C9 (complement C9) is the pore-forming component of the membrane attack complex (MAC), a critical effector of the complement cascade 12345678. During complement activation triggered by classical, alternative, lectin, and GZMK pathways, C5 is cleaved into C5b, which initiates MAC assembly 91078. C9 associates with the C5b8 intermediate complex and polymerizes to complete the pore structure, inserting into target cell membranes to cause lysis 311512136. This mechanism enables pathogen elimination and strengthens adaptive immunity through signaling cascades 78. CD59, an abundant immune regulator, protects human cells by binding C8 and deflecting C9 β-hairpins to prevent membrane perforation and MAC polymerization 14. Similarly, disulfide-locked C9 variants and horse C9 inhibit MAC assembly through competitive binding mechanisms 15. Clinically, C9 deficiency impairs complement-mediated immunity, while dysregulation contributes to age-related macular degeneration. C9 represents a validated therapeutic target for conditions where MAC-mediated tissue damage occurs.