Researchers found that a 30-minute sauna session increased the number of every type of circulating white blood cell. Participants briefly cooled down under a cold shower halfway through the session.

Levels of neutrophils and lymphocytes, two important 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," says Ilkka Heinonen, an Academy Research Fellow at the University of Turku.

A similar release of white blood cells happens during physical exercise. By sending more of these cells into circulation, the body may improve immune surveillance and strengthen its ability to respond to possible threats.

Cytokine Changes Varied With Temperature

The researchers also examined several cytokines, signaling molecules that help coordinate immune activity. Overall, sauna bathing produced only small changes in the average levels of cytokines circulating in the blood.

However, individual cytokine responses appeared to depend partly on how much a participant's body temperature increased during the sauna session.

"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," says Professor Jari Laukkanen, who led the study at the University of Eastern Finland.

Longer Term Effects Remain Unclear

Regular sauna use has previously been linked to a range of health benefits. The temporary immune changes observed in this study could help researchers understand some of the biological processes behind those associations.

Still, the researchers caution that the experiment measured only the immediate effects of a single sauna session. The findings cannot establish whether repeated sauna bathing produces 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.