The myth persists: Those who train hard are automatically "untouchable" by infections. Science paints a more nuanced picture. Regular, moderate exercise strengthens the immune defense – however, excessive strain without recovery can temporarily dampen it and promote infections ^{[1] [2]}. Those who hit the right intensity window gain health, energy, and resilience.

Our immune system is not a simple on/off switch but a finely regulated network. Exercise acts like a conductor: it enhances "immune surveillance" – the constant patrol of natural killer cellsimmune cells that early eliminate infected or degenerative cells and T-lymphocytescentral white blood cells of the adaptive defense. The dose is crucial. Moderate activity harmonizes cytokinessignaling molecules that regulate immune responses, reduces chronic low-grade inflammation, and slows down immunosenescenceage-related decline in immune function ^{[3] [1]}. In contrast, very intense, long sessions without adequate recovery can shift the balance of T-helper cellssubgroups that direct immune responses – resulting in temporarily reduced cell-mediated defense ^[2]. Context also matters: PM2.5very fine air particles from pollution and dehydration increase stress signals that can hinder immune performance ^{[4] [5] [6]}.

Those who engage in regular moderate exercise lower inflammatory markers, improve the mobility of immune cells, and thus strengthen the defense against respiratory infections – while simultaneously slowing immunological aging processes ^{[3] [1]}. Aerobic training reduces systemic inflammation and correlates with lower risks for inflammation-associated cancers; concurrently, there are more natural killer cells and T-cells – an immunological upgrade, especially in older individuals ^[7]. Conversely, excessive, poorly recovered training increases the risk of upper respiratory infections, often 3–72 hours after very long or intense exertions ^[2]. Environment and behavior also modulate the effect: Exercise on heavily polluted days promotes inflammatory responses in the airways, particularly in allergy sufferers ^[4], and fine dust can impair lung function and vascular health – with consequences for performance and immune balance ^[5]. Dehydration weakens neutrophil function after exertion and disrupts recovery processes ^[6].

Review articles on training dose and immune function show a clear pattern: Moderate intensity improves immune surveillance, antibody responses, and vaccine reactions, while excessive or prolonged intensities promote temporary suppression due to stress hormone shifts ^{[1] [3]}. These works are designed as narrative reviews and condense large evidence landscapes into practice-relevant dose recommendations. Mechanistically plausible is the harmonization of the cytokine milieu and better cell mobility. A large transnational cohort analysis links aerobic training with lower systemic inflammation and reduced risk for several inflammation-associated cancers. Complementary laboratory analyses show fewer immunosenescent signatures and more NK and T-cells after aerobic training, particularly in older animals – indicating that training increases "immune reserve" ^[7]. This combination of epidemiology and multimodal biology provides both relevance and plausible mechanisms.

On the flip side, overtraining research provides evidence that extreme exertions are associated with shifted T-helper cell polarization and temporarily weaken cell-mediated defense – clinically observable in more upper respiratory infections following marathons and during phases of very high training volumes ^[2]. Together, this creates a consistent picture: The dose makes the immune medicine movement.

- Plan for 150–300 minutes of moderate training per week: brisk walking, easy cycling, gentle swimming. This stabilizes cytokines, dampens chronic inflammation, and supports a robust immune response ^{[3] [1]}.
- Incorporate three aerobic sessions (20–60 minutes of jogging, cycling, or rowing): ideally between 117–500 minutes per week, depending on tolerance. Goal: reduce systemic inflammation, slow immunosenescence, strengthen NK and T-cells ^[7].
- Integrate 2–3 yoga or Tai Chi sessions (10–45 minutes): breath work, gentle flows, short meditation. Stress and inflammation modulation supports immune function, mentally and physically ^[8].
- Break sitting times every 30–60 minutes with 1–3 minutes of movement: standing up, 20 squats, stairs, or a short walk. This reduces sitting-related inflammatory profiles and promotes anti-inflammatory signals in the vascular system ^[9].
- Manage intensity and recovery: after intense days, plan for 24–48 hours of regeneration. Take warning signs like persistent fatigue, decreased performance, or frequent infections seriously – signs of impending immune suppression ^[2].
- Hydrate wisely: pre-load 5–7 ml/kg, drink according to sweat rate during exercise; add electrolytes in heat. This protects neutrophil function and accelerates recovery ^{[10] [6]}.
- Avoid poor air: check PM2.5/ozone in your app. Move intense sessions to times/places with better air quality (early morning, parks, indoor filters). This is particularly beneficial for allergy or asthma sufferers ^{[4] [5]}.
- Prioritize sleep (7–9 hours, consistent times): Good sleep quality correlates with higher mucosal defense markers and fewer infections after competitions ^[11].

The next big questions: What does the optimal, personalized "immune dose" look like – depending on age, genetics, microbiome, and environmental stress? Studies linking training protocols with air quality, sleep, and digital biomarkers could provide precise plans that maximize immune strength and minimize risks ^{[3] [5] [11]}.