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The immune system consists of a vast number of cells, tissues, and messengers—for example, cytokines—that play a key role in the protection of the body against infection and in healing after injury. It is becoming increasingly evident that it is highly integrated with our neurological and endocrine systems, and research now seeks to understand and exploit these interactions.1 The need for an active immune system is self evident if an athlete is to continuously produce peak performances, but often intense exertion and treatment for inflammation lead to partially reduced immune capacity and consequently potential infection or disease. To circumvent such problems, it is of major importance to understand how to achieve the optimum balance of the immune system.
A review of the literature highlights the fact that sports immunology is now becoming a significant subdiscipline of sports science in terms of publications produced, symposia, and the development of specialist journals. Research has generated a plethora of interesting results on the effects of exercise on the immune system. It is also evident that there are variations and, in some cases, conflicts in the results published. The parameters that may modulate immune responses during exercise include nutritional status, changes in circulating levels of cytokines, the expression of adhesion molecules, changes in chemotaxis/mobility, and the generation of reactive species. However, many of these factors are closely interlinked—for example, the role of cytokines and adhesion molecules in modulating leucocyte mobility. Immune responses are also affected by factors such as age, sex, biological rhythms, and lifestyle, and there are technical variations in the methods used to extract, purify, store, and analyse samples. There are many reports of increases/reductions in immune related cell numbers, and, although these may reflect alterations in cytokine and other levels, it is important to relate these to functional tests of activity. This is true not just in the case of sports immunology. The design of all aspects of sampling needs to ensure that results generated are highly controlled, physiologically relevant, and technically accurate.2, 3
In general, the literature suggests that acute exercise—for example, marathon and ultramarathon running—results in an associated reduction in aspects of immune competence so that such athletes may be at increased risk of illness and need to pay particular attention to their nutritional state, hygiene, and exposure to infections.3 It has been suggested that exercise induced reductions in particular lymphocyte subsets in runners is more dependent on training intensity than volume and is transient.4 Other researchers consider that immunosuppression caused by stress as a result of acute exercise is not due to reallocation of scarce metabolic components but may represent a mechanism to reduce the potential of an autoimmune response.5 Fallon and colleagues6 concluded from their recent research that intense exercise (ultramarathon running) results in a range of alterations in haematological parameters consistent with the normal acute phase response to injury. This should not, in their view, be confused with disease, and athletes can adapt to such situations. However, for inadequately fit people, or those unwell or under medication, undertaking very strenuous exercise could be deleterious. It is also known that anaphylactic reactions may be induced by exercise in some.
There are a number of reported beneficial clinical applications of exercise to immunology in aging, cerebrovascular disease, management of acute viral infection (such as AIDS), cancer, and chronic fatigue syndrome.7,8 Studies in humans and experimental animals indicate that a combination of dietary restriction and physical exercise can retard age associated reductions in immunological reactivity,9 and enhancement of some immune factors was also induced by exercise in patients with cerebrovascular disease.10 Clearly the exploitation of exercise as a treatment modality is deserving of further study, and, for the athlete, the rapidly developing molecular and cellular laboratory research approaches may lead to exciting insights into how the ideal balance of the immune system may be achieved and exploited to maximise performance and health.
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