (low urine output, fatigue)
  • Eye problems
  • Other infections

What causes C3G?

C3G occurs when the complement system is too active. The complement system consists of proteins in the blood that are part of your immune system. They help to fight foreign invaders like bacteria and viruses.

If your child has C3G, their complement system is working harder than normal, which damages complement 3 (C3) proteins. Damaged parts of C3 proteins then become trapped in the glomeruli (kidney filters). This makes it more difficult for glomeruli to filter blood and create urine. As a result, toxins may start to build up in the blood. Without treatment to help slow down the damage, C3G will continue to hurt the kidneys and hinder their functioning. Kidney failure may occur over time.

In most cases of C3G, healthcare providers aren’t sure what exactly causes your child’s complement system to stop working as it should. In some cases, mutations in C3, CFH, and other complement system genes cause C3G. Researchers suspect that autoantibodies may also affect the complement system. Autoantibodies are proteins made by the immune system that target normal body tissues by mistake.

Diagnosis & Treatments

How is C3G diagnosed?

We may use various tests to assess your child’s kidney health. These tests include:

  • Blood tests: To evaluate kidney function (eGFR) and the complement system
  • Urine analysis: To evaluate the amount of protein or blood in urine
  • Kidney biopsy: A kidney biopsy can help determine what type of C3G your child has, according to the absence or presence of dense deposits
  • Genetic testing and complement functional panel: Blood tests like these further elucidate causes of C3G

How is C3G treated?

Treatment for C3G includes managing blood pressure, proteinuria, and cholesterol with medications; lifestyle changes; and complement-targeting therapies.

Currently, it is difficult to predict which people will progress to kidney failure and which will not.

Medications

Medications used to slow kidney damage from C3G may include the following:

  • ACE inhibitors and ARBS
  • Corticosteroids and immunosuppressive drugs
  • Complement inhibitors

In March 2025, the FDA approved iptacopan for the treatment of C3G. Iptacopan is a complement factor B inhibitor. This drug works by binding to factor B, which regulates the breakdown of C3.

In July 2025, the FDA approved pegcetacoplan for the treatment of C3G. Pegcetacoplan is a complement C3 inhibitor. It works by binding to and inhibiting C3, which helps regulate C3 breakdown. This approval also included pegcetacoplan for the treatment of immune-complex membranoproliferative glomerulonephritis (IC-MPGN). IC-MPGN is similar to C3G but is a distinct disease.

What is the long-term outlook for C3G?

Until recently, the prognosis of this rare disease was poor. But now that we have effective new complement-directed therapies, there is reason to believe that patients can preserve their kidney function longer and avoid dialysis. Your child’s quality of life is our top priority, and we will do everything we can to support your family’s care journey.

How we care for C3G at Boston Children’s Hospital

Our team in the Division of Nephrology at Boston Children’s Hospital has extensive experience treating C3G over the years. We have successfully used the recent complement-directed therapies in children with C3G. We also participate in multicenter research studies on C3G and continue to move the needle forward to improved care and outcomes.

The newly established GlomCare Clinic is focused solely on disorders of the glomeruli, groups of tiny blood vessels in your kidneys that filter blood and make urine.