Complement system of bony and cartilaginous fish

Accumulating evidence indicates that the complement system experienced a di scontinuous development at an early stage of vertebrate evolution. Inverteb rates such as echinoderms and ascidians, and the most primitive extant vert ebrates, the cyclostomes, seem to have a primitive complement system equipp ed only with the alternative and lectin pathways. In contrast, cartilaginou s fish and higher vertebrates seem to have a modern complement system which has two additional pathways, namely the classical and lytic pathways. Rece nt molecular analyses of the complement system of bony and cartilaginous fi sh have not only confirmed the above conclusion, but also revealed a unique characteristic of the complement system of fish, where certain key compone nt genes are duplicated. The complement system seems to play a more pivotal role in body defence in fish, whose adaptive immunity is considered to be at a relatively undeveloped state. (C) 2000 Academic Press.