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 properdine, 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).

See Also

• Classical complement pathway
• Mannose-binding lectin pathway

Further reading

• Immunobiology. Janeway, et al. 5th 3ed. ISBN 0-8153-4101-6. (5th ed. text online at [1] (http://www.ncbi.nlm.nih.gov/books/bv.fcgi?call=bv.View..ShowTOC&rid=imm.TOC&depth=10).)
• BioCarta's diagram of the alternative pathway, [2] (http://www.biocarta.com/pathfiles/h_alternativePathway.asp)