The alternative complement pathway is one of the three complement pathways, the other two being classical and lectin. These pathways work innately to opsonize pathogens and kill them. The alternative pathway is activated with the complement C3 protein is cleaved spontaneously in the blood. The C3b component is then free to covalently bond to the surface of pathogens or apoptotic cells, acting as a tag for other parts of the immune system. Complement factor B is cleaved into factors Ba and Bb, and factor Bb can then bind to complement factor C3b on the surface of the pathogen along with a water molecule. This complex is known as fluid-phase C3 convertase, and it cleaves many more C3 proteins into C3a and C3b. Properdin is another compound that is important for complement activation, and it binds to the C3bBb complex, stabilizing it and forming the C3bBbP complex. This complex then can bind another C3b protein, and it then functions as a C5 convertase, splitting C5 into C5a and C5b. At this point, the remainder of the pathway is the same between the alternative and classical pathways. The complement C5b protein binds to and forms a complex with component C6, followed by C7, C8 and C9. Multiple molecules of C9 end up binding to this complex, and this is what forms the membrane attack complex pore that allows for uncontrolled diffusion of the cell’s contents, and if enough pores are formed, the cell will be killed.

Alternative Complement Pathway References