Living organisms have strong defense mechanisms against invading microorganisms as survival strategies. One of the defense mechanisms is the complement system, composed of more than 30 serum and cell surface components. This system collaborates in recognition and elimination of pathogens as a part of both the innate and acquired immune systems. The two collagenous lectins, mannose-binding lectin (MBL) and ficolins, are pattern recognition proteins acting in innate immunity and, upon recognition of the pathogens, they trigger the activation of the lectin complement pathway through attached serine proteases (MASPs). A similar lectin-based complement system, consisting of the lectin-protease complex and C3, is present in ascidians, our closest invertebrate relatives and in lamprey, the most primitive vertebrate. Furthermore, a lamprey N-acetylglucosamine (GlcNAc)-binding lectin was identified as the orthlogue of mammalian C1q, and lamprey MASP is suggested as the prototype of MASP-2/C1r/C1s, indicating that the classical complement pathway arose as a part of the innate immune system. Thus, the complement system is one of the most highly organized innate immune systems in invertebrates and jawless vertebrates, and this system has survived in vertebrates with its core components little changed for 600-700 million years.