The Complement System and AMD: How Immune Activity Damages the Retina
What the Complement System Does
The complement system is a network of proteins in your blood that helps your immune system fight infections and clear damaged cells. These proteins activate in a chain reaction, tagging invaders for destruction and cleaning up cellular debris. Under normal conditions, your body keeps this system in check with regulator proteins that prevent it from attacking healthy tissue.
In age-related macular degeneration (AMD), this regulation breaks down in the retina. The complement system begins attacking healthy retinal tissue, contributing to drusen formation, retinal pigment epithelium (RPE) damage, and ultimately vision loss.
When complement proteins activate without proper regulation, they form a structure called the membrane attack complex (MAC, also known as C5b-9). This complex punches holes in cell membranes. In the macula, uncontrolled MAC formation damages RPE cells and the tissue beneath them.
Complement activation products accumulate within drusen deposits. According to research published in PMC (2011), eyes from donors who carried the high-risk genetic variant showed69 percent higher levels of membrane attack complexcompared to eyes from donors with the protective variant. This demonstrates that complement-driven damage at the macula is measurably worse in genetically susceptible people.
The complement cascade has several key steps. C3 is a central protein that, when activated, splits into fragments that amplify the immune response. C5 is activated downstream of C3 and leads to MAC formation. Both of these steps are targets for AMD treatments.
Regulators like complement factor H (CFH) normally prevent excessive complement activation. When CFH is impaired by genetic variants, complement activity in the sub-RPE space goes unchecked, accelerating drusen formation and retinal cell damage.
The Genetic Link: CFH and AMD
In 2005, researchers identified a common genetic variant in the complement factor H (CFH) gene, called Y402H, as a major risk factor for AMD. According to Klein et al. (Science, 2005), people who carry two copies of the risk version of this gene face a7.4-fold increased likelihood of developing AMDcompared to non-carriers.
The population-attributable risk for CFH Y402H is estimated at25 percent, meaning roughly one quarter of all AMD cases can be attributed to this single genetic variant (PMC, 2007). This discovery reshaped the understanding of AMD as a disease with a strong immune component.
The Y402H variant changes a single amino acid in the CFH protein, impairing its ability to bind to protective molecules in the extracellular matrix of the macula. According to PMC (2018), this reduced binding causes impaired complement regulation in the space beneath the RPE.
Without proper CFH regulation, chronic low-grade complement activation persists at the macula. Over years, this ongoing immune activity drives drusen formation and RPE damage, setting the stage for geographic atrophy or wet AMD.
Beyond CFH, genome-wide studies have identified pathogenic variants in complement factor B (CFB), complement factor I (CFI), C2, and C3 that also modify AMD risk. Some variants increase risk, while others are protective. Your combined genetic profile across these genes shapes your overall susceptibility.
Genetic testing for AMD risk variants is available but is not yet standard practice. Your doctor may discuss genetic testing if you have a strong family history of AMD or if you are considering complement-targeting treatments.
Complement-Targeting Treatments for AMD
Pegcetacoplan, approved by the FDA in February 2023, targets complement protein C3. By blocking C3, this medication reduces the downstream cascade that leads to MAC formation and RPE damage. Your doctor injects it into your eye on a monthly or every-other-month schedule.
In clinical trials, pegcetacoplan slowed the growth of geographic atrophy lesions. It was the first drug ever approved for this condition. However, it does not restore vision that has already been lost and carries a risk of converting dry AMD to wet AMD.
Avacincaptad pegol, approved in August 2023, targets complement protein C5, a step further down the cascade from C3. By blocking C5, it prevents MAC formation while leaving earlier complement functions intact. Your doctor injects it into the eye on a monthly schedule.
Like pegcetacoplan, avacincaptad pegol slows GA lesion growth without improving visual acuity. The two drugs target different points in the cascade, giving your doctor options based on your response and risk profile.
The approval of complement-targeting drugs validates decades of research linking the immune system to AMD. For patients with geographic atrophy, these treatments offer the first chance to slow the disease beyond lifestyle changes and supplements.
Your doctor weighs the benefit of slower lesion growth against the treatment burden (regular eye injections) and the risk of wet AMD conversion. Starting treatment before the fovea is affected may preserve central vision longer.
Questions About the Complement System and AMD
No. Carrying the risk variant increases your likelihood of AMD but does not guarantee it. Other genes, lifestyle factors like smoking and diet, and environmental exposures all influence whether AMD develops. Many people with the risk variant live into old age without AMD.
Genetic testing can reveal whether you carry high-risk complement variants. However, the results do not change the standard monitoring or treatment approach for most patients. Your doctor may recommend testing if you have a strong family history or if complement-targeting treatments are being considered.
No supplement targets complement regulation in a clinically meaningful way. AREDS2 vitamins support retinal health through antioxidant pathways, not complement modulation. The complement-targeting treatments are prescription medications delivered by injection, not oral supplements.
Long-term safety data is still being collected. The known risk of wet AMD conversion requires ongoing monitoring. Side effects include inflammation inside the eye and infection risk common to all eye injections. Your doctor discusses these risks before starting treatment.
Researchers are studying additional complement targets and delivery methods, including sustained-release implants that could reduce the frequency of injections. Gene therapy approaches that would allow your eye to regulate complement activity on its own are also in development.
A family history of AMD increases your risk regardless of whether you know your genetic status. Get regular dilated eye exams after age 50. If you want more information, discuss genetic counseling with your eye doctor. Knowing your risk can motivate preventive steps like quitting smoking and maintaining a healthy diet.
Understanding the Science Behind Your Treatment
The complement system's role in AMD explains why the new treatments work and why research continues. Ask your eye doctor whether complement-targeting therapy is appropriate for your stage of AMD and what new options may become available in the coming years.