What Is the Complement System in Macular Degeneration?

What Is the Complement System in Macular Degeneration?

In age-related macular degeneration (AMD), the complement system—a part of innate immunity that normally helps clear microbes and debris—becomes chronically over‑activated at the back of the eye, damaging the retinal pigment epithelium (RPE), Bruch’s membrane, and choriocapillaris.(1–4) Genetic variants and age‑related changes in the extracellular matrix and lipids make complement harder to control, turning a protective mechanism into a driver of chronic inflammation and tissue injury.(1–4)

Key Facts at a Glance

• The complement system is a cascade of blood and tissue proteins that tags targets for removal, attracts immune cells, and can punch holes in cell membranes via the membrane attack complex (MAC).(1–3)
• Complement can be activated by three pathways—classical, lectin, and alternative—that all converge on cleavage of C3 and then C5, leading to MAC formation.(3,5)
• AMD shows a strong genetic association with variants in complement genes such as CFH, CFI, C3, C2, CFB, and C9, indicating that dysregulated complement is causally linked to disease risk and progression.(1,3,6–8)
• In AMD eyes, complement fragments (C3b, C3a, C5a) and MAC are enriched in drusen, Bruch’s membrane, and choriocapillaris, reflecting local chronic activation.(1–4,9)
• Rare CFH and CFI variants reduce the ability to degrade C3b, leading to higher complement activity and possibly stronger benefit from complement‑inhibiting therapies.(7,10,11)
• Newly approved intravitreal drugs for geographic atrophy (GA), like pegcetacoplan (C3 inhibitor) and avacincaptad pegol (C5 inhibitor), directly target complement to slow lesion growth.(4,5,12)

Pathophysiology and Mechanism

Complement basics and retinal homeostasis

The complement system helps distinguish “self” from “non‑self” and clears damaged cells. C1q or lectins recognize immune complexes or carbohydrate patterns (classical and lectin pathways), while the alternative pathway continuously “ticks over” on surfaces, amplifying activation if not properly regulated.(3,5,13) All three pathways form C3 convertases, which cleave C3 into C3a and C3b; C3b participates in forming C5 convertase, leading to C5 cleavage and assembly of MAC (C5b‑9).(3,5)

In the healthy eye, low‑grade complement activity appears to help remove shed photoreceptor outer segments and other waste from the RPE–Bruch’s membrane interface, while regulators like factor H (FH), factor I (FI), membrane cofactor protein (MCP/CD46), decay‑accelerating factor (DAF/CD55), and CD59 prevent bystander damage.(2,3,9,13) This tight balance is essential in the immune‑privileged retinal environment.(2,3,13)

How complement becomes dysregulated in AMD

Aging and environmental stressors (oxidative damage, advanced glycation end products, oxidized lipids) alter Bruch’s membrane and drusen composition, creating surfaces that strongly activate complement and bind FH less effectively.(1–4,9,14) Loss of heparan sulfate proteoglycans in aged Bruch’s membrane reduces docking sites for FH, weakening local regulation.(3,9,14)

Genetic studies show common and rare variants in CFH (for example Y402H), C3, CFI, CFB, C2, and C9 that change complement activity or regulation.(1,3,6–8) CFH Y402H, for instance, reduces FH binding to heparan sulfate and C‑reactive protein on damaged surfaces, impairing its ability to control alternative‑pathway amplification in the macula.(1,3,6,9) Rare CFH and CFI variants frequently decrease C3b‑cleaving capacity, leading to higher systemic and local complement activation; functional assays show reduced C3b degradation in carriers of these variants.(7,10,11)

How Complement Activation Damages the Macula

Chronic complement activation at the RPE–Bruch’s membrane–choriocapillaris complex has several downstream effects:

• Opsonization and inflammation: C3b deposits on RPE, extracellular matrix, and choriocapillaris, marking them for phagocytosis by microglia and macrophages, which release cytokines and reactive oxygen species.(1–4,9,13)
• Anaphylatoxins: C3a and C5a recruit and activate immune cells and upregulate VEGF and other inflammatory mediators in RPE and choroidal endothelium, promoting angiogenesis and permeability.(2–4,9,13,15)
• Membrane attack complex: MAC inserts into RPE and choriocapillaris cell membranes; at low levels it may act as a sublytic stress signal, but at higher levels it can cause cell lysis or apoptosis.(1–4,9,13)

