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This topic has always been of interest and lately, for obvious reasons, this has become even more so.

While I won’t focus solely on COVID-19 in this, there will be some parallels. However, I will save Long COVID for another blog post.

This blog will start to explore the influence that exercise has on our immune system.

NOTE: Origin Physiotherapists are not virologists or immunologists. This is a nice introduction and exploration into an interesting and important topic that might help feed your curiosity or inspire to dust off those running shoes maybe.

I hope to discuss a few different areas…

  • What is our immune system?
  • What is being classified as exercise exactly?
  • What effects does exercise have on our immune system?

So what exactly is our immune system?

The immune system is large and complex and has a wide variety of functions.

Put simply it is our bodies protective system and defence against outside invaders such as viruses, fungi, bacteria and toxins.

Our first line defence includes our skin, mucous membranes, body temperature, low pH and special chemical mediators.

There are several factors which influence or affect the daily function of the immune system: age, gender, eating habits, medical status, training and fitness level.

The mammalian immune system can be divided into two main arms: innate and adaptive.

Innate – characterised by cells focussed on rapid recognition and identification of infectious agents and initiating a pro-inflammatory response.

The first line of defence. White blood cells.

These can be cells such as neutrophils, leukocytes, macrophages, monocytes etc. These incredible cells work on patrolling the body, trapping and destroying foreign invaders.

Adaptive – informed and directed by the innate arm, the adaptive system consists of natural killer, T or B cells. These are examples of lymphocytes – a type of white blood cell found in lymph (the fluid flowing around the lymphatic system).

B cells make Y-shaped antibodies specific to each pathogen. They lock onto the surface of an invading cell and mark it for destruction by other immune cells.

T cells are divided into helper and killer (NK) cells. Helper T cells, as the name suggests, help B cells to make antibodies and the killer cells to develop. Killer cells directly destroy cells that have already been infected by a foreign invader. Finally, T cells use cytokines as chemical messenger molecules.

Cytokines affect interactions and communications between cells, the process needed to mount and coordinate an effective immune response. There are both pro-inflammatory and anti-inflammatory cytokines.

The adaptive system takes longer to develop because the behaviour is learned and evolves from previous experiences. Adaptive immune cells live longer than innate cells. Upon re-infection the cells mediate a rapid and protective immune response (Pennock et al. 2013).

What are we classifying as ‘exercise’?

Exercise and the positive evidence behind its benefits is more and more compelling. There is strong evidence in adults demonstrating the protective effect with physical activity on a range of chronic conditions including coronary heart disease, obesity and type 2 diabetes, mental health problems and social isolation.

“Exercise” is a component of physical activity refers to physical activities that are planned, structured, repetitive, and undertaken for the purpose of improving or maintaining components of physical fitness and/or sporting performance.

“Physical activity” refers to activities undertaken during leisure time, at home, as part of employment, or for transport purposes.

The government recommendation to maintain overall health is 150 minutes (21/2 hours) of moderate intensity activity or 75 minutes of vigorous activity per week. You can also have a combination of moderate, vigorous and very vigorous (Department of Health & Social Care, 2019).

Exercise and our immune system

When reviewing the evidence linking exercise and the immune system, there is plenty to reinforce the positive link between the two.

So what exactly does exercise do to benefit our immune systems?

Primarily physical activity stimulates the immune system and strengthens the infection defence. There are indications that untrained people who start exercising regularly get a progressively stronger immune system and become less susceptible to infections (Nash MS 1994).

The immune system is very responsive to exercise with the extent and duration reflecting the degree of physiological stress imposed by the workload.

Physical activity has been found to have an effect on lowering the incidence, intensity of symptoms and mortality in viral infections observed in people who practice physical activity regularly. The practice of physical exercises acts as a modulator of the immune system.

Some of the mechanisms include:

  • Muscle fibre activation increases the release of calcium, which in turn promotes the synthesis of pro-inflammatory cytokines. These cytokines attract neutrophils to the site. Neutrophils released from the bone marrow during physical activity is influenced by a release of cortisol. The volume released can depend on the volume of contractile mass involved, the duration and the intensity of exercise.
  • Leukocytes concentration is increased due to shearing of immune cells in blood vessels during physical activity. Increased concentration of leukocytes can persist for up to 24 hours after exercise.
  • Blood flow increases during physical activity to maintain supply of metabolic material to the human body. An increase in the recruitment of natural killer cells occurs through cellular stress promoted by exercise.
  • Lymphocyte concentrations increase in the vascular bed during moderate physical exercise. After strenuous exercise, these levels decrease below the pre-exercise period.

