The immune system is essential in the defence against infections, as well as in maintaining good health, in all developmental stages of fish. The farming of new fish species is hampered by serious disease problems. The intensive salmon production would greatly benefit from more effective immune prophylactic measures, and the lack of basic knowledge on fish immune cell functions is one of the major obstacles in fish health for rational design of vaccines and use of immunostimulants, especially for intracellular pathogens and viruses. Fish immunology is an active and growing area of research, often combining basic and applied aspects