The complement cascade can be viewed as one that can be triggered by a series of proteases, causing it to assemble into convertases that cleave key proteins such as C3 and C5, thus amplifying the cascade and, ultimately, attacking the target. The cascade is triggered via the CP (primarily by antigen-antibody complexes) or the lectin pathway (by carbohydrates on surfaces binding to pattern recognition molecules such as mannose-binding lectin [MBL] or ficolins, and C2 and C4 cleavage by mannose-associated serine proteases [MASPs] or small MBL-associated proteins [sMAP]), to form an enzyme that cleaves C3, called the C3 convertase (C4bC2b). The alternative pathway can be initiated in the fluid phase by the conversion of C3 to C3(H₂O), which binds to factor B, and in the presence of factor D, can generate C3b from C3. C3b binds to hydroxyl (-OH) or amine (-NH2) groups on carbohydrates or proteins on cellular surfaces via its thioester bond. Alternatively, C3b deposits directly on a surface and binds to factor B, which is then cleaved into Bb by factor D to form the (alternative pathway) C3 convertase, C3bBb. C3 convertases cleave C3 to C3a and C3b, facilitating the formation of a C5 convertase, which cleaves C5 to form C5a and C5b. C3a and C5a serve as anaphylatoxins, promoting vasodilation and chemotaxis by binding to their cognate receptors (C3aR, and C5aR1, although the role of C5aR2 continues to be clarified). C3b facilitates opsonophagocytosis and can also bind to factor B to amplify the alternative pathway. C5b binds to C6, C7, and C8 and subsequently C9 to form the membrane attack complex (MAC, C5b-9). At each step, a series of membrane regulators and fluid-phase regulators keep this system in check.