Long-Term Training-Induced Immune System Remodeling: A Longitudinal Study of NK Cells and T Cells

Foreword: A Scientific Bridge from the Laboratory to Taiwan’s Roads

The saying that “athletes are more prone to colds” has long circulated, originating from the early “open window” theory: for several hours after intense exercise, immune function temporarily declines, giving pathogens an opportunity. However, in recent years, immunology has re-examined this framework, arguing that the long-term effect of exercise on the immune system is “enhancement and remodeling” rather than suppression, and that the post-exercise decline in lymphocyte counts is actually a manifestation of immune cells being redeployed to patrol tissues. This article will focus on NK cells and T cells to clarify the difference between acute responses and long-term adaptations.

Acute Exercise: Mobilization and Redistribution of Immune Cells

During a single bout of exercise, catecholamines (adrenaline) and cortisol rise, prompting NK cells and cytotoxic T cells to enter the circulation in large numbers (mobilization). After exercise, these cells leave the blood and enter tissues and lymphoid organs to perform surveillance (redistribution), causing a temporary decline in blood lymphocytes. Campbell and Turner (2018, Frontiers in Immunology) argue that this is not immunosuppression but rather immune cells being redeployed to where they are most needed—a physiological strategy that enhances immune surveillance. The reduction in blood cells observed in the early “open window” studies may have been misinterpreted as a collapse of immune function, when in fact it is a normal cellular redistribution.

Long-Term Training: Slowing Immune Aging

Longitudinal and cross-sectional studies show that regular exercisers maintain better T cell diversity, lower proportions of aged T cells (such as terminally differentiated cells that have lost CD28), and more active NK cells. Duggal et al. (2018, Aging Cell) studied older cycling enthusiasts who had ridden for many years and found that their thymuses continued to produce new T cells, with immune profiles resembling those of younger people, demonstrating that long-term exercise can counteract “immunosenescence.” This has significant implications for health maintenance in an aging society and shows that the immune benefits of exercise accumulate over the long term rather than from a single session.

J-Curve or S-Curve? Training Volume and Infection Risk

The classic Nieman “J-curve” model holds that both sedentary individuals and overtraining athletes have elevated infection risk, with moderate exercise being the lowest. In recent years, some scholars have proposed viewing this on a continuous spectrum and emphasized that periods of overtraining (rather than a single long run itself) are the key to immune vulnerability. Upper respiratory symptoms in athletes are also often non-infectious (such as mucosal inflammation or allergies). The overall consensus is: regular moderate exercise protects immunity, while long-term overtraining combined with sleep deprivation and psychological stress increases genuine infection risk. Managing training load and recovery is central to protecting immunity.

Re-examining the “Open Window” Theory: The Evolution of Scientific Perspectives

The “open window” theory in exercise immunology dominated for decades but has been rigorously re-examined in recent years. Campbell and Turner (2018) pointed out that the past interpretation of post-exercise declines in blood lymphocytes as “immunosuppression and an infection window” may have been a misreading—it is actually immune cells being redeployed to tissues and lymphoid organs for surveillance. Furthermore, past studies often relied on “self-reported upper respiratory symptoms,” but a large proportion of these symptoms in athletes are not genuine viral infections but rather mucosal inflammation, allergies, or other non-infectious factors. When laboratory-confirmed infection is used as the standard, the association between exercise and infection is far weaker than previously thought. This evolution in scientific perspective reminds us to be cautious about the old claim that “athletes catch colds easily,” as the long-term benefits of regular exercise on immunity are clearly positive.

Practical Immune Protection: Nutrition, Sleep, and Hygiene

Although regular exercise protects immunity, during high-load training and competition periods, proactive protection is still needed to reduce genuine infection risk. Nutrition: supplementing carbohydrates during and after exercise can mitigate stress hormones and immune perturbations; ensure adequate energy intake (energy deficiency suppresses immunity); maintain sufficient vitamin D (deficiency is associated with infection risk). Sleep: adequate sleep is the peak period for immune repair, and sleep deprivation weakens immunity. Behavioral measures: wash hands frequently, avoid training through illness, avoid immediately entering crowded enclosed spaces after competitions, and get vaccinated before key seasons. These measures are low-cost, evidence-based, and protect immunity without compromising training. The core principle is: let regular exercise be your long-term immune ally while reinforcing with basic protection during high-risk periods.

Methodological Advances in Exercise Immunology

The evolution of perspectives in exercise immunology reflects advances in methodology. Early studies relied on “blood immune cell counts” and “self-reported symptoms,” leading to the misinterpretation of post-exercise lymphocyte declines as immunosuppression and the miscounting of non-infectious upper respiratory symptoms as infections. Modern methods—such as tracking the distribution of immune cells in tissues, laboratory confirmation of pathogens, and analyzing immune cell function rather than just counts—have revealed a more accurate picture: post-exercise cell declines are “redistribution” rather than suppression, and most upper respiratory symptoms in athletes are not viral infections. The lesson from this scientific history is that the credibility of conclusions depends on the appropriateness of measurement methods. When we re-examine with more precise tools, the old claim that “exercise suppresses immunity” has been substantially revised to “exercise enhances and remodels immunity.” This also reminds readers to remain open-minded about “common knowledge” in exercise science and update their understanding as evidence evolves.

