Chapter 14 Development of the immune response in relation to bacterial disease in the growing fish

Publisher Summary This chapter reviews the knowledge of the functional development of the immune system of fish for the strategic use of vaccines in relation to defences against bacterial diseases. Newly hatched fish do not have a functionally developed specific immune system and protection against bacterial infection is provided by non-specific mechanisms and factors, some of them derived from maternal origins. In the larvae, lymphocytes first appear in the thymus and T cells differentiate before B cells that develop first in the kidney. The gut becomes populated with T and B cells quite early in development, with T cells being present in the mucosal epithelium and B cells in the lamina propria. The ability to produce specific immune responses begins at the time of first feeding in fish with large eggs and larvae. This is a time when the fry are very susceptible to bacterial infection. The onset of maturation of the specific immune response relates to the rate of general differentiation of the tissues and this is temperature related in fish. The timing of maturation of the immune system in fish is of importance in defining the earliest time at which different fish species can be vaccinated. As fish grow more slowly at lower temperatures, development correlates better with size rather than age.