FAB-2 Labeling Of Immunoglobulin-Containing Lymphocytes Within The Head Kidney And Epidermis Of Channel Catfish (Ictalurus punctatus)     J. Maki1, B. Steffens2, M. Ard2, and H. Dickerson1.   1Department of Medical Microbiology and Parasitology, 2Electron Microscopy Facility, College of Veterinary Medicine, The University of Georgia, Athens, GA USA 30602     Channel catfish (CC) (Ictalurus punctatus) respond to antigenic stimulation by producing a single class of antibody (Ab). This tetrameric immunoglobulin (Ig) is present on the surface membranes of B-lymphocytes, in sera and mucosal secretions. Monoclonal Abs against CC Ig heavy and light chains identify Ig+ cells in suspensions but fail to recognize their epitopes in fixed tissues. CC Ig-specific goat FAB fragments (FAB-2) were used as a primary Ab to label Ig+ cells in resin-embedded CC tissues examined by transmission electron microscopy. Normal rabbit serum was used as a blocking agent. Sites of FAB-2 binding were detected with a commercially prepared rabbit FAB anti-goat FAB-biotin labeled Ab followed by avidin-gold beads. In the head kidney, Ig+ lymphocytes, Ig- lymphocytes and macrophages were observed. Ig+ lymphocytes were infrequently identified in CC epidermis. The staining pattern of the epidermal Ig+ cells was similar to those found in the HK. In conclusion, these results demonstrate Ig+ lymphocytes in CC head kidney and skin. Based on their morphological similarity to other fish plasma cells, we propose they are Ab-secreting lymphocytes. Ultrastructural identification of Ig+ lymphocytes in various CC tissues will provide insight as to the origin of their mucosal surface Ab and the development of protective mucosal immunity.