Evolution and diversity of the complement system of poikilothermic vertebrates

In mammals the complement system plays an important role in innate and acqu ired host defense mechanisms against infection and in various immunoregulat ory processes. The complement system is an ancient defense mechanism that i s already present in the invertebrate deuterostomes. In these species as we ll as in agnathans (the most primitive vertebrate species), both the altern ative and lectin pathway of complement activation are already present, and the complement system appears to be involved mainly in opsonization of fore ign material. With the emergence of immunoglobulins in cartilaginous fish, the classical and lytic pathways first appear. The rest of the poikilotherm ic species, from teleosts to reptilians, appear to contain a well-developed complement system resembling chat of homeothermic vertebrates. However, im portant differences remain. Unlike homeotherms, several species of poikilot herms have recently been shown to possess multiple forms of complement comp onents (C3 and factor B) that are structurally and functionally more divers e than those of higher vertebrates. It is noteworthy that the multiple form s of C3 that have been characterized in several teleost fish are able to bi nd with varying efficiencies to various complement-activating surfaces. We hypothesize that this diversity has allowed these animals to expand their i nnate capacity for immune recognition.