The alternative pathway of the complement system is a humoral component of the immune system's natural defence against infections which can operate without antibody participation. The cascade begins with the spontaneous activation and attachment of the complement protein C3 to a pathogen, creating a catalytic activator that decays both spontaneously and quickly, but can lead to the creation of a C3 convertase. The alternative pathway convertase consists of the activated B and D factors, forming an unstable compound that can become stable after binding properdin, a serum protein. The C3-Factor B-Properdin complex is also known as the alternative pathway C5 convertase. After the creation of C5 convertase, the complement system follows the same path regardless of the means of activation (alternative, classical, or MBL).

The alternative pathway is one of three complement pathways that opsonizes and kills pathogens. The alternative pathway does not require a specific antibody to commence.

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.

Since C3b is free and abundant in the plasma, it can bind to either a host cell or 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 CD55) and DAF (decay accelerating factor) 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 works with C3b-biding proteins cofactors such as CR1 and Membrane Cofactor of Proteolysis (MCP or CD46)

• Another complement regulatory protein is Factor H which either competes with factor B, displaces Bb from the convertase, acts as a cofactor for Factor I, or preferentially binds to C3b bound to vertebrate cells (because of affinity to sialic acid residues)

See also

• Classical complement pathway
• Mannan-binding lectin pathway

Further reading

• Immunobiology. Janeway, et al. 5th 3ed. ISBN 0-8153-4101-6. (5th ed. text online at ^*.)
• BioCarta's diagram of the alternative pathway, ^*

Complement system