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Before discussing on effects of exercise on immune function, immune system needs to be addressed. The immune system is a network of cells, cellÂ products, and cell-forming tissues that protects the body from foreign invaders. Immune system can identify and remember millions of different enemies, abolishes infected and malignant cells, and removes cellular debris. The thymus, spleen, lymph nodes, lymph tissue, stem cells, white blood cells, antibodies are part of immune system. Immune system is very important to maintain good health. Disorder of immune system can lead to many different diseases. Immune system consists of two components, the innate immune system and the adaptive immune system.
The Innate system and the Adaptive system
The innate system is the first line of defence and is also called as non-specific immune system. Innate immune system includes external defence like mucous membrane; skin etc. microorganisms that escape the external defence came in contact with adaptive immune system. Adaptive immune system is made up of T and B cells. The adaptive immune system launches attacks specific to the attacking pathogen and requires some time to adapt its custom-made response. The adaptive system "remembers" antigens it has encountered and reacts more fast and competently the next time that antigen is found but more slowly than the innate system.
Sports immunology is quite new in field that observes the physical and environmental stress on immune function. More than 600 articles are issued in this area over the last 100 years. Lately scientist and clinicians have started to recognise the complex interaction between exercise and immune function.
Acute effect of exercise on immune system
"Exercise is defined as the leisure-time application of physical activity" (P.G.Brolinson Date). Exercise has major effects on immune system. Lots of scientists support this concept of bad and good effects of exercise on immune function. All these effects vary depending on the type, nature, frequency, intensity and duration of exercise. Different studies have describes that acute session of prolonged, tireless exercise has short-term effect on human immune function whereas other studies show that extreme form of exercises like marathon and ultra-marathon were related with increased risk to infections of the upper respiratory tract.
Heavy program of training and competition can lead to immune impairment in athletes which is related with increased exposure of infections, mostly upper respiratory tract infections (URTI), sepsis or trauma. There is increase in the number of leukocyte mainly lymphocyte and neutrophil due to amount and period of exercise.
In addition, there is also rise in the plasma concentration of various substances that are known to influence leukocyte functions such tumor Necrosis Factor (TNF), macrophage inflammatory protein-1 and interleukin-1-receptor antagonist (IL-1ra), IL-6, and IL-10 which are known to act as anti-inflammatory cytokines, and acute phase proteins including C-reactive proteins (Michael Gleeson 2007). During exercise concentration of plasma IL-6 is increased as well due to the release of cytokine from contracting muscle fibers however during exercise IL-6 production by monocytes was noticed to be withdrawn.
In response of exercise hormonal changes also occur, including increase in plasma concentration of several hormones like cortisol, growth hormone and prolactin known as immunomodulatory effects. During prolonged exercise IL6 is responsible for the increased secretion of cortisol.
T lymphocyte is another section that was known to be influenced in cell mediated immunity, T lymphocytes can be classified as Type-1 T cells depending on which cytokines they produce. Type-1 largely produces interferon (INF) and TNF for the activation of macrophages and starts of the killing mechanism by T cytotoxic cells for the purpose to stimulate the inflammation by the immune system against infectious (Michael Gleeson 2007) whereas type 2 mainly producesIL-4,IL-5,IL-10 and IL-13 which are important for raise of humoral immunity. Type 1 T cell can be inhibited together with IL-4 and IL-10. The percentage of type 1 cell decreases due to acute, vigorous and very prolonged exercise whereas Type 2 cell does not change. Type 1 cell can suppress due to the release of cortisol and epinephrine whereas IL-6 directly stimulate type 2 T cells. In athletes phagocytic properties of neutrophils seems to decrease during intense period of exercise as compared with light activity. This effect is seen in the swimmers as well during time of training.
Acute exercise also shows a role in increases the circulating natural killer cells but after exercise the number of natural killer cell will drop to the half of its normal level for few hours where standard values of inactive persons are normally restored within 24 hours. However if the exercise is vigorous and prolonged natural killer cell decreases and cytolytic activity started during exercise session. Also the number of circulating lymphocytes might decrease below the pre-exercise level for few hours after prolonged exercise and T-lymphocytes CD4+/CD8+ ratio decreases.
Furthermore vigorous and long exercise or unusual exercise usually produces an inflammatory reaction. During urinary secretion of IL-1, IL-6 and IFN were noted after prolonged exercise. All of these three cytokines where constantly raised in urine of trained versus untrained person.
