Three different immunoglobulin (Ig) isotypes can be found in teleost fish， IgM， IgD， and the teleost-specific IgT. IgM is considered to have a systemic activity， and IgT is attributed a mucosal role， similar to mammalian IgA. In this study， the complete sequence of gilthead sea bream IgM and IgT in their membrane (m) and soluble (s) forms are described for the first time in a perciform fish. Their constitutive gene expression is analyzed in different tissues， and their regulation upon viral， bacterial， parasitic， mucosal vaccination and dietary challenges are studied. GCB IgM and IgT have the prototypical structure when compared to other fish Igs. The constitutive expression of was the highest overall in all tissues， whereas expression was highest in mucosal tissues， such as gills and intestine. and were differentially regulated upon infection. was highly upregulated locally upon infection with the intestinal parasite or systemically after Nodavirus infection. Long-term intestinal parasitic infections increased the serum titer of both isotypes. Mucosal vaccination against subsp. finely regulated the Ig response inducing a systemic increase of IgM titers in serum and a local IgT response in skin mucus when animals were exposed to the pathogen by bath challenge. Interestingly， plant-based diets inhibit IgT upregulation upon intestinal parasitic challenge， which was related to a worse disease outcome. All these results corroborate the mucosal role of IgT and emphasize the importance of a finely tuned regulation of Ig isotypes upon infection， which could be of special interest in vaccination studies.