In a bit more detail…

Increased immune surveillance

Increased immune surveillance against infections has been proposed as a mechanism responsible for improving the immune response related to physical exercise. Moderate intensity physical activity is responsible for providing an increase in the anti-pathogenic activity of macrophages, at the same time as elevations in the circulation of immune cells, immunoglobulins and anti-inflammatory cytokines occur (Neiman D and Wentz L. 2019).

During regular physical exercise practices, inflammatory responses and stress hormones are decreased. At the same time lymphocytes, NK cells, immature B cells and monocytes are at high levels. This means greater vigilance of the immune system as well as a reduction in the systemic inflammatory process, factors that strengthen the link that regular physical activity helps to improve the immune system (Nieman DC 2020).


Respiratory infections

Regular exercise of moderate intensity has already been associated with a reduction in respiratory infections compared to sedentariness. However, exhaustive physical practices before or during an infectious condition, such as influenza or COVID-19, can trigger severe illness due to changes in the immune system. This occurs due to the production of Th2 anti-inflammatory cytokines in order to reduce muscle tissue damage, but in strenuous activities this effect can reach immunosuppression levels, thus providing the opportunity for infections. (Neiman D and Wentz L. 2019) (Gleeson M et al 2011).

Note: Exercise should be delivered and conducted at an appropriate level, especially when undertaking exercise that is new or at a new level of difficulty.

Age, exercise and the immune system

The population of more developed countries currently is living longer, becoming more obese and, consequently, less active when it comes to physical exercise.

As a result the immune system undergoes negative changes; that is, there is a functional impairment of innate immunity and adaptive immunity called immunosenescence, which results in greater susceptibility to infectious diseases and systemic inflammatory processes, decreased response to antibodies and, therefore, compromised immunological surveillance (Nieman DC. 2020).



Therefore, for the elderly population, physical activity is even more essential, as these individuals generally have greater comorbidities and are more vulnerable to contracting disease. Damiot et al. (2020) suggested that individuals who have remained active throughout their lives have less pronounced immunosenescence characteristics.



The downside to exercise?

There are some potential negative responses to exercise.

  • After aerobic physical activity, there is a short-term reduction in neutrophil production, during which the opportunistic activity of infectious microorganisms can occur.
  • If physical activity lasts more than 3-hours, the concentration of natural killer cells returns to the pre-exercise level, or even drops this level as the cells migrate to sites of muscle injury, potentially leading to a decrease in our immune function.
  • However these are entirely normal responses to some activity.

In summary

Exercise is (and will always be) king.

Exercise must be prescribed but can be incredibly varied.

There are multiple mechanisms behind the improvement in our immune systems through exercise.

We benefit from continuing to exercise as these improvements can be reversed.

References

Campbell J and Turner J. 2018. Debunking the Myth of Exercise-Induced Immune Suppression: Redefining the Impact of Exercise on Immunological Health Across the Lifespan. Frontiers in Immunology. 9: 648

Damiot A, Pinto AJ, Turner JE & Gualano B. 2020. Immunological implications of physical inactivity among older adults during the covid- 19 pandemic. Gerontology 66: 431-438

Gleeson M, Bishop NC, Stensel DJ, Lindley MR, Mastana SS, & Nimmo MA. 2011. The anti-inflammatory effects of exercise: mechanisms and implications for the prevention and treatment of disease. Nature Reviews Immunology. 11(9):607–15

Jun-Ming Z and Jianxiong A. 2007. Cytokines, Inflammation and Pain. International Anesthesiology Clinics. 45 (2): 27-37

Nash MS. 1994. Exercise and immunology. Medicine and Science in Sports and Exercise. 26(2):125-7

Neiman D and Wentz L. 2019. The compelling link between physical activity and the body’s defense system. Journal of Sport and Health Science. 8 (3): 201-217

Nieman DC. 2020. Coronavirus Disease-2019: a tocsin to our ageing, unfit, corpulent, and immunodeficient society. Journal of Sport and Health Science. 9 (4): 293-301

Pennock N, White J, Cross E, Cheney E, Tamburini B and Kedl R. 2013. T cell responses: naïve to memory and everything in between. Advances in Physiology Education. 37 (4): 273-283

UK Chief Medical Officer’s Physical Activity Guidelines. 2019. Accessed 06/11/2020. https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/832868/uk-chief-medical-officers-physical-activity-guidelines.pdf

Windsor M, Bailey T, Perissiou M, Meital L, Golledge J, Russell F and Askew C. 2018. Cytokine Responses to Acute Exercise in Healthy Older Adults: The Effect of Cardiorespiratory Fitness. Frontiers in Physiology. 9: 203

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