Fish Diseases and Management

3.1.7.Factors affecting the immune response

1. Temperature : Temperature is the most important extrinsic factor that affects the immune response in fish. Generally, within the physiological range of any species, higher the temperature, faster the onset and magnitude of immune response. Carps could be immunologically suppressed at temperatures below 15 ° C. In coldwater species like salmonids, suppression occurs at temperatures below 6 ° C.

2. Antigen dose : Level of antibody production in the primary response is positively correlated with the dose of antigen administered. For memory effect, the dose appears to depend on the nature of the antigen.

3. Nature of antigen: The immunogenicity of different antigens differs between species. This is also related with dose of the antigen. Soluble protein antigens generally need to be administered with adjuvant to elicit antibody production.

4. Route of administration : Intramuscular injection of antigen elicits some what higher antibody titres than intraperitoneal injection. Following immersion vaccination of vibrio bacterin, serum antibody levels are not detected though higher protection was achieved. Oral presentation results in uptake of antigen by epithelium of lower intestine but antigen cannot be detected systematically. Oral vaccines do not elicit appearance of antibody in serum but it may appear in the gut mucus while inducing suppression of the systemic response.

5. Adjuvants and immunostimulants: These are substances which are administered along with antigens to enhance the immune response. Complete Freund’s adjuvant (CFA) has been shown to enhance the immune response in many fish species. Dimethylsulphoxide (DMSO), ETE (an extract of a tunicate) are also effectively used as adjuvants. Immunostimulants on the other hand include glucans, chitins, bacterial lipopolysaccharides, light oils etc.

6. Seasonal effects: Under constant light and temperature, fishes were found to produce lower antibody titres when immunized prior to winter. The mechanisms of endogenous rhythms are not yet fully understood.

7. Environmental effects: A large number of factors are known to suppress the immune response in fish. Stress factors like crowding would affect immune modulation in fish causing immune suppression. Pollutants, antibiotics and dietary deficiencies also have deleterious effects on the fish immune response.

8. Antigenic competition : This is defined as inhibition of immune response to one antigen or antigenic determinant caused by administration of other antigen in fish. However polyvalent vaccines are not yet affected by antigenic competition.

Intrinsic factors

1. Antibody feed back inhibition : There exists an innate property in fish to limit the constant production of antibody when the levels of the antibody produced reaches specific levels.

2. Immune complexes – rapidly trapped in spleen are priming fish with immune complexes. This leads to improved immune induction and produce antibodies of higher affinity.

3. Helper and suppressor activity : The role of thymus in affecting the immune response has been studied in fish.  It  is showed that the antibody response has been lowered in cases where specific antigens were administered (thymus dependent antigen) after thymectomy was performed. Similarly, presence of suppressor activity was also noticed in normal fish compared to thymectomized fish in case of certain antigens.

4. Ontogenetic maturation : Fish become immunologically mature in a few days of hatching. During ontogeny, the lymphoid organs and immune responses are sequentially developed with non-specific immunity developing first followed by cell- mediated and then humoral immunity. Tolerance induction in young fish has been demonstrated only to injected antigen but not to the antigens presented by direct immersion.