# Complement system

The complement system is a squad of more than 30 blood and cell-surface proteins that form a fast-acting arm of your innate immune system. It switches on through three routes that all converge: the classical (antibody-triggered), lectin, and alternative pathways. Each leads to cutting two proteins (C3 and C5) and, finally, building a 'membrane attack complex' (C5b-9) that punches holes in targets. Its main jobs: tagging germs for eating (opsonization), directly bursting some bacteria and viruses, calling in inflammatory cells with alarm signals (the anaphylatoxins C3a and C5a), and clearing away dead-cell debris. With age, complement goes out of tune. It can both weaken and, at the same time, simmer at a chronic low-grade level. That dysregulation is implicated in age-related macular degeneration (AMD), where variants in a gene called complement factor H (CFH) strongly shift risk, and in brain inflammation, where complement-driven pruning of synapses may go too far in conditions like Alzheimer's.

## Sources

- Zheng R, Zhang Y, Zhang K, Yuan Y, Jia S, Liu J. (2022). The Complement System, Aging, and Aging-Related Diseases. International Journal of Molecular Sciences. https://doi.org/10.3390/ijms23158689

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_Canonical: https://longevity-switzerland.com/en/glossary/complement-system · Part of Longevity Cities · Updated 2026-06-22_