The leucocytes of fish: A review

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Abstract
The data available in the literature concerning fish thrombocytes, lymphocytes, monocytes, macrophages and granulocytes is discussed and the dearth of reliable information concerning the functional identity of most fish leucocytes is underlined. The methods of applying nomenclatures to fish leucocytes encountered in the literature is discussed in some detail and found to be unsatisfactory. An attempt is made to simplify the confused and conflicting reports by applying the concepts which have recently arisen in the field of mammalian immunology. Nomenclatures are assigned to cells on a functional and morphological basis where sufficient data exists.Thrombocytes are responsible for clot formation and are considered to be distinct, separate and unrelated to lymphocytes. Lymphocytes are recognised as immuno‐competent cells though as yet little information exists on their role in immune mechanisms. Recent work is furnishing interesting results in this direction, especially in the origin of lymphocyte populations and their co‐operative responses in immunity. Previous references to monocytes and macrophages in fish are confused and the literature is discussed in the light of the new classification of the Mononuclear Phagocyte System.Very few experimental data exist concerning granulocytic leucocytes in fish, indeed the presence of such cells with granulocytic functions still requires proof. Cells with morphological similarity to mammalian granulocytes do exist in fish but information essential for an understanding of their role in defence mechanisms is lacking. The state of the literature concerning the eosinophil, basophil and mast cell in fish is so confused that an entirely new approach to a study of these cells is warranted. It is hoped that this review will supply guidelines for future research.
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