Editing Alternative complement pathway (section)

==Regulation==
Since C3b is free and abundant in the plasma, it can bind to either a host cell or a pathogen surface.  To prevent complement activation from proceeding on the host cell, there are several different kinds of regulatory proteins that disrupt the complement activation process:
* Complement Receptor 1 (CR1 or [[CD35]]) and DAF ([[decay accelerating factor]] also known as [[CD55]]) compete with Factor B in binding with C3b on the cell surface and can even remove Bb from an already formed C3bBb complex
* The formation of a C3 convertase can also be prevented when a plasma protease called [[complement factor I]] cleaves C3b into its inactive form, iC3b.  Factor I requires a C3b-binding protein cofactor such as complement factor H, CR1, or membrane cofactor of proteolysis (MCP or [[CD46]])
* [[Factor H|Complement factor H]] can inhibit the formation of the C3 convertase by competing with factor B for binding to C3b;<ref>{{cite journal | vauthors = Conrad DH, Carlo JR, Ruddy S | title = Interaction of beta1H globulin with cell-bound C3b: quantitative analysis of binding and influence of alternative pathway components on binding | journal = The Journal of Experimental Medicine | volume = 147 | issue = 6 | pages = 1792–1805 | date = June 1978 | pmid = 567241 | pmc = 2184316 | doi = 10.1084/jem.147.6.1792 }}</ref> accelerate the decay of the C3 convertase;<ref>{{cite journal | vauthors = Weiler JM, Daha MR, Austen KF, Fearon DT | title = Control of the amplification convertase of complement by the plasma protein beta1H | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 73 | issue = 9 | pages = 3268–72 | date = September 1976 | pmid = 1067618 | pmc = 431003 | doi = 10.1073/pnas.73.9.3268 | bibcode = 1976PNAS...73.3268W | doi-access = free }}</ref> and act as a cofactor for factor I-mediated cleavage of C3b.<ref>{{cite journal | vauthors = Pangburn MK, Schreiber RD, Müller-Eberhard HJ | title = Human complement C3b inactivator: isolation, characterization, and demonstration of an absolute requirement for the serum protein beta1H for cleavage of C3b and C4b in solution | journal = The Journal of Experimental Medicine | volume = 146 | issue = 1 | pages = 257–70 | date = July 1977 | pmid = 301546 | pmc = 2180748 | doi = 10.1084/jem.146.1.257 }}</ref> Complement factor H preferentially binds to vertebrate cells (because of affinity for sialic acid residues), allowing preferential protection of host (as opposed to bacterial) cells from complement-mediated damage.
* [[CFHR5]] (Complement factor H-Related protein 5) is able to bind to act as a cofactor for factor I, has decay accelerating activity and is able to bind preferentially to C3b at host surfaces.<ref>{{cite journal | vauthors = McRae JL, Duthy TG, Griggs KM, Ormsby RJ, Cowan PJ, Cromer BA, McKinstry WJ, Parker MW, Murphy BF, Gordon DL | title = Human factor H-related protein 5 has cofactor activity, inhibits C3 convertase activity, binds heparin and C-reactive protein, and associates with lipoprotein | journal = Journal of Immunology | volume = 174 | issue = 10 | pages = 6250–6 | date = May 2005 | pmid = 15879123 | doi = 10.4049/jimmunol.174.10.6250 | doi-access = free }}</ref>