Fish Immunology (1+1)

Unit 5- Specific defence mechanism in Fish

5.2.2. Primary lymphoid organs
Primary lymphoid organs mainly contains thymus and anterior kidney.
Thymus
It is situated in the dorsolateral region of the gill chamber close to the opercular cavity. Thymus is as the primary lymphoid organ and mostly contains T cells and few populations of B cells. The thymus starts developing from 24 hours after fertilization and it is the first lymphoid tissue to become populated by lymphocytes during development. Once developed the thymocytes migrate from thymus to spleen and kidney. Along with sIg– and sIg+ lymphocytes thymus also possess epithelial cells monocytes, macrophages, neuroendocrine cells etc.
Anterior kidney
The anatomy of the kidney varies from species to species. The kidney of teleost fish not only function as the excretory organ, it also contains the medullary and cortical adrenal homologs and hematopoietic tissue. Kidney is situated along the dorsal wall of the body cavity. Based on location and function kidney of teleost fish has been classified into two types they are:- 1) anterior or head kidney, and 2) posterior or trunk kidney.
Anterior kidney losses its excretory role after the fish matures and Mature anterior kidney is the primary hematopoietic organ in adult teleosts. It is the first lymphoid organ to possess sIg+ cells. Based on the species the anterior kidney may be bifurcated or single extension. B cells will generate and mature in head kidney. Anterior kidney can even act as secondary lymphoid organ.