Physical Activity Although exercise immunology is considered relatively new, since 1990 a significant literature has been established[123,124]. Studies have assessed the effect of individual bouts of exercise of varying duration on immune markers, the clinical effect of infection prevention after exercise interventions, and assessed the effect of exercise on the ageing immune system. Generally, there is consensus within the literature that increased physical fitness and participation is associated with a lower risk of infection, and more favourable inflammatory, oxidative stress and immune marker profile.
Findings indicate that an acute bout of exercise, less than 60 minutes in duration, results in increased immunosurveillance, represented by an increase in anti-pathogen activity of macrophages, an enhancement of innate and adaptive immune system activity, increases the release of anti-inflammatory cytokines, improves glycaemic control. In brief, enhancing pathogen fighting capacity while reducing barriers to healthy immune function, i.e. anti-inflammation[126–130]. However, bouts of intensive exercise longer than 120 minutes, elicit an increase in oxidative species, muscle tissue injury, a pro-inflammatory cytokine profile and strong innate immunity response[131,132].
Exercise interventions that have compared a control/’no exercise’ group to those engaged in regular physical activity, show that those who are exercising regularly experience significantly lower incidence of upper respiratory infections even after a period of 8 weeks, with potentially increasing benefits from 9 to 12 months of intervention[133–136]. Long term adaptations of greater anti-inflammatory action, higher frequency of increased immunosurveillance, greater metabolic and glycaemic control and reduced oxidative stress are considered to drive the lower susceptibility to infection. These long term effects are also realised by older individuals, such that habitual exercise is capable of regulating the immune system and delaying immunosenescence[137–139]. In contrast, very high volume training over time leads to immunosuppression and an increased susceptibility of upper respiratory tract infection compared to those who exercise at moderate volume over the same time period. Considering the modifiable nature and multi-faceted benefits that accompany physical activity, it is an important consideration when aiming to enhance the immune response of a population or individual.
Sleep Deprivation Sleep is essential to normal functioning of the immune system and overall health maintenance[140,141]. Effects of inadequate or poor quality sleep influence 33-45% of adults in Australia. Sleep deprivation in rodent models is fatal, with immune deficiency resulting in infection being one of the key hypotheses for this result[142,143]. In humans, short habitual sleep (<6 hours per night) is associated with reduced lifespan, increased vulnerability to viral infection, with sleep deprivation negatively impacting antibody titers after influenza vaccination.
The initial findings have investigated the effect of sleep duration on markers of immune function. On balance, the majority of studies indicate a suppression of immune markers, with clinical studies indicating a higher susceptibility of infection for lower amounts of sleep[142,143]. When compared to individuals sleeping 7 to 8 hours per night, those sleeping consistently less than 5 hours per night experience a higher incidence of viral and/or upper respiratory tract infections[146,147]. Poor sleep has been associated with metabolic disease, cardiovascular disease[148–150], neurodegenerative conditions[151–154], and overall mortality[155–157].
A growing literature that describes the relationship between sleep and immune function continues to confirm the findings presented above[158,159]. Addressing the factors that contribute to adequate quality sleep is an important consideration when addressing population and individual immunity[143,160,161].
Gut and the Microbiome Among the many factors that influence metabolic homeostasis, dysbiosis has some to light in recent years as an important contributor to immune-related diseases[115,162]. A healthy microbiome should be diverse, stable, resistant, and resilient[163–166]. The dynamic multispecies community that make up the microbiome of the digestive tract differ from person to person[167,168] and yet there are common factors found to have a strong impact on our homeostasis and immunostasis[169–171].
When states of disease, oxidative stress, chronic inflammation, food intolerances or other factors destroy commensal bacteria, allow pathobionts to flourish, or reduce the diversity of the gut biome, it leaves the host open for infection and a reduced ability to mount a proper and appropriate immune response. Reducing the state of dysbiosis has been shown to improve conditions of auto-immune diseases[172–174], allergic reactions[175,176], neurodegenerative diseases[177–179] among others.
As such, dietary and lifestyle interventions that address the dysbiosis-associated diseases will directly and indirectly reduce the prevalence and severity of immune-mediated and immune-associated diseases as mentioned above.
Pollution While a non-modifiable risk factor for individuals, there is a substantial literature base describing the effect of air pollution on immune health. Airborne participate matter and ozone, in both short and long term studies, are associated with mortality and cardiovascular and respiratory hospital admissions[180–182]. A growing evidence now exists for a relationship between immune function and airborne pollutant levels. Air pollution has an adverse effect on oxidative stress, macrophage activity and other related markers of the immune system. Individuals with a higher air pollution exposure have a greater susceptibility to respiratory viral infections including pneumonia and influenza, i.e. a lower immunity, . The pollutants that have been shown to have this effect include: nitrogen dioxide[183,184], ozone[185,186], and particulate matter[187,188].
In addition to air pollutants, studies have found that industrial products can have direct and indirect impacts on overall health and immunity[189–191]. Though there are many exposures that may be beyond one’s control, the limitation of food additives is one that can be promoted. Due to the impact certain food additives can have on inflammatory, autoimmune diseases, digestive health and overall immunological homeostasis[192–194], encouraging the selection of quality whole food is one approach to minimizing exposure to pollutants.
Oxidative Stress The detrimental effects of free radicals on cellular health are known, and evidence indicates that protecting tissues against environmental oxidative stress can help with natural barriers to infection[195,196]. Even in conditions of acquired infections, when the body increases its metabolic output to mount an immune response, antioxidant intake has shown promise in the prevention and treatment of respiratory and systemic infections. Furthermore, though many might opt for supplementation[197–199] in the hopes of finding nutrient adequacy, evidence points to the synergistic effects of nutrients from whole foods that contain flavonoids, anthocyanins, fibre and other beneficial nutrients, further encouraging the approach of healthy diet and lifestyle for longevity[200–205].
Plant foods offer protection against the development of chronic diseases related to oxidative stress[206,207], whereas high doses of exogenous antioxidants may not. The reason for the preference for naturally antioxidant and nutrient-rich foods is because of their synergistic effects in cellular repair and removing cellular damage[209,210].