Fish Immunology (1+1)

Unit 6- Ontogeny of Fish Immune System

6.1.Ontogeny of Fish Immune System
In fish, as in all vertebrates, the thymus is the first lymphoid organ to develop and to become lymphoid. Later kidney and spleen develops as lymphoid organ and remaining throughout life. In the early life of the carp, the thymus contribute majorly to lymphoid cell pool (70% or about 3 X l0⁵ cells at day 28). Later as the organism grows say at 2 months of age, the total lymphocyte cell pool (5 x l0⁶ cells) was distributed among the lymphoid organs with 38% in the mesonephros, 32% in the thymus, 15% in the pronephros, 12% in the peripheral blood, and 3% in the spleen.
The development of the lymphoid system in fish is explained by two hypotheses.

• Natural selection will favor individuals in which development of the immune system coincides with their first exposure to potential pathogens either from their environment or in their food (i.e., hatching in oviparous species, parturition is viviparous species, or at the time of first feeding).
• Development of the lymphoid organs occurs at a preset time during embryogenesis. Once a certain "physiological age" has been reached lymphoid organs develop and is independent of first exposure to antigenic challenge.

The state of immunological maturity of the fish may be correlated with the age, weight, and water temperature. The weight of fish is approximately 0.24 g when the first antibody is detected.