The main objective of this study was to evaluate the effects of sodium sulfite (SoS) treatment of maize and its impact on the porcine immune system in the presence of an LPS-induced systemic inflammation. Control maize (CON) and Fusarium -toxin contaminated maize (FUS) were wet-preserved (20% moisture) for 79 days with (+) or without (−) SoS and then included at 10% in a diet, resulting in four experimental groups: CON−, CON+, FUS−, and FUS+ with deoxynivalenol (DON) concentrations of 0.09, 0.05, 5.36, and 0.83 mg DON/kg feed, respectively. After 42-day feeding trial (weaned barrows, n = 20/group), ten pigs per group were challenged intraperitoneally with either 7.5 μg LPS/kg BW or placebo (0.9% NaCl), observed for 2 h, and then sacrificed. Blood, mesenteric lymph nodes, and spleen were collected for phenotyping of different T cell subsets, B cells, and monocytes. Phagocytic activity and intracellular formation of reactive oxygen species (ROS) were analyzed in both polymorphonuclear cells (PMN) and peripheral blood mononuclear cells (PBMC) using flow cytometry. Our results revealed that the impact of DON was more notable on CD3+CD4+CD8+ T cells in lymphoid tissues rather than in blood T cells. In contrast, SoS treatment of maize altered leukocyte subpopulations in blood, e.g., reduced the percentage and fluorescence signal of CD8high T cells. Interestingly, SoS treatment reduced the amount of free radicals in basal ROS-producing PMNs only in LPS-challenged animals, suggesting a decrease in basal cellular ROS production ( p SoS*LPS = 0.022). Electronic supplementary material The online version of this article (10.1007/s12550-020-00403-x) contains supplementary material, which is available to authorized users.