Fish & Shellfish Immunology (1997) 7, 235–245 Immunohistochemistry of gut-associated lymphoid tissue of the sea bass Dicentrarchus labrax (L.) LUIGI ABELLI*, SIMONA PICCHIETTI,NICLA ROMANO,LUCIA MASTROLIA AND GIUSEPPE SCAPIGLIATI Department of Environmental Sciences, Faculty of Sciences, Tuscia University, Viterbo 01100, Italy (Received 20 August 1996, accepted in revised form 9 December 1996) Histology revealed scattered lymphoid cells at all levels of the digestive tract of the sea bass Dicentrarchus labrax (L.), notably in the mucosa and sub- mucosa. Immunohistochemistry with monoclonal antibodies against T cells (DLT15) and immunoglobulin-bearing cells (DLIg3 and DLIg13) demonstrated the presence of a gut-associated lymphoid tissue. Quantitative analysis showed that the concentration of DLT15-positive cells largely exceeded that of Ig-bearing cells, while the number and the distribution of DLIg3- and DLIg13- immunoreactive cells were superimposable. An apparent gradient in the number of lymphoid cells was present, concentrating them towards the anus. This suggests that, as in other fish species, the posterior gut acquired higher immunological relevance.  1997 Academic Press Limited Key words: gut-associated lymphoid tissue (GALT), T-lymphocytes, immunoglobulins (Ig), monoclonal antibodies (MAbs), fish, Dicentrarchus labrax. I. Introduction The mucosal surfaces of fish skin, gills and gut are protected by both humoral and cellular mechanisms (Peleteiro & Richards, 1985; Hart et al., 1988; Rowley et al., 1988; Roberts, 1989). Leucocytes occur in all parts of the teleost digestive system, most extensively in the intestine, where lymphocytes, plasma cells, granulocytes and macrophages are present in and under the epithelium. Although large lymphoid centres are lacking, many lymphoid cells, either scattered or in small groups, were reported to be present in the epithelium and lamina propria (Zapata, 1979; Davina et al., 1980; Temkin & McMillan, 1986; Hart et al., 1988). Various studies indicated that regional di#erences occur in organisation of the gut-associated lymphoid tissue (GALT) at di#erent levels of the digestive tract, possibly reflecting specialised immunological functions. Uptake and transport of antigens have been shown to occur mainly in the second gut segment of carp, Cyprinus carpio L. (Rombout et al., 1985, 1986; Rombout & Van den Berg, 1989), and trout, Oncorhynchus mykiss (Georgopoulou et al., *Author to whom all correspondence should be addressed at: Department of Environmental Sciences, Faculty of Sciences, Tuscia University, Via San Camillo de Lellis, block D, 01100 Viterbo, Italy. 235 1050-4648/97/040235+11 $25.00/0/fi960079  1997 Academic Press Limited