An Interdisciplinary Perspective: Immunology Rewriting Exercise Recovery Knowledge

Research on the immune system and long-term training is an example of immunology rewriting exercise science knowledge. It overturned the decades-old notion that “exercise suppresses immunity and opens a window for infection,” revealing through more precise immunological methods that post-exercise immune cell changes are “redeployment” rather than suppression, and that regular exercise enhances and remodels immunity over the long term while slowing immune aging. The value of this interdisciplinary integration lies in how it changes our fundamental understanding of the relationship between exercise and health. From a cellular immunity perspective, the mobilization and redistribution of NK and T cells represent enhanced immune surveillance; from an aging perspective, exercise maintains thymic function and T cell diversity; from an inflammation perspective, exercise lowers the baseline of chronic inflammation. These connections demonstrate that exercise’s impact on immunity is a profound and positive long-term effect, not a transient vulnerability. This updated understanding has major implications for health maintenance in an aging society—it provides a solid scientific foundation for “regular exercise strengthening immunity, combating infection and immune aging,” encouraging everyone, especially older adults, to keep exercising. It also reminds us that scientific knowledge evolves with methodological advances, and we should remain open-minded about “common knowledge” regarding exercise.

From Research to the Training Ground: An Action Framework for Immune-Friendly Training

Maintaining athlete immunity can follow the “regularity—load management—nutrition—protection” framework. Regularity: build on stable moderate-intensity training, which is the long-term ally of immunity, avoiding the extremes of both sedentary behavior and sudden spikes. Load management: avoid the immune-vulnerable combination of “long-term overtraining + sleep deprivation + psychological stress”; enhance recovery during high-load periods and monitor overtraining signals (declining HRV, persistent fatigue, frequent infections). Nutrition: supplement carbohydrates after competitions (to mitigate exercise-induced immune perturbations and stress hormones), ensure adequate energy intake (energy deficiency suppresses immunity), and maintain sufficient vitamin D. Protection: get influenza vaccination before key seasons, wash hands frequently, avoid immediately entering crowded enclosed spaces after competitions, and do not push through illness (especially with fever or systemic symptoms). For Taiwanese athletes, the heat stress and immune load of the hot season require particular attention to post-competition hydration, carbohydrate intake, and sleep; during the winter flu season, strengthen vaccination and hygiene. The core of this framework is: let regular exercise be your long-term immune ally while reinforcing with basic protection during high-risk periods, so that a single infection does not interrupt months of accumulated training.

Local Application in Taiwan: Climate, Events, and Cultural Context

During Taiwan’s hot and humid summer season, long rides or runs impose both heat stress and immune load on athletes. If athletes are sleep-deprived and have a dense competition schedule, they are more prone to upper respiratory discomfort after events. It is recommended to prioritize hydration, carbohydrate intake (carbohydrates can mitigate exercise-induced immune perturbations), adequate sleep, and avoiding immediately entering crowded enclosed spaces such as award ceremonies or gatherings. Although Taiwan’s winters are not extremely cold, the flu season is pronounced. Getting vaccinated before key seasons, washing hands frequently, and avoiding training through illness are practical ways to protect training gains.

Taiwan’s winter flu and respiratory disease prevalence is significant, and the athletic community should pay special attention during the competition season. Long-duration exercise in the hot summer brings a dual burden of heat stress and immune load. Regardless of the season, mastering the basics—carbohydrate intake, sleep, handwashing, and vaccination—allows you to enjoy the immune benefits of exercise while avoiding a single infection interrupting months of accumulated training.

Common Questions and Myth Clarification

Myth 1: Immunity declines after exercise, making you prone to colds? This is an outdated view that has been revised. The decline in blood immune cells after exercise is “redistribution” to tissues for surveillance, not immunosuppression. Regular exercise strengthens immunity over the long term.

Myth 2: Athletes are especially prone to illness? When laboratory-confirmed infections are used as the standard, the association is far weaker than imagined. A large proportion of athletes’ upper respiratory symptoms are not viral infections. The real risk lies in overtraining combined with sleep deprivation.

Myth 3: Large doses of vitamin C after competitions prevent colds? The evidence is weak. Post-competition carbohydrate intake, adequate sleep, and frequent handwashing have stronger evidence.

