The immune response of piglets to a vaccination against the Porcine Reproductive and Respiratory Syndrome Virus (PRRSV) was tested after feeding different feed additives. The trial included four groups of ten piglets and lasted 39 days. Feed additives were added to a control diet (C). Tested substances included an autolyzed yeast preparation from Saccharomyces cerevisiae (YA) at 0.2%, dried bovine colostrum (CM) at 0.9% and the probiotic Bacillus cereus var. toyoi NCIMB 40112 (BC) at 10(9) colony forming units/kg diet. Three-week-old piglets were vaccinated against PRRS-Virus, blood samples were taken before vaccination and 7, 21, and 35 days after vaccination for complete blood counts, flow cytometry for phenotyping of different lymphocyte subsets in peripheral blood, measurement of specific IgG antibodies and ELISpot assay to detect specific interferon gamma-producing lymphocytes. Animal performance was not affected by the experimental diets. Hematological parameters and complete blood counts displayed time-dependent changes, feed-related effects were not observed. The analysis of the expressed lymphocyte surface markers showed distinct time dependent shifts but no dietary effects. CD4(-)CD8 alpha(+) cells reached highest levels on day 21 post vaccination and decreased thereafter (P=0.037), the CD2(+)CD5(-) cells decreased in all groups towards day 35 (P < 0.001). CD2(+)CD5(+) cells were determined at constant levels during the observation period, while CD2(-)CD5(+) decreased at day 21 (P=0.039) and CD4(+)CD8 alpha(-) cells decreased over time (P=0.003). CD4(+)CD8 alpha(+)-double positive cells increased age-dependently in all groups (P=0.006). CD21(+) cells and CD14(+) cells were not affected by age. Increasing levels of specific IgG were determined (P < 0.001), but group effects could not be demonstrated as for the interferon gamma producing T cells. The development of the immune system in piglets seems to follow a typical time-dependent pattern, the lack of dietary effects on the immune response in this trial might be due to a balanced diet and appropriate sanitary conditions. (C) 2015 Elsevier B.V. All rights reserved.