Respiratory and cardiovascular system are commonly effected due to acute bout of exercise. However, extensive research indicates that the immune system also responds to physical activity both acutely and chronically. Exercise induces humoral as well as neuroendocrine response, with lead to increase in plasma cytokines, adrenaline and cortisol, ultimately impacting immune function. The recent epidemiological data collected during international competition events has revealed that 2%–18% of elite athletes experience illness episodes, with higher proportions for females and those engaging in endurance events.

Both sympathoadrenal and hypothalamic-pituitary-adrenal (HPA) axes are activated at exercise onset that drive the immune response. The first and immediate acting one is the sympathoadrenal system, which leads to the release of adrenaline. This is the main signal that induces the mobilization and redistribution of leucocytes, particularly NK cells and cytotoxic T-lymphocytes, which results in a transient increase in circulating leucocyte numbers during exercise. In contrast to the immediate acting sympathoadrenal system, the HPA axis is responsible for the release of cortisol in a more delayed manner. Cortisol exerts an immunosuppressive effect during and after exercise by reducing lymphocyte activation and proliferation as well as circulating lymphocyte counts. It also reduces pro-inflammatory cytokine production by T CD4+, T CD8+ and NK cells and attenuates cytotoxic activity.

Two models have been described in literature to understand the relationship between exercise and the immune system. The “open window theory” suggests that immune function is compromised in the first hours after vigorous exercise and therefore, susceptibility to infections during this time is higher. The “J-shaped curve” model shows the relationship between the intensity of regular exercise and the risk of upper respiratory tract infection (URTI). Whereas moderate physical activity decreases the risk of URTI when compared to that of a sedentary individual, the same is higher with increasing levels of exercise.

High exercise training workloads, competition events, and the associated physiological, metabolic, and psychological stress are linked with transient immune dysfunction, inflammation, oxidative stress, muscle damage, and increased illness risk. IOC has also focused on load management of both internal (e.g., psychological responses) and external factors (e.g., training and competition workloads), and lifestyle strategies (e.g., hygiene, nutritional support, vaccination, regular sleep) to reduce illness incidence and associated downturns in exercise performance, interruptions in training, missed competitive events, and risk of serious medical complications. Several nutritional strategies like, increased intake of carbohydrates, polyphenols etc. have also been mentioned in literature for athletes.