Looking at the Teleostei advanced fish family we find a big jump in the development of the immune system. Lymphocytes are still produced by intestinal GALT tissue, there is still no presence of bone marrow, but there is a clear distinction between the function of T cells and B cell production of antibody. Teleostei have kidneys and spleens. Most importantly, we see the first presence of a key lymphoid organ, the thymus. So advanced fish are showing compartmentalization of the immune system into specialized organs and use of specialized cell types to defend against different pathogen challenges.
Amphibians also have immune cells arranged into organs.
They also have areas which look like primitive lymph nodes in the kidney, liver, gills and elsewhere. Some, but by no means all, amphibian species have bone marrow. So while some species produce their lymphocytes in from stomach GALT, others have progressed to producing blood cells in the better protected bone marrow regions. We also see the subdivision of T cells into different specialist functional ability. With a few dukeing it out in the ring and a large audience shouting encouragement.