The aim of this study was to examine the early effects of low dose 12C6+ irradiation or X-ray on peripheral blood lymphocytes (PBL) of patients with alimentary tract cancer and to explore mechanisms that may be involved in an antitumor immune response. We found that the percentage of T lymphocyte subsets, the mRNA expression levels of IL-2 and IFN- γ in PBL, and their protein levels in supernatant were significantly increased 24 hours after exposure to low dose radiation. The effects were more pronounced in the group receiving 0.05Gy 12C6+ ion irradiation than the group receiving X-ray irradiation. There was no significant change in the percentage of NK cell subsets and TNF-α production of PBL. Our study suggests that low dose irradiation could alleviate immune suppression caused by tumor burden and that the effect was more pronounced for 0.05Gy high linear energy transfer (LET) 12C6+ irradiation.