C6 and infection: Specifically, in this study, several differentially expressed proteins, such as fibrinogen proteins (FGA, FGB and FGG), complement proteins (C3 and C6) expressed in mammary gland participated in many pathways encompassing the MAPK, complement and coagulation cascades, focal adhesion, etc. The complement system plays a fundamental role in innate immunity in addition to enhancing adaptive immune responses and is therefore a primary line of defense against infection [21].