Complement System: Evolution

Abstract

The human complement system is one of the principal effector systems of innate immunity and consists of more than 30 serum and cell surface proteins. Most complement components show a striking modular structure, which makes evolutionary studies feasible. The evolutionary origin of the complement system can be traced back to the common ancestor of eumetazoa, predating by far the origin of the canonical adaptive immunity unique to the jawed vertebrates. Although the complement system has been conserved by all deuterostomes analysed thus far, it has been lost multiple times independently in the protostome lineage. Sophistication of the complement system from a simpler system by gene duplications and exon shuffling occurred in the vertebrate lineage. Key Concepts: The complement system is one of the most ancient body defence mechanisms of eumetazoa. Most complement components have a characteristic modular structure. The modern complement system was established in a common ancestor of jawed vertebrates by gene duplication and exon shuffling of the primitive complement genes. The complement system has been lost multiple times independently in protostome lineages, whereas it has been retained by all deuterostomes analysed so far. The thioester‐containing protein family is subdivided into two subfamilies, the C3 and A2M subfamilies.