It is well-known that the innate immune mechanisms in fish serve as the first line of defence against wide variety of pathogens. The innate immune mechanisms or responses are more pronounced and important in fish than in higher vertebrates. Since they have a ready source of non-specific immune components to handle the pathogens, do not need precise recognition of the surface structures of the invader unlike in the case of specific immune responses. In most of the situations, innate responses get induced and enhanced after the pathogen invasion. It would be interesting to look into the inducibility of various innate immune mechanisms and the level of enhancement after the entry of the pathogen. Hence, in the present investigation, modulation of the innate immune responses in the striped murrel, Channa striata on experimental challenge with either live virulent or heat killed Aeromonas hydrophila at a dose of 1× 107 CFU mL-1 were measured. Most of the non-specific (both humoral and cellular) immune responses tested were substantially induced or enhanced in both the experimental groups in comparison with the unchallenged control group. Significant increase in the lysozyme, total peroxidase, antiprotease and respiratory burst activities were observed after the pathogen challenge. However, the serum ACH50 activity was not modulated and its level remained at basal level throughout the study period. Thus, most of the innate non-specific immune responses are inducible though they are constitutive of fish immune system exhibiting a basal level of activity even in the absence of pathogen challenge.
Key words: Striped murrel, non-specific immune response, Aeromonas hydrophila, experimental challenge
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