In mammals, reactions following C5b formation are common to the classical and alternative complement activation pathways, both lead to formation of the membrane attack complex (MAC), which forms pores in the target cell membrane resulting in cell lysis. Assembly of MAC is initiated by proteolytic cleavage of C5 by C5 convertases at the target cell surface, generating C5a and C5b. C5b has the transient ability to associate tightly with C6. The C5b:C6 complex subsequently interacts with C7, C8, and up to 18 molecules of C9 to create MAC.
All terminal complement component (TCC) genes are present in mammalian, avian, and amphibian genomic sequences, except for the avian C9 gene, which is not found in the draft chicken genome [Nonaka M and Kimura A 2006]. Chicken MAC structural (C6, C7 and C8 alpha, beta, gamma) and regulatory genes (CD59, vitronectin and clusterin) are expressed in a wide range of adult chicken tissues, most abundantly in the liver [Mikrou A and Zarkadis IK 2010].