Abstract: To investigate the immunoenhancement mechanism in Ruditapes philippinarum during secondary infection by Vibrio anguillarum at varying concentrations. In this study, we documented survival rates of R. philippinarum following secondary infection with varying concentrations of V. anguillarum, investigated dynamic changes in lysozyme and superoxide dismutase (SOD) activities, and analyzed mRNA expression levels of six immune-related genes in hepatopancreatic tissues under secondary infection conditions. Results indicated that the van1 group injected with V. anguillarum at a concentration of 5×10⁵ CFU/mL exhibited the highest survival rate (85.3%), while the van5 group injected with V. anguillarum at a concentration of 5×10⁹ CFU/mL showed the lowest (17.5%). Lysozyme activity peaked at 24h in the control group (van) and secondary infection groups van1, van2, and van3 (P<0.05), whereas peaked at 72h in groups van4 and van5 (P<0.05). Between 24h and 96h, SOD activity in groups van3, van4, and van5 was significantly higher than that in the primary infection group (control van) (P<0.05), whereas, at 48h and 72h, SOD activity in groups van1 and van2 was significantly lower (P<0.05). Furthermore, quantitative Real-time PCR (qPCR) analysis revealed that secondary infection with V. anguillarum significantly upregulated the expression of immune-related genes, including mannose receptor C-type 2 (MRC2), ubiquitin-like protein ATG12 (Atg12), Toll-like receptor 4 (TLR4), and lysozyme (P<0.05). The study reveals that these genes play a critical role in the immune defense mechanism of R. philippinarum against secondary infection with V. anguillarum.