Sauna as immune training? A new study shows a surprising immune response after just one sauna session

One 30-minute Finnish sauna session increased circulating white blood cells, but without a strong cytokine-driven response.

Sauna as immune training? A new study shows a surprising immune response after just one sauna session

Table of contents

    Sauna and immunity: what did the researchers test?

    Sauna bathing has appeared in epidemiological research for years as a practice associated with better health outcomes. Regular sauna use, especially in Finnish populations, has been linked to lower risks of cardiovascular disease, stroke, dementia, pneumonia, and all-cause mortality. The problem is that an observed association does not yet explain what exactly happens inside the body.

    This new study focused on one possible mechanism: how the immune system responds to short-term exposure to high heat. The researchers examined whether a single Finnish sauna session could change the number of circulating white blood cells and the levels of cytokines, which are signaling molecules used by immune cells to communicate.

    The key finding was that even one sauna session triggered rapid, transient mobilization of white blood cells. However, this was not a classic, strong inflammatory response driven by large cytokine changes. This suggests that sauna may act as a mild physiological stressor that temporarily shifts immune cells into the bloodstream.

    Study details

    • Publication title: Acute Finnish sauna heat exposure induces stronger immune cell than cytokine responses.
    • Authors: I. H. Heinonen, T. Koivula, M. Hollmén, J. Immonen, S. K. Kunutsor, S. Jalkanen, J. A. Laukkanen.
    • Publication year: 2026.
    • Journal: Temperature.
    • DOI: 10.1080/23328940.2026.2645467.
    • Study type: Experimental study assessing the acute physiological response to a single sauna exposure.
    • Population and sample: 51 middle-aged adults, including 27 women and 24 men; the mean age was around 50 years. Most participants had at least one cardiovascular risk factor but no active cardiovascular disease.
    • Intervention: A single 30-minute Finnish sauna session at 73°C and 10–20% relative humidity. Participants were allowed to drink 0.5 liters of water during the session.
    • Measurements: Counts of different white blood cell types, levels of 37 cytokines, and changes in body temperature before sauna, immediately after sauna, and 30 minutes after the session.
    • Primary finding: Total white blood cell count increased after sauna, and selected immune cell subpopulations became more abundant in the bloodstream. Most cytokines did not change significantly.

    This study did not test the long-term effects of regular sauna bathing. However, it shows that even a single heat exposure can quickly change the visible immune activity profile in the blood.

    White blood cells moved into the bloodstream

    The clearest effect of the sauna session was an increase in white blood cells, the immune cells responsible for threat detection, defense responses, and immune surveillance. The researchers observed an increase in total WBC count immediately after sauna in both women and men.

    Specific immune cell groups also increased. Neutrophils and lymphocytes, including T cells and B cells, rose immediately after the session but returned to baseline within about 30 minutes. The MXD cell group, which includes monocytes, eosinophils, and basophils, stayed elevated slightly longer, especially in women.

    In practical terms, this means that sauna did not activate one highly specific branch of immunity. The response looked more like general mobilization across multiple immune cell types. The researchers interpret this as a possible movement of immune cells from tissues or reservoirs into the bloodstream.

    The most important observations were as follows:

    • Total white blood cell count increased immediately after sauna, showing that the immune system reacts very quickly to short-term heat stress.
    • Neutrophils and lymphocytes rose transiently, but returned to baseline after 30 minutes. This suggests a short, dynamic response rather than prolonged inflammatory activation.
    • Monocytes, eosinophils, and basophils remained elevated for longer, especially in women. This may point to sex-related differences in the immune response to heat, although this needs further research.
    • The proportions between immune cell types did not change significantly, which is important because it suggests non-selective mobilization of the overall white blood cell pool rather than a response aimed at a specific threat.

    This partly resembles the response observed after physical exercise. Exercise also causes a temporary increase in circulating white blood cells. Sauna may therefore act as a mild stress stimulus that temporarily increases immune cell “patrolling” throughout the body.

