| In recent years the information regarding the morphological and organization of fish immune systems has increased dramatically. Study on the fish immune systems will not only contributes to the understanding of the origin and evolution of human immune system, but also to the farming of the fish.Flounder, Pamlichthys olivaceus, was an important species in the aquaculture in China. This paper studied the structure of the immune system, including head kidney, thymus, spleen, blood and mucosa-associated lymphoid tissues, such as gill and gut for the first time. In addition, the ontogeny of the immune system in the flounder from hatching to 13 month was also studied. The thymus involuted quickly than any other fishes. The immunoglobulin was purified from serum of the flounder. It was found that the heavy chain was 78kd, and the two light chains were 29.9kd and 28.2kd. The ultrastructural and biochemical study of the flounder infected with Edwardsiella tarda were also studied. The changes in the leucocytes number and the biochemical parameters were determined, and they could serve as candidates of biomarkers for surveillance of infection of the flounder.The immune system of the flounder included kidney (head kidney), thymus, spleen, blood and mucosa-associated lymphoid tissues, gill and gut et al. Head kidney of the flounder was restricted in the foremost parts of kidney, which was predominantly a lympho-myeloid compartment with aglomerular. The parenchyma of head kidney was supported by the stroma of the kidney. The stroma consisted of reticular cells and fibroblasts. The cells responsible for the nonspecific and specific immune defenses included lymphocytes, monocytes, granulocytes (two types) and macrophages. The results showed that the immune cells in head kidney were abundant and head kidney might be an important germinal center. There had no distinct inner or outer zones in thymus and red or white pulp in spleen.The micro- and ultrastructure of peripheral blood cells of the flounder were studied. Under the light microscopy, six main types of blood cells could be recognized: erythrocytes, monocytes, thrombocytes, lymphocytes, neutrophils and eosinophils. Basophils were not found. Among these cells, lymphocytes were the smallest, while monocyteswere the biggest; Lymphocytes were the most abundant leucocytes and eosinophils were the least. The direct division of thrombocytes could be formed in the peripheral blood. The transmission electron microscopy showed that cytoplasm of erythrocytes was even and only has a few mitochondria, vacuoles and many small black granules. Lymphocytes were characterized by the presence of short pesudopodia, the big center nucleus and many free ribosornes, while monocytes were characterized by eccentric nucleus, vacuoles and many organelles. Two distinctive types of granules leucocytes were also described as type I and type II.The gill lamella was composed of squamous epithelium cells and pillar cells. The blood sinusoid cavity was very large. Functionally, the gill lamella could be divided into gas exchange region and immune region. Gas exchange region located in the upper half with many erythrocytes in the blood sinusoid, while the immune region located in the lower half with lymphocytes, monocytes, neutrophils and eosinophils in the blood sinusoid. Moreover, the chloride secreting cells and mucous cells were also observed. The results indicated that the immune cell types existed in the gill tissue was completed and they might play some extent roles in the fish local immune competence.Five kinds of leucocytes in the gut-associated lymphoid tissue (GALT) of the flounder were distinguished under the transmission electron microscope. They were lymphocytes, macrophages, monocytes, plasma-like cells and granulocytes. The leucocytes in the GALT all showed more diversified than that in the hematogenic organs. Lymphocytes were identified by many long pseudopodia. Macrophages had irregular shapes. Many vacuolus occurred in the monocytes' cytoplasm and plasma-like cells wer...
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