A study of the structure, distribution and function of phagocytic cells in the immune system of fish
Dogfish leucocytes were characterised by histochemistry, light and electron microscopical methods and qualitative phagocytic studies. This enabled the recognition of the major phagocytic cells and their role in the immune response of the fish was further evaluated. In addition to in vitro and in vivo studies of phagocytosis, the phagocytic cells were examined for certain surface properties such as the presence of Fe and C3 receptors. Attempts were also made to demonstrate delayed hypersensitivity by in VIVO skin tests and in vitro migration inhibition tests. All the major leucocytic elements found in mammals were demonstrated in dogfish blood together with some, as yet, unidentified cells. The main phagocytic cells of the blood were the neutrophil and the monocyte. Clearance of antigen and other particulate material from the circulation was effected chiefly by the spleen and to a lesser extent by the gill. In contrast to mammals, the liver showed no phagocytic activity. Although in vitro studies indicated that the identified phagocytes were efficient at endocytosing antigenic material, in VIVO clearance studies showed that low numbers of viable bacteria and yeasts persisted in the circulation for long periods after injection. This suggests a functional deficiency in the phagocytic ability of dogfish compared to higher vertebrates. This may however be augmented by natural elements in their circulation. Delayed hypersensitivity reactions and possession of receptors (which may mediate cellular interactions) were not demonstrated in fish. This may suggest a less sophisticated system is operating in these fish compared with mammals or conversely that these phenomena can not be demonstrated by the methods employed in this study.