Transcriptome alterations of leukocytes from soldiers who underwent 8 weeks of Army Ranger training (RASP, Ranger Assessment and Selection Program) were analyzed to evaluate impacts of battlefield-like stress on the immune response. About 1400 transcripts were differentially expressed between pre- and post-RASP leukocytes. Upon functional analysis, immune response was the most enriched biological process, and most of the transcripts associated with the immune response were downregulated. Microbial pattern recognition, chemotaxis, antigen presentation and T-cell activation were among the most downregulated immune processes. Transcription factors predicted to be stress-inhibited (IRF7, RELA, NFκB1, CREB1, IRF1 and HMGB) regulated genes involved in inflammation, maturation of dendritic cells and glucocorticoid receptor signaling. Many altered transcripts were predicted to be targets of stress-regulated microRNAs. Post-RASP leukocytes exposed ex vivo to Staphylococcal enterotoxin B showed a markedly impaired immune response to this superantigen compared with pre-RASP leukocytes, consistent with the suppression of the immune response revealed by transcriptome analyses. Our results suggest that suppression of antigen presentation and lymphocyte activation pathways, in the setting of normal blood cell counts, most likely contribute to the poor vaccine response, impaired wound healing and infection susceptibility associated with chronic intense stress.