THE FISH IMMUNE SYSTEM
Immunity in fish is mediated by two major systems: the innate or non-specific immune system and the adaptive or specific immune system. The innate im-mune system is thought to be of ancient origin and acts as the first line of defense against invading pathogens. It has no memory component and is ac-tive against a variety of microbial antigens. On the other hand, the adaptive immune system is present only in the vertebrates above the level of the agnathans. The most distinctive characteristics of the adaptive immune re-sponse are high specificity for microbial antigens and memory. Unlike the in-nate immune response, the adaptive response is not immediate since it would take time to synthesize specific antibodies against an invading antigen. Thus it constitutes the second but more specific line of defense. A summary of the components of the fish immune system is shown in Table 9 - 1.