In mammals, the alternative pathway is activated either by spontaneous hydrolysis of the internal thioester bond of C3 or by covalent attachment of C3b to target surfaces. Factor B binds to both hydrolyzed forms of C3 (or C3i) and surface-bound C3b. Factor B is subsequently cleaved by factor D to generate C3bBb or C3i:Bb, the alternative C3 convertases.
Antibody independent complement activity in chicken shows characteristics similar to those of the mammalian alternative complement pathway. Thus, hemolytic activity of chicken serum against horse erythrocytes (HRBC) required the presence of Mg2+, but not Ca2+ ions. The lysis of HRBC remained unaffected by the treatment with carrageenan, which acts as a C1 inactivator via classical pathway [Otha H et al 1984]. Besides, normal chicken serum, which lacked viral-neutralizing antibody, was found to cause C3 deposition in Fowlpox virus-infected chicken embryonic cells. This C3 deposition occured independently of Ca2+ ions [Otha H et al 1983].
The major proteins of the human alternative pathway are C3, factor B, factor D, properdin and regulatory factors I and H. Complement component C3 and factor B-like protease were purified and characterized in chicken [Laursen I and Koch C 1989; Mavrodis M et al 1995; Koch C 1986; Kjalke M et al 1993]. Predicted chicken factors H (CFH) and I (CFI) show 38% and 51% aminoacid sequence identity with their human counterparts respectively. Factor D and properdin are not found in the chicken genome, but the absence of factor D may reflect technical problems in identifying it due to its simple domain structure [Nonaka M and Kimura A 2006].
Here we assume that chicken processes of the alternative pathway might be occurring in a similar fashion to that in human, forming fluid-phase and surface-bound C3 convertases.