Finnish researchers led by Professor Jari Laukkanen at the University of Eastern Finland observed that a single 30-minute sauna session, including a brief cold shower halfway through, causes white blood cells to surge into the bloodstream. The study measured increases in multiple types of circulating white blood cells, including neutrophils and lymphocytes, both important to immune defense. These increases were temporary, returning to baseline levels within 30 minutes after the session ended.
According to Ilkka Heinonen, an Academy Research Fellow at the University of Turku, the findings suggest that "sauna bathing mobilizes additional white blood cells into the bloodstream from tissues, where they are then redeposited after the session." This process, known as immune surveillance, allows immune cells to patrol the body more effectively and respond to pathogens. A similar response occurs during physical exercise, though the researchers did not claim the immune benefits are identical.
The researchers also examined cytokines, proteins that help immune cells communicate. Overall cytokine levels showed little change, but certain cytokines changed in relation to how much body temperature rose during the session. White blood cell counts, by contrast, showed no relationship with temperature increases. This suggests sauna bathing may influence different parts of the immune system through different processes.
The study involved 51 adults with an average age of 50 and was published in the journal Temperature. The researchers emphasized that the findings do not establish whether sauna bathing produces lasting improvements in immune function or reduces illness risk. The observed changes were immediate and short-lived. Because the session combined sauna heat with cold exposure, the results reflect the combined routine rather than heat alone.