Introduction
Enhancement of the immune system seems to be the most
promising method of preventing fish diseases. This modulation
can be achieved with vaccines, which enhance the acquired (or
specific) immune response of the fish and are considered to be
themosteffectiveagents,butasingle vaccineiseffectiveagainst
only one type of pathogens, and in the case of many pathogens
(like the bacterium Aeromonas hydrophila) no effective vaccine
is available due to the complex antigenic structure