Exercise also affects the antigen processing cell function by decreasing its volume. Exercise also made decrease in macrophage, MHC II expression. During exercise both T-memory and T-naÃ¯ve cells increases provisionally. Also production if immunoglobulin's is inhibited due to vigorous exercise. The body temperature and PH of blood also alters during exercise.
Chronic effect of exercise on immune system
Following an acute bout of exercise, change in circulation leukocyte number, function normally returns to the resting state within 3-24hr.
Heavy exercise training influences infection resistance, numerous recent studies have tried to identify these effects. Some of these findings are altered numbers of circulating leukocytes, plasma cytokine concentrations, natural killer cell activity, rate of secretory immunoglobulin A (IgA) secretion, as well as neutrophil and macrophage phagocyte activity. It's been noted that after a prolonged or intense exercise salivary IgA is decreased. After various type of exercise Serum immunoglobulin generally do not change or increase. Due to intense or prolonged exercise production of antibody in blood circulation is inhibited and local production of IgA in mucosa is also inhibited. IgA is the major antibody present in secretion of the mucosal immune system and is the first line of defence against upper respiratory pathogens. IgA have been related with an increase incidence of upper respiratory tract infection (URTIs).
Neutrophil concentration increases during severe exercise. The lymphocyte concentration also increase during exercise due to the recruitment of all lymphocyte subpopulation to the blood.CD4+ T cells, CD8+Tcells,CD19+B cells CD16+natural killer cells and CD25+natural killer cells increased in number during exercise and drops following intense exercise (Exercise n immune system).
In recent year numerous studies have done in investigating the effect of short period of increased training on resting immune function and on immunoendocrine responses to strength exercise. These studies shows relatively increase in leukocyte function which includes neutrophil and cytotoxic activity during the training load in athletes who are already well-trained. Even in short period of intense of training shows reduction in the amount of neutrophil function, lymphocyte proliferation and circulation number of T cell. Therefore both innate and adaptive immune system are depressed with chronic period of heavy exercise, however athletes are not clinically immune deficient. In other words we can say exercise cannot put athletes in danger but it could be easy to increase the risk of infection in athletes.
There is a relationship between cytokines and the muscle damage. Athletes can damage their muscles that can lead to necrosis and inflammation due to eccentric exercise. Inflammatory response is expanded due to the production of inflammatory cytokines. Neutrophils are gathered in a muscle which is monitored by macrophages. Prostaglandins are produce due to macrophage induced by cytokines that brings out the muscle pain. However some studies suggest that after exercise cells do not migrate to skeletal muscles. Those studies also show that muscular alteration to exercise may occur in non-inflammatory style.
Muscle adaption through interfaces between leukocyte and endothelial cell adhesion molecule can also affect the immune system and also release of cytokines and growth factor.
The muscle affected is penetrated by neutrophil which is den followed by macrophage. Macrophages become activated due to free radicles that are stored in the neutrophil. Pathways like compliment, the coagulation and fibrinolytic are activated. To control the inflammation and decrease the muscle soreness icing is used that usually affects the muscle and ingestion of antioxidant.
Icing causes the decrease in blood flow that slow down the transport of neutrophils and monocytes to the site of injury. Icing also reduces the enzyme activities that continue optimally at body temperature and the whole inflammatory process is slow down. However these procedures can help in control inflammation but will not stop the many symptoms associated with it.
Comparison between concentric and eccentric exercise and create an association between increased IL-6 level and muscle damage are noted by increase in creatine kinase. Therefore the post-exercise cytokine production is related to skeletal muscle damaged. Recently it's been found that IL-6-mRNA was found in the skeletal muscle biopsies that were obtained from the athletes after a marathon run. This shows that IL-6 is produces due the vigorous exercise or exercise cause muscle damage. However IL-1mRNA was expressed by blood mononuclear cells (BMNC) but was not present in skeletal muscle after marathon not before the run which indicates that IgA produce persuade a systematic anti-inflammatory response. Exercises resemble that seen in response to trauma is due to cytokine cascade and thus exercise may be considered a model of trauma. (P.Gunnar, 2007).
Nutrition and Immune function
Nutrition plays an important role in immune function. Nutritional deficiency can lead to the weak immune function and there is a enormous evidence that lots of infection is caused by the nutritional deficiency. However excess consumption of fatty acids, vitamins, zinc, and iron can cause damage to the immune system and leads to increase the possibility of different infection. This harmful effect on immune system might be associated with the increase in the amount of oxygen produced due to hectic exercise, leading to the extreme production of free radical. (P.Gunnar, 2007). Those free radicles are released by increased number of monocyte and neutrophils that might affect the function of lymphocyte that leads to the impaired function of later cells.
