The complement system is the first line of defense against invading microbes. It consists of a large number of distinct proteins, which circulate in the blood stream in functionally inactive states. When activated, complement components assemble on the surface of a target cell. The activation of one component induces its proteolytic function that acts on the next component in the cascade, cleaving it into biologically active fragments. In mammals, the complement system is activated via three distinct pathways: the classical, lectin and alternative pathways. All three pathways merge at the proteolytic cleavage of component C3 to form the key molecule C3b. The activation of the complement system leads to four principal outcomes: (1) opsonization of target cells to enhance phagocytosis (2) lysis of target cells via an assembly of the membrane attack complex (MAC) on the pathogen surface (3) production of anaphylatoxins that are involved in the host inflammatory response (4) clearance of antibody-antigen complexes.
Avian species (chicken, turkey and duck) have been reported to induce functional complement pathways in response to immunization with sheep red blood cells (SRBC) [Ellis MG et al 1989; Koppenheffer TL et al 1999; Baelmans R et al 2005]. Complement activation in chicken was also shown to mediate host response against bacterial and viral infections [Skeeles JK et al 1979a, b; Ohta H et al 1983; Laursen SB and Nielsen OL 2000]. Immune competence of the distinct chicken ecotypes was assessed by measuring complement hemolytic activity after immunization with SRBC [Baelmans R et al 2004; Baelmans R et al 2005]. Both classical Ca2+ dependent complement pathway (CPW) and alternative calcium-independent complement pathway (APW), as well as total Ig (IgG and IgM antibody) responses were detected. However, the type and magnitude of immune response varied for individual chickens even within the same ecotype.
Analysis of genome data revealed that mammals and aves seem to share practically the same set of complement genes [Nonaka M and Kimura A 2006]. Indeed, most of the components of the classical and alternative complement pathways have been found in the chicken genome. However, an absence of some components such as chicken C9, factor D, properdin, MASP-1 has been also reported [Barta O and Hubbert NL 1978; Lynch et al 2005; Koch C 1986; Mikrou A and Zarkadis IK. 2010]. In this project we assume that antimicrobial functions of chicken complement are similar to those of human, although the mechanism of the chicken complement activation remains to be clarified.
This Reactome module refers to the larger complement fragments as "b" and the smaller "a", based on the nomenclature of the complement proteins .