Alternative complement pathway

The alternative pathway of the complement system is an innate component of the immune system's natural defense against infections, which can operate without antibody participation.

The alternative pathway is one of three complement pathways that opsonize and kill pathogens. The alternative pathway does not require a specific antibody to commence, and, so, can be effective much faster than if antibody synthesis had to take place, as in the classical pathway. The caveat of this faster activation is that only specific types of antigens can activate this pathway.

Cascade
It is initiated by the spontaneous hydrolysis of C3, which is abundant in the plasma in the blood. "Tickover" occurs through the spontaneous cleavage of the thioester bond in C3 to form C3(H2O).

This change in shape allows the binding of plasma protein Factor B, which allows Factor D to cleave Factor B into Ba and Bb.

Bb remains part of the C3(H2O) to form C3(H2O)Bb. This complex is also known as a fluid-phase C3 convertase. This convertase, although only produced in small amounts, can cleave multiple C3 proteins into C3a and C3b.

The alternative pathway C3-convertase consists of the activated B and D factors, forming an unstable compound that can become stable after binding properdin, a serum protein.

After the creation of C3 convertase, the complement system follows the same path regardless of the means of activation (alternative, classical, or MBL). Binding of another C3b-fragment to the C3-convertase of the alternative pathway creates a C5-convertase analoguous to the MBL or classical pathway.

The C5-convertase of the alternative pathway consists of C3bBbC3b also referred to as C3b2Bb (instead of C4b2a3b in the other pathways)

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 Factor I cleaves C3b into its inactive form, iC3b. Factor I requires a C3b-binding protein cofactor such as complement factor H, CR1 and Membrane Cofactor of Proteolysis (MCP or CD46)
 * Complement Factor H can inhibit the formation of the C3 convertase by competing with factor B for binding to C3b; accelerate the decay of the C3 convertase; and act as a cofactor for Factor I-mediated cleavage of C3b. 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.