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Background: Certain immune functions are known to be impaired in human beings exposed to Antarctic winter; in particular, decreased amounts of serum proinflammatory cytokines, such as TNF-alpha and IL-1, were noted. It is not known, however, whether this exposure has any effect on T-cell-mediated acquired immune functions., Objectives: This study aims to investigate whether exposure to Antarctic winter has any effect on T cell-dependent immune functions., Methods: We assessed changes in various immunologic indicators, including serum levels of various cytokines, peripheral blood Valpha24Vbeta11 natural killer T cell numbers, and T(H)1/T(H)2 ratios of 40 Japanese personnel exposed to an Antarctic winter. Also, a 2-month inland traverse was executed during the isolation, and the effect on the above indicators was assessed., Results: All subjects were healthy during the Antarctic isolation. The levels of serum TNF-alpha, IL-1Ra, IL-6, and IL-1beta were dramatically reduced and remained at low levels throughout the isolation. The decrease in the levels of TNF-alpha and IL-1Ra was more pronounced during the inland traverse than during the rest of the isolation. The percentage of Valpha24Vbeta11 natural killer T cells was significantly increased at the midpoint of the isolation. Most interestingly, T(H)1/T(H)2 ratio was increased significantly, and this T(H)1 bias was most prominent at the late point of the isolation., Conclusions: Exposure to an Antarctic winter appeared to induce T(H)1-skewed immunity in human beings.