The complement cascade can be initiated by three distinct pathways: the classical, lectin, and alternative pathways. The classical and lectin pathways are triggered when recognition molecules bind to structures such as antibody complexes and carbohydrates on pathogen surfaces, leading to the activation of their associated proteases C1s/C1r and MASP-1/2. These proteases cleave C4 and C2, generating the C3 convertase (C4b2b), which then processes C3 into the anaphylatoxin C3a and the opsonin C3b. Accumulation of C3b induces the formation of the C5 convertase, which cleaves C5 into C5a and C5b. The subsequent interaction of C5b with C6, C7, C8, and C9 leads to the assembly of a lytic pore, known as the terminal complement complex (TCC). The alternative pathway is initiated when factor B (FB) interacts either with water-hydrolyzed C3, C3(H₂O), or with deposited C3b to form the C3(H₂O)Bb or C3bBb C3 convertases following factor D (FD) cleavage. These proteases, and in particular, the surface-bound and properdin/factor P–stabilized (FP-stabilized) C3bBbP C3 convertase act as an amplification loop for complement, generating most of the activated C3 fragments regardless of the initiating pathway.