A new schematic drawing of the complement cascade. The new feature of this complement scheme is the absence of trimolecular C5 convertases. Instead, it is envisaged that the bimolecular C3 convertases C3bBb (or C3bBbP) and C4b2a proteolytically activate C5 once it is conformationally prepared for cleavage (or primed) on highly dense surface clusters of C3b alone or C3b and C4b.