File:Tickover Initiation of the Alternative Complement Pathway.png

עברית: Tickover Initiation of the Alternative Complement System. Spontaneous C3 cleavage into soluble C3(H2O) drives the formation of transient C3 Convertase Complexes C3(H2O)Bb. This complex is dependent on the proteolysis of factor B by factor D. Upon infection, C3b fragments that were created by this C3 Convertase bind the pathogen membrane, a core turning point toward the widespread formation of an immune response. Soluble factor B binds the membrane bound C3b, creating a C3bB complex. After further cleavage by factor D, and stabilization by the protein Properdin, a highly effective C3 Convertase is formed - C3bBb. This complex cleaves many C3 molecules, some turn into C3a anaphylatoxin, some into C3b opsonin, and some C3b bind the C3bBb complex - transforming it into a C3bBbC3b-Properdin-Stabilized structure. This is the alternative C5 convertase, which structurally - but not functionally - differs from the classic/lectin C4bC2aC3b C5 Convertase.