Investigations of the complement system have been performed across the scope of the animal kingdom. Evidence of complement activity has been detected early in the phylogenetic tree. Complement-like activity homologous to the mammalian alternative pathway has been reported in coelomic fluid of invertebrates, including the sea star (Asterias forbesi) and the sea urchin (Strongylocentrotous droebachiensis). A component comparable to mammalian C3 which acts as an opsonin in phagocytosis of foreign cells has been isolated from one of the most primitive of vertebrates, the lamprey. Classical and alternative complement activity and their functional components have been identified in nurse shark serum. Similar functional and structural components have been found in studies among other non-mammalian vertebrates; osteichthyes, amphibians, reptiles and avian species. Complement proteins have been investigated in some of the more common domestic terrestrial mammals such as the rabbit, dog, rat, horse and cow.

In the current study the complement system of the bottle-nosed dolphin (Tursiops truncatus) was examined. Dolphins, and other cetaceans, lack factor XII (Hageman factor); an important activator of inflammation, fibrinolysis, as well as the intrinsic coagulation and complement systems in other mammals. Whether the lack of factor XIII influences activity of the complement system of dolphins was a primary goal of this study. Additionally, the physiological adaptations of the dolphin to an aquatic environment were also of interest in terms of identifying changes in such a vital effector mechanism.

The classical and alternative pathways of the complement system of the bottle-nosed dolphin (Tursiops truncatus) was investigated by evaluation of serum hemolytic activity. Guinea pig serum was used for comparison to dolphin serum complement activity. Dolphin serum lysed sheep red blood cells coated with rabbit anti-sheep red cell antibody. Lysis was inhibited by EDTA and EGTA, and was destroyed by preheating serum to 56°C for 1 hour. The addition of fresh dolphin serum to heated serum restored hemolytic activity. Hemolytic activity of sera from dolphins immunized with sheep red cells was similar to that of sera from non-immunized dolphins incubated with antibody sensitized sheep red blood cells. Dolphin serum lysed rabbit red blood cells in an Mg++ -EGTA buffer system. Pre-incubation of dolphin serum with zymosan abrogated hemolytic activity.

These results indicate that the dolphin possesses a classical and alternate complement system with functional activity similar to terrestrial mammals.