The Complement Systems of Fish

Abstract

The mammalian complement system consists of about 30 distinct proteins and is activated through the classical pathway (CCP) or the alternative pathway (ACP). The biological importance of this system has stimulated many scientists to study the complement system of fish. It has been reported that jawless fish (hagfish and lamprey) lacks cytolytic activity of the complement and seems to have only the ACP of C3 activation. The complement system of cartilaginous fish (nurse shark) possesses the CCP which is composed of 6 functionally distinct components, but there is no indication of the existance of an ACP. On the other hand, bony fish have both of the pathways, directly comparable to those of mammals and thus far a number of complement components have been isolated from fish species. We have isolated C1, C4, C2, C3, C5, C6, C7, C8, C9, factor D and factor B from carp serum and have shown that these components interact basically in the same manner as their mammalian counterparts. It was also found that the ACP activity (ACH50) of bony fish was extremely high as compared with those of mammals, and that the function of the CCP of bony fish was only to fix C3 on target cells and triggers the activation of the ACP. These results suggest that the role of the ACP, which works effectively in the early stages of infection, is much more important in fish than in mammals. This paper describes the complement systems of fish concentrating upon that of bony fish.