Figure 2: Regulation and dysregulation of complement activity on host cells. C3b is generated by the classical, lectin, or alternative pathways. (a) To protect normal host cells, C3b is inactivated by membrane regulators, such as factor H (CFH) and membrane cofactor protein (MCP). Factor I (CFI) cleaves C3b into iC3b and other C3 degradation products with the activity of cofactor regulators. (b) If genetic and/or environmental risk factors reduce the efficiency of membrane complement regulators, C3 convertase (C3bBb) can accumulate on cell surfaces, creating a C3b amplification loop. Formation of C5 convertase (C3bBbC3b) triggers C5 cleavage into C5b, which interacts with C6, C7, C8, and C9 to generate membrane attack complex (MAC) leading to cell damage.