Phylogenetic origins of immune recognition: lymphocyte surface immunoglobulins and antigen binding in the genus Carassius (Teleostii)

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Abstract
Virtually all lymphocytes (from thymus, spleen and head nephros) of two closely related cyprinoid fish, C. auratus and C. carassius, were shown to bear surface immunoglobulin (Ig) detectable by immunofluorescence. Nonlymphoid cells (for example, those of the granulocytic and erythrocytic series) showed insignificant staining compared with lymphocytes. Rabbit antisera used in these studies were raised to purified serum IgM of C. auratus. The serum IgM of C. auratus and C. carassius expressed identical determinants as recognized by this antibody in radioimmunoassay, but with a lower level of expression on the IgM of C. carassius. Similarly, with the use of these reagents in quantitative cytofluorometry, the lymphoid cells of C. carassius stained less brightly than those of C. auratus. However, in both species, a hierarchy of fluorescence intensity could be observed, such that spleen ⩾ head nephros > thymus.A small percentage of lymphocytes in all organs of both species bound either myoglobin, keyhole limpet hemocyanin, or horse spleen ferritin antigens. All antigen‐binding lymphocytes were also positive for membrane Ig. Analysis of the distribution of antigen and Ig patches on these cells showed that antigen patches were always coincident with Ig patches.These results provide further evidence of lymphoid heterogeneity in the genus Carassius, and are compatible with the suggestion that lymphocyte surface Ig functions as a primary recognition molecule for all lymphocytes in Carassius species.