Figure 2.

Complement cascade schematic illustration. There are three pathways involved in complement-mediated renal involvement, namely the classical, lectin, and alternative pathways. The classical pathway can be activated by the complex formation of antigens and antibodies. When mannose-binding lectin (MBL) binds to serine proteases, known as mannose-associated serine proteases (MASP1 and MASP2), the lectin pathway is activated. The alternative pathway is initiated when complement C3 covalently binds to microbial surfaces and undergoes cleavage, resulting in the formation of C3b. Each pathway ultimately generates active C3 convertases, leading to the cleavage of C3 into C3a and C3b fragments. C3b can then interact with C4b2a or C3bBb, generating C5 convertases. Under the action of C5 convertases, C5 is cleaved into C5a and C5b. C5b binds to cell membranes and, along with C6, C7, C8, and C9, forms the membrane attack complex (MAC), leading to cell lysis. Additionally, complement products such as C3a, C4a, and C5a act as anaphylatoxins and chemotaxis, attracting and activating immune cells, thereby inducing an inflammatory response.