A single 30-minute sauna session, followed by a brief cold shower, triggers a temporary surge of white blood cells into the bloodstream, according to a new study from researchers in Finland that adds to a growing body of evidence on the physiological effects of heat exposure.
The study, published in the journal Temperature, found increases in every type of circulating white blood cell the researchers measured, including neutrophils, which are often the first responders to infection, and lymphocytes, which recognize specific threats and help coordinate the immune system's broader response. The increase was short-lived: both cell counts returned to their baseline levels within 30 minutes of finishing the sauna session.
Cells on Patrol, Not Permanently Added
"This may indicate that sauna bathing mobilizes additional white blood cells into the bloodstream from tissues, where they are then redeposited after the session," said Ilkka Heinonen, an Academy Research Fellow at the University of Turku. "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." Researchers describe that patrolling process as immune surveillance, and note that a broadly similar short-term boost in circulating immune cells occurs during physical exercise.
The researchers, who studied 51 adults with an average age of 50, also looked at cytokines, the small proteins immune cells use to communicate with each other. Average cytokine levels in the blood changed little overall, but the team found that levels of several cytokines varied depending on how much a person's body temperature rose during the session — a relationship that did not hold for white blood cell counts. "Interestingly, however, the levels of several cytokines changed in relation to how much body temperature rose during sauna bathing," said Professor Jari Laukkanen of the University of Eastern Finland, who led the study.
Not Yet Proof of Lasting Benefit
The researchers caution that their findings describe only the immediate aftermath of a single session and do not establish whether regular sauna bathing produces lasting improvements in immune function or lowers the risk of illness. Because the protocol combined heat exposure with a cold shower partway through, the results reflect that combined routine rather than the isolated effect of heat alone — a distinction the team says future studies will need to untangle.