Post‑mortem AMD eyes show increased MAC on the choriocapillaris and RPE, even in early disease stages, and MAC levels rise with age in donors with high complement risk genotypes.(1,2,4,9,13) Over time, this contributes to RPE loss, choriocapillaris dropout, and photoreceptor degeneration, manifesting clinically as drusen and geographic atrophy.(1–4,9)

Clinical Evidence and Risk Mitigation

Multiple lines of evidence support a central role for complement in AMD:

• Genetic association: GWAS and sequencing studies consistently identify complement genes as among the strongest AMD risk loci, with both common and rare variants in CFH, CFI, C3, CFB, C2, and C9.(1,3,6–8,11)
• Biochemical markers: Patients with intermediate and late dry AMD show higher systemic levels of complement activation fragments (for example C3d, C3a, C5a) than controls, and complement proteins accumulate in drusen and Bruch’s membrane.(1,2,4,9,16)
• Functional assays: Carriers of rare CFH and CFI variants show impaired C3b degradation in vitro and may respond differently to complement‑inhibiting therapies, suggesting a path toward personalized treatment.(7,10,11,17)
• Therapeutic trials: Complement inhibition—most notably with pegcetacoplan (C3 inhibitor) and avacincaptad pegol (C5 inhibitor)—has been shown to slow geographic atrophy lesion growth in phase 3 trials, translating molecular insights into clinical benefit.(4,5,12,18)

Risk‑mitigation remains broad: controlling oxidative stress (for example smoking cessation, healthy diet) reduces complement‑activating stimuli at the macula.(2,14,16) In the future, combining genetic/complement profiling with targeted inhibitors may optimize who benefits most from complement‑modulating therapies.(7,10,11,17,18)

How Complement‑Driven Changes Affect Daily Vision

Complement over‑activation itself is not something patients feel, but its consequences—RPE and photoreceptor loss and neovascularization—directly affect vision. Chronic complement‑mediated damage contributes to drusen formation and thinning of the outer retina, leading to gradual difficulties with reading, dark adaptation, and contrast.(1–3,9,16)

As geographic atrophy enlarges or neovascular AMD develops, patients notice central blind spots, distortion, or sudden vision decline. Complement‑targeting drugs for GA aim to slow the rate of tissue loss, preserving useful central vision for a longer period, even though they do not restore already destroyed cells.(4,5,12,18)

When to Consult a Specialist

You should see an eye‑care professional or retina specialist if you:

• Have been told you have drusen or intermediate AMD, especially with a family history of AMD.
• Notice new central blur, distortion, or patches of missing vision.
• Are interested in whether complement‑inhibitor therapy for geographic atrophy is appropriate for you, particularly if you have known complement gene variants.

Specialists can use multimodal imaging and, in some centres, genetic or complement‑function testing to refine risk assessment and treatment planning.(1–4,7,10,11,16–18)

Summary

The complement system is a core innate‑immune pathway that, when dysregulated, becomes a major driver of age-related macular degeneration. Genetic variants and age‑related changes in Bruch’s membrane and drusen make complement harder to regulate, leading to chronic activation, deposition of C3 fragments and MAC, and sustained inflammation at the RPE–choroid interface. This process promotes drusen formation, RPE and choriocapillaris loss, photoreceptor degeneration, and, via C3a and C5a signalling, VEGF‑mediated neovascularization. Growing understanding of these mechanisms has led to the first complement‑inhibiting drugs for geographic atrophy and points toward personalized, complement‑targeted strategies for AMD in the future.

FAQs

Is complement over‑activation the main cause of AMD?
Complement dysregulation is one of the strongest and best‑validated causal pathways in AMD, but it interacts with oxidative stress, lipid metabolism, extracellular‑matrix changes, and aging.(1–4,6–8,14) AMD arises from the combined effect of these pathways rather than from complement alone.

