Fish Immunology (1+1)

Unit 5- Specific defence mechanism in Fish

5.2.1. Lymphoid organs in fish
There are two types of lymphoid organs can be seen in fish. Primary lymphoid organs include thymus and head kidney that produce and mature stem cells' The secondary lymphoid organs include kidney, spleen, and Mucosa lymphoid tissue. Besides, liver, skin, intestine and heart are also important organs that take part in the defence. The endothelial cells and macrophages present in these organs are highly endocytic towards 'self' or 'non-self' substances. The development lymphoid organs does not necessarily correspond to the maturation of immune functions' Even though the organs develop simultaneously in trout and salmon, the surface IgM positive cells appear 8 days pre-hatching in frout and 45 days post-hatch in salmon.