The fish lymphoid organs.

In this study, we summarise the available information on fish lymphoid organs, paying special attention to teleosts. We review data on the first lympho-haematopoietic loci, the unique characteristics of the teleost thymus, the origin of B lymphocytes in vertebrates without a haematopoietic bone marrow, the organisation of the spleen in the absence of lymph nodes and germinal centres, and the significant presence of lymphoid tissue in fish mucosae. This provides some suggestions for future studies.