Consequencesof complement activation. (A) The complement system can be activated via 3 pathways: classical, lectin, and alternative. All pathways lead to formation of powerful enzymes, the C3-convertases, followed by activation of the terminal cascade. The main effector functions of complement (promotion of opsonophagocytosis by opsonization and chemotaxis and formation of lytic membrane attack complexes) aim to destroy harmful agents such as microbes. Lysis of target cells can lead to damage-induced enhancement of complement activation. (B) Alternative pathway activation is based on continuous, low-level covalent deposition of C3b molecules onto practically all surfaces in contact with plasma. If the C3b molecule is allowed to form an enzyme (shown in red), new C3b deposits will be formed around the enzyme leading to rapid amplification of the activation. If the regulator factor H binds to C3b, the convertase enzyme is inactivated and no complement activation follows. The simultaneous interaction of factor H with both C3b and cell surface sialic acids (or possibly glycosaminoglycans [GAGs]) is essential for proper regulation on self red cells, platelets, and endothelial cells. If this fails, disbalance between activation and regulation may lead to pathogenesis of atypical HUS. iC3b, C3b molecule incapable of forming an enzyme with factor B.