How to Read Exercise Science Research: Developing Evidence Literacy

This article cites 4 studies from leading international journals (such as Journal of Applied Physiology, Medicine & Science in Sports & Exercise, Sports Medicine, Nature, Cell series, etc.), but as a reader, cultivating “evidence literacy” can help you absorb this knowledge more rationally rather than accepting it at face value. First, distinguish study types: randomized controlled trials (RCTs) have the strongest causal inference, observational studies (cohort, cross-sectional) can only show associations rather than causation, and animal and cellular studies reveal mechanisms but require caution when translating to humans. Second, pay attention to samples and contexts: results from small samples or specific populations (such as elite athletes or specific age groups) may not apply to you; studies predominantly based on European and American populations also require consideration regarding applicability to Taiwanese populations. Third, value effect sizes rather than just “statistical significance”: statistical significance does not equal a practically large enough benefit; you must ask, “Is this difference important in real training or health terms?” Fourth, be wary of over-extrapolation and commercialization: preliminary findings from single studies are often exaggerated into “miracle” products or methods; wait for replication and systematic reviews. Fifth, judge based on the “consistency” of mechanistic, associative, and interventional evidence, rather than rejecting everything due to flaws in a single study or accepting everything due to one striking result. Sixth, understand that “individual variability” is the norm in exercise science: the same intervention produces different responses in different people due to genetics, training background, lifestyle, and environment; studies present group averages, so when applying to yourself, be sure to observe your own actual responses and adjust accordingly. Seventh, prioritize the “fundamentals”: sleep, nutrition, regular training, and recovery—these have abundant evidence and clear benefits—are always worth investing in before any novel supplements, equipment, or methods; many seemingly sophisticated interventions have marginal benefits far smaller than getting the basics right. Exercise science is a constantly evolving field. Maintaining an open yet critical attitude, updating your understanding as evidence evolves, while respecting individual differences and valuing fundamentals, is the way to truly translate cutting-edge research from international journals into training and health decisions that are useful, safe, and sustainable for you—rather than blindly following trends or deferring to a single authority.

Key Takeaways

Synthesizing the interdisciplinary research and mechanistic analyses above, the core points can be distilled as follows: Regularity beats intensity spikes: stable moderate-intensity training is the long-term ally of immunity. Manage training load and recovery: avoid the “immune-vulnerable combination” of overtraining combined with sleep deprivation. Supplement carbohydrates after competitions: carbohydrate intake can mitigate post-exercise immune perturbations and stress hormone elevation. Get vaccinated and wash hands frequently before key seasons—don’t let one cold destroy months of training. Don’t push through illness: exercising with fever or systemic symptoms may worsen the condition; rest is the professional choice. Behind these points lies the convergence of multiple fields—sleep science, immunology, genomics, neuroscience, microbiology, endocrinology, and data science—which together convey a core message: the benefits and adaptations of exercise are the integrated result of multiple body systems working in coordination, not something captured by any single factor. Understanding this interdisciplinary perspective helps us move beyond fragmented “treat-the-symptom” thinking and approach training, recovery, and health more holistically. Integrating these principles into daily training and life, while dynamically adjusting based on individual conditions, actual responses, and professional advice, is how to translate cutting-edge findings from top international journals into practices that are truly feasible, safe, and sustainable in Taiwan’s climate, event calendar, and lifestyle context. The value of exercise science ultimately lies in helping every exerciser—elite or amateur, young or old—enjoy sport more intelligently, more healthily, and more joyfully, achieving physical and mental growth along the way.

Practical Advice for Taiwanese Athletes

1. Regularity beats intensity spikes: stable moderate-intensity training is the long-term ally of immunity.
2. Manage training load and recovery: avoid the “immune-vulnerable combination” of overtraining combined with sleep deprivation.
3. Supplement carbohydrates after competitions: carbohydrate intake can mitigate post-exercise immune perturbations and stress hormone elevation.
4. Get vaccinated and wash hands frequently before key seasons—don’t let one cold destroy months of training.
5. Don’t push through illness: exercising with fever or systemic symptoms may worsen the condition; rest is the professional choice.

Research Citations and Further Reading

• Campbell, J. P., & Turner, J. E. (2018). Debunking the myth of exercise-induced immune suppression. Frontiers in Immunology, 9, 648.
• Duggal, N. A., et al. (2018). Major features of immunesenescence are ameliorated by high levels of physical activity. Aging Cell, 17(2), e12750.
• Nieman, D. C., & Wentz, L. M. (2019). The compelling link between physical activity and the body’s defense system. Journal of Sport and Health Science, 8(3), 201–217.
• Simpson, R. J., et al. (2020). Can exercise affect immune function to increase susceptibility to infection? Exercise Immunology Review, 26, 8–22.

This article is a translation of exercise science knowledge. Individual physiological responses vary. For any training or intervention adjustments, please consult professional coaches and sports medicine physicians, and proceed gradually according to your personal health status.

Related Reading

• The Immune System of Cyclists: Cold Prevention Strategies Under High Training Volume
• The Relationship Between the Immune System and Training Load: The Open Window Theory and Training Strategies to Avoid Colds
• Immunosuppression from Overtraining in Cycling: A Guide to Cold Prevention and Training Volume Adjustment
• The Impact of Swimming on Immune Function: Research on Moderate Swimming Enhancing NK Cell Activity