    Cytokines did not play the main role

    Cytokines are often associated with inflammation, immune cell communication, and coordination of the body’s response to stress or infection. This is why the researchers measured 37 cytokines to see whether they were driving the white blood cell mobilization after sauna.

    The result was surprising. Despite the clear increase in immune cell counts, only two of the 37 measured cytokines changed significantly. This suggests that the observed immune cell mobilization was not strongly driven by a classic cytokine response visible in the bloodstream.

    There are two possible explanations. First, the mechanism may rely on other pathways, such as hemodynamic changes, hormonal responses, or signals related to acute heat stress. Second, cytokine responses may have been too local, too brief, or too subtle to be captured at the selected measurement timepoints.

    It is also important that people whose body temperature rose more showed different cytokine trajectories than those who heated up less. However, a similar relationship was not observed between temperature increase and white blood cell count. This suggests that immune cells and cytokines may respond to sauna partly independently.

    What does this mean for health and longevity?

    This study fits well with the concept of hormesis. Hormesis describes a situation in which a small dose of a stressor may stimulate adaptive protective mechanisms, while too much of that same stressor may be harmful. Sauna, like exercise, cold exposure, or temporary energy restriction, may be one of the stimuli that briefly disrupts homeostasis in order to activate an adaptive response.

    However, this does not mean that one sauna session “boosts immunity” in a simple marketing sense. The study showed a change in the number of immune cells in the blood, but it did not test whether participants got sick less often, fought infections more efficiently, or achieved better long-term health outcomes.

    The most reasonable interpretation is therefore moderate:

    • Sauna produces a measurable physiological response, and is not merely a form of relaxation. The body responds to heat with changes in temperature and immune cell mobilization.
    • The effect is rapid and transient, which may matter in the context of repeated regular stimuli, but by itself does not prove lasting immune improvement.
    • The mechanism may partly resemble physical exercise, because both stimuli are associated with transient white blood cell mobilization.
    • The long-term benefits of sauna still require careful interpretation, because observational links with lower disease risk do not prove causality.

    In the context of longevity, sauna may be an interesting tool to support a healthy lifestyle, but it does not replace the fundamentals: sleep, movement, nutrition, stress regulation, and proper treatment of actual health problems.

    Limitations and practical context

    The most important limitation is that the study examined the acute response to one sauna session. It is not known whether the same response occurs after weeks or months of regular sauna use, or whether this immune cell mobilization translates into a lower risk of infections or chronic disease.

    A second limitation is the way temperature was measured. The researchers used tympanic temperature, measured in the ear. On average, it increased from 36.4°C to 38.4°C after the session, indicating real heat stress. However, this is not an ideal measure of deep core body temperature, for which rectal or esophageal measurements are considered more accurate but are harder to use in larger studies.

    It is also worth noting that participants were allowed to drink water during the sauna session, and the researchers corrected their results for possible plasma volume changes. This matters because sweating can concentrate the blood and artificially increase measured cell and protein concentrations per unit of volume. In this study, the average plasma volume change was not significant, but the correction strengthens the reliability of the findings.

    From a practical perspective, the most reasonable conclusions are:

    • Sauna is a biological stimulus, so people with cardiovascular disease, blood pressure problems, arrhythmias, infection, or poor general condition should approach it cautiously and consult a physician when needed.
    • Hydration matters, because high-heat exposure increases sweating and may affect plasma volume and how a person feels after the session.
    • More is not always better, because hormesis works only within a certain range. Sessions that are too long, too hot, or too frequent may stop being a beneficial adaptive stimulus.
    • Sauna fits best as an addition to a healthy lifestyle, not as a standalone intervention meant to compensate for poor sleep, sedentary behavior, or chronic stress.

    The most interesting conclusion from this study is therefore not “sauna boosts immunity.” It is rather: just one sauna session is enough for the immune system to respond in a measurable, rapid, and more cellular than cytokine-driven way.

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