Acute cold exposure mobilizes immune cells without showing improved resistance to infection.
Two hours of cold-air exposure can acutely mobilize several immune-cell populations. Seven healthy young men completed four experiments in which they sat for two hours in a chamber maintained at 5°C (41°F). Before the cold exposure, they either rested at a neutral temperature, underwent passive heating, or exercised under conditions designed to produce different changes in body temperature.
Cold exposure increased total leukocytes and granulocytes, raised natural killer cell number and activity, and increased circulating interleukin-6. Exercise performed without increasing core temperature amplified some leukocyte, granulocyte, and monocyte responses. The investigators proposed that increases in norepinephrine may have helped mobilize these cells.
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These results demonstrate an acute stress response, not improved immunity. The study included only seven men and did not measure colds, influenza, pathogen clearance, or any other clinical outcome. A rise in circulating immune cells can reflect redistribution into the blood and does not by itself show that the cells became more effective or that participants would be less likely to become sick.