Dose–response curves for IL-1β-induced IL-8 production 6 months post-deployment for the fatigued and non-fatigued group. Whole blood samples obtained 6 months after deployment (T2) from participants assigned to the fatigued group (black circles, n = 62) and non-fatigued group (white squares, n = 401) were stimulated for 24 h with increasing concentrations of IL-1β. The amount of IL-8 in the culture supernatant was measured by ELISA. IL-1β induced a dose-dependent increase in IL-8 production in both groups (dose: F(2.00, 924.02): 602.82, P < 0.001), but the dose–response curve differed between the two groups (group: F(1,461): 4.55, P < 0.01; dose x group: F(2.00, 924.02): 7.94, P < 0.001). After applying Bonferroni correction, the fatigued group had significantly higher IL-8 production than the non-fatigued group after administration of 10 ng/mL (P < 0.01) and 30 ng/mL IL1β (P < 0.01). Data are presented as mean ± SEM. #P < 0.05, significant before Bonferroni correction, but not after Bonferroni correction, **P < 0.01, significant after Bonferroni correction.