Bb

The complement system includes three separate upstream activation pathways, all converging on a common terminal pathway. Two of the pathways are induced by specific and distinct mechanisms: the classical pathway (CP) is engaged when antibodies bind to antigens, and the lectin pathway (LP) is activated by carbohydrate residues on the surface of pathogens. The alternative pathway (AP) is unique in that it is continuously active at a basal level, referred to as "AP tick-over"; its activity can be greatly increased by a variety of signals on foreign surfaces and damaged cells via a positive-feedback amplification loop. The primary driver of the AP amplification loop is the AP convertase (C3bBb). This enzyme is formed when the zymogen factor B is cleaved to generate the split product fBb, which rapidly associates with surface -bound C3b to form active enzyme C3bBb. C3bBb then continues to cleave additional molecules of the central C3 protein, leading to the generation of opsonins (C3b, iC3b), anaphylatoxins (C3a and C5a) and the terminal lytic complex (MAC; C5b-9).