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Not AvailableFish is continuously exposed to changing environmental conditions diurnally and seasonally. Climate variability affects the fish habitat with rise in water temperatures. Higher temperatures make the fish susceptible to various diseases. Immunoglobulin M (IgM) is the component of specific humoral immune response in fishes which respond to change in environmental factors, such as temperature, DO, pH, salinity etc. Following study attempts to understand the expression of IgM which fights against infections. Tilapia, Oreochromis mossambicus of average length 6.5 ± 0.5cm and average weight 6.4 ± 0.7g were exposed to acute thermal stress. The water temperature was raised to 40°C at the rate of 0.3°C/min. At this temperature the fish attained loss of equilibrium (LOE) representing the critical thermal maxima (CTmax) of the fish. All fishes survived the acute thermal stress. Fish recovered after water temperature reached 25-27°C which represented the initial zero hour sampling. The fish were sampled at 0, 2, 6, 24, 48 hrs and were analyzed for impact of acute thermal stress on the expression of Immunoglobulin gene IgM. It is observed that IgM expression in the fish skin tissue declines rapidly from zero hours to 48 hours. The IgM expression decreased due to acute thermal stress and could not attain the normal expression levels even after 48 hours after onset of stress. The thermal stress suppresses the expression of IgM gene. High temperature acclimation results in decrease in IGM concentration indicating that the fish possess an optimal thermal range for synthesis of immunoglobulins. The above result suggests that specific immune system of Oreochromis mossambicus gets altered due to abiotic stress of temperature in the aquatic environment. This makes the fish susceptible to infectious disease in the event of sudden and drastic changes in the climatic conditions.Not Availabl