Ancient origins: complement in invertebrates

Abstract

Proteins with obvious similarities to mammalian complement are widely distributed in the animal kingdom. In the vertebrate lineage, deuterostomes like sea urchins and tunicates express proteins that are homologues of C3, the central component of the vertebrate complement cascade. Their genomes also encode molecules resembling factor B from the “alternative” complement activation pathway; and tunicates have collagenous lectins of the type that can activate complement in the absence of antibodies. This suggests that the core components of the complement system evolved before antibodies, which first appear in jawed fish.