Simple SummaryAs a consequence of the increased use of plant ingredients in marine fish diets, there is a trend for a reduction in long-chain polyunsaturated fatty acid content and an increase in carbohydrates in aquafeeds. Thus, it is important to understand the ways in which such dietary modifications impact the health status and disease resistance of fish. Considering this, we evaluated the effects of high dietary inclusions of arachidonic acid (ARA) or docosahexaenoic acid (DHA) together with high or low inclusion of digestible carbohydrates (CHO) before and after a bacterial challenge with killedPhotobacterium damselaesubsp.piscicida(Phdp) on selected immune parameters. Our results showed that high dietary ARA levels improved the fish immune response to a bacterial challenge. The dietary CHO provides important energy to promote a fast response of the gilthead sea bream immune system.AbstractThis study aims to assess the effects of different dietary n-6/n-3 long-chain polyunsaturated fatty acid ratios and CHO content in the immune response of gilthead seabream. For that purpose, gilthead sea bream juveniles (initial body weight = 47.5 g) were fed for 84 days with four isoproteic (47% crude protein) and isolipidic (18% crude lipids) diets with high (20%) or low (5%) level of gelatinized starch (HS or LS diets, respectively) and included approximately 2.4% ARA or DHA. At the end of the trial, the DHA-enriched groups presented increased red blood cell (RBC) count, hemoglobin, plasmatic nitric oxide (NO) content, and antiprotease and alternative complement activities. The ARA groups had increased thrombocyte count, and plasmatic bactericidal activity againstVibrio anguillarumwas lower in the fish fed the ARA/LS diet. After the feeding trial, the fish were challenged with an intraperitoneal injection (i.p.) of killedPhotobacterium damselaesubsp.piscicida(Phdp) and sampled at 4 and 24 h after the challenge. At 4 h after i.p., the ARA groups presented increased plasma total immunoglobulins (Ig) and bactericidal activity againstV. anguillarum. In addition, the fish fed the ARA/LS diet presented lower white blood cell (WBC) and alternative complement activity. At 24 h after i.p., the ARA groups presented increased RBC, WBC, and thrombocyte numbers, total IG, plasma peroxidase activity, and casp3 expression in the distal intestine. The HS groups presented increased plasma NO content and bactericidal activity againstPhdpand decreased protease, antiprotease activity, and bactericidal activity againstV. anguillarum. In conclusion, high dietary DHA levels seemed to improve the immune status of unchallenged gilthead sea bream juveniles, while high dietary ARA levels improved the fish immune response to a bacterial challenge. The energy provided by dietary starch seems to be important to promote a fast response by the fish immune system after a challenge.