ries of lymphocytes based on the antigen receptor they carry: T-lymphocytes and B-lymphocytes. The binding of antigens to their receptors triggers lymphocytes to become active in an immune response, a complex process usually referred to as lymphocyte activation. An activated B lymphocyte starts to manufacture large quantities of immunoglobulin molecules, which are then released into the blood. These soluble forms of immunoglobulin are commonly called antibodies.
THE FISH IMMUNE S YSTEM 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 immune 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 active 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 response are high specificity for microbial antigens and memory. Unlike the innate 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.
Innate or Non-specific Immunity Innate or non-specific immunity refers to various physical and cellular attributes that collectively represent the fish first lines of defense against infectious disease. These defense mechanisms come into play immediately or within hours of an antigen’s or an invaders appearance in the body. The nonspecific immune resistance includes the following levels:
Table 9 - 1. The major components of the immune response in fish
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Innate immune response
Adaptive immune response
Soluble Factors
Acute phase proteins Enzyme inhibitors Cell lytic enzymes Agglutinins
Antibodies
Cells
Phagocytes Natural killer cells
B – lymphocytes T – lymphocytes
Health Management in Aquaculture