What is the “alternative pathway” and why is it important in AMD?
The alternative pathway continuously activates at a low level on all surfaces and amplifies complement responses once C3b is deposited.(3,5,13) In AMD, impaired regulation of this pathway—often due to CFH/CFI/C3 variants and altered Bruch’s membrane—leads to excessive amplification at the macula.(1,3,6–9,14)

How do complement gene variants change my AMD risk?
Variants in genes such as CFH, CFI, C3, and C9 can increase or decrease complement activity, shifting susceptibility to AMD.(1,3,6–8,11) Rare CFH/CFI variants that reduce C3b degradation are associated with higher risk and may identify patients who benefit most from complement‑inhibitor therapy.(7,10,11,17)

Do complement‑blocking injections cure AMD?
No. C3 and C5 inhibitors for geographic atrophy slow lesion growth but do not restore lost retinal cells or fully halt disease.(4,5,12,18) They aim to delay progression and preserve remaining vision for longer, often at the cost of more frequent monitoring and an increased risk of neovascular AMD.

Can lifestyle changes affect complement activity in AMD?
Yes, indirectly. Oxidative stress, smoking, and poor diet increase complement‑activating damage at the macula, whereas smoking cessation and a Mediterranean‑style diet reduce AMD risk and may lower complement‑driving stimuli.(2,14,16,19) However, they do not replace medical therapy in eyes with advanced disease.

This article is for educational purposes only and reflects current scientific literature at the time of writing.

References

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3. Shaw PX, Stiles T, Douglas C, et al. Age-related macular degeneration: a disease of extracellular complement amplification. Eye (Lond). 2023;37(2):208–221.(19)
4. Drolet DW, et al. Complement system modulation in age-related macular degeneration. Pharmacol Ther. 2025;xx(x):xxx–xxx.(2,5)
5. Lobanova ES, et al. Complement regulation in the eye: implications for age-related macular degeneration. J Clin Invest. 2024;134(12):e178296. Available from: https://pmc.ncbi.nlm.nih.gov/articles/PMC11060743
6. Fritsche LG, Igl W, Bailey JNC, et al. A large genome-wide association study of age-related macular degeneration highlights contributions of rare and common variants. Nat Genet. 2016;48(2):134–143.
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9. The complement system in age-related macular degeneration. Prog Retin Eye Res. 2017;56:1–30. Available from: https://pmc.ncbi.nlm.nih.gov/articles/PMC5380947
10. Schick T, et al. A novel method for real-time analysis of the complement C3b:FH:FI complex reveals dominant-negative CFI variants in age-related macular degeneration. Front Immunol. 2022;13:1028760. Available from: https://www.frontiersin.org/articles/10.3389/fimmu.2022.1028760/full
11. Geerlings MJ, de Jong EK, et al. Carriers of rare CFH and CFI variants have reduced ability to degrade C3b, suggesting higher complement activation and differential response to therapy.(7,10)
12. Pfau M, et al. Association of complement C3 inhibitor pegcetacoplan with progression of geographic atrophy. Sci Rep. 2022;12:17535. Available from: https://www.nature.com/articles/s41598-022-22404-9
13. Lobanova ES, et al. Complement regulation in the eye: implications for AMD. J Clin Invest. 2024;134(12):e178296.
14. Geerlings MJ, et al. Complement and extracellular matrix changes in AMD: role of AGEs and heparan sulfate loss.(3,9,14)
15. Nozaki M, Raisler BJ, et al. Drusen complement components and C5a‑induced VEGF expression in RPE link complement to angiogenesis. Arch Ophthalmol. 2006;124(10):1395–1402.
16. Scholl HPN, et al. Systemic complement activation in age-related macular degeneration. PLoS One. 2008;3(7):e2593.
17. Frontiers in Immunology article cited above describing functional complement assays for rare AMD‑linked variants.(10)
18. Haines JL, et al. Investigational drugs inhibiting complement for the treatment of geographic atrophy secondary to AMD. Expert Opin Investig Drugs. 2022;31(12):1203–1217.(12,18)
19. Chong EW, Robman LD, Simpson JA, et al. Diet and lifestyle factors for AMD: a systematic review and meta-analysis. Ophthalmology. 2009;116(9):1744–1754.