It is widely demonstrated that farmed sh, prawn and shrimp are more susceptible to disease agents than their wild counterparts due to the articial conditions posed by intensive rearing. The immune system of aquatic organisms, such as sh, prawn and shrimp is continuously affected by periodic or unexpected changes of their environment. Adverse environmental situations may acutely or chronically stress the sh, prawn and shrimp altering some of their biochemical parameters and suppressing their innate and adaptive immune responses. Nonspecic defense mechanisms play an important role at all stages of infection. Fish, prawn and shrimp particularly, depend more heavily on these non-specic mechanisms than do mammals. Hence, there has been increasing interest in the modulation of the non-specic immune system of sh, prawn and shrimp as both a treatment and prophylactic measure against disease. When infectious outbreaks appear they may be fought by means of chemotherapeutants, vaccines or immunostimulants