A half-hour in the sauna may do more than leave people sweating. Finnish researchers say it briefly shifts more white blood cells into the bloodstream, including cells that help fight infection.

The study found that a single 30-minute sauna session increased the number of every type of circulating white blood cell. Participants also took a brief cold shower halfway through the session.

Levels of neutrophils and lymphocytes, two key parts of the immune system, returned to their usual levels within about 30 minutes after the sauna ended.

“This may indicate that sauna bathing mobilizes additional white blood cells into the bloodstream from tissues, where they are then redeposited after the session. This kind of periodic release of white blood cells into the bloodstream is beneficial, as once they leave their storage sites, they are better able to patrol the body and respond to pathogens,” said Ilkka Heinonen, an Academy Research Fellow at the University of Turku.

The researchers said a similar release of white blood cells happens during physical exercise. They said sending more of these cells into circulation may improve immune surveillance and strengthen the body’s ability to respond to possible threats.

The team also looked at cytokines, signaling molecules that help coordinate immune activity. Overall, sauna bathing produced only small changes in the average levels of cytokines in the blood.

“Interestingly, however, the levels of several cytokines changed in relation to how much body temperature rose during sauna bathing. No similar association was observed between white blood cell counts and changes in body temperature,” said Professor Jari Laukkanen, who led the study at the University of Eastern Finland.

The researchers said the temporary immune changes may help explain some of the biological processes behind earlier links between regular sauna use and a range of health benefits.

They also said the study only measured the immediate effects of a single sauna session. The findings cannot establish if repeated sauna bathing causes lasting changes in immune function or long-term health.

The study included 51 adults with an average age of 50. The findings were published in Temperature, a scientific journal focused on the medical physiology of body temperature regulation.

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