Fig. 4.

Complement pathway. C3 convertase generated either by the classical, lectin, or alternative pathway, cleaves C3 into C3a and C3b, and results in C3b amplification loop. Cascade then proceeds to the activation of a C5 convertase of the classical (C4b2aC3b) or alternative (C3bBbC3b) pathway. Further, C5 convertase initiates the activation of late components of the complement system to form membrane attack complex (MAC). MAC formation is the final event occurring downstream of complement cascade inducing dysfunction of the target cells by forming membrane-spanning pores. Both complement factor H (CFH) and factor I (CFI) promote the cleavage of C3b to its inactive form and act as negative regulators. Genetic studies have identified associations of multiple SNPs associated complement genes including CFH, CFI, CFB, C2, C3, and C9, and AMD susceptibility