Therefore nutritional supplementation with protein, energy, mineral and carbohydrates may affect exercise associated immune function.
Protein and Energy
It's well-recognised that an insufficient consumption of protein damages host immunity with mainly effect on the T-cell system result in an increased rate of adaptable infection. Protein deficiency weakens immune function as immune defences are reliant on fast cell replication and the production of protein with important biological activities like immunoglobulin's, acute phase protein and cytokines. (Gleeson et al, 2003). In protein- energy malnutrition number of mature and full differentiate T-lymphocyte are found to be depressed. The ratio of T- lymphocyte CD4+/CD8+ are noticeably decreased in protein-energy deficiency. It also effects the phagocytic function of cell, cytokine production and the compliment fixation. However athletes are not likely to touch a stage of such malnutrition unless they have been dieting very extremely; however even in a moderate protein deficiency some of the deficiency of defence mechanism is observed. (Gleeson et al, 2003).
Energy-restricted diet is very common in sports where strength or low body mass is supposed to consult a performance or aesthetic advantage or is essential to meet certain body weight criteria. In athletes even short term dieting can damage the immune function.
Vitamins and Minerals
Numerous vitamins are important for normal immune function. Lack of fat soluble vitamins A and E, folic acid, B6, B12 can decrease resistance of various infections. Several studies investigated that some athletes take 600mg of vitamin C before their ultra-marathon. These athletes suffer with URTIs than the nonsupplemented athletes during few weeks of competition. Some minerals like zinc, iron, copper and magnesium are known to be exert modulatory effect on immune function. Also exercise has a prominent effect on both zinc and iron metabolism. Because of increased losses in sweat and urine athletes require these minerals more than the normal individual. However excess intake of some minerals can lead to damaging the immune function and increase the risk of infection. Immune function can also be affected by taking excess vitamin E and beta-carotene and can cause the increase in the oxidative stress cells. This effect was monitored over 14000 Scandinavian men taking excess of vitamin E and beta-carotene and that shows increase in the risk of URTIs while experiencing heavy exercise.
Iron deficiency has also little effect on immune function by affecting antibody generation, wherever some other researches are conflicting about its effect on cell-mediated immunity. No effect is shown on Supplementation of glutamine in athletes. Glutamine has a beneficial effect on the gut function, morbidity, mortality and some features of immune function, however if glutamate is taken in excess amount during and after exercise it causes the decrease in the lymphokine-activated killer activity and neutrophil function. Therefore intake of vitamin and minerals should be taken only when it's required and athletes should their supplements from proper diet.
Carbohydrates and lipids
Carbohydrates also have immunosuppressive effect of exercise. Some research shows that intake of carbohydrates have positively influenced blood cortisol, number of lymphocyte and natural killer cell activity during the exercise period. Ingestion of carbohydrates throughout exercise also attenuates increase in ACTH, catecholamine and cortisol. There is increase in neutrophil and lymphocyte ratio during exercise due to the intake off carbohydrate. It's been noted that the intake of 30-60g of carbohydrates per hour during vigorous exercise stop both the decline the percentage and the number of IFN T-lymphocyte and also the destruction of IFN production. (Gleeson.M, 2007).
In response to exercise there is an argument regards the decrease in plasma IL-6, as IL-6 has numerous metabolic effects and shared mechanisms concerning immune damage. Lipolysis will also inhibit due to decrease in IL-6 response which also decrease the anti-inflammatory effect of exercise and also decrease the amount of metabolic genes in exercised muscles. ( Gleeson.M, 2007). It's also likely that intake of carbohydrates during exercise might limit the change of training.
Intake of carbohydrates make athletes work harder and work for longer.
However there is no proof that physiological and performance of athletes are damaged by carbohydrate intake during exercise. The likely interaction between extreme acute exercises, known to destroy the immune system.
Several factors including the type of infection and quality and quantity of exercise are involved in the association between exercise and the exposure to infection. Regular reasonable exercise improves immune function whereas intense and vigorous exercise can destroy the immune function.
Viral Upper Respiratory Tract Infections
Upper respiratory tract infections are very common in athletes. Most of the upper respiratory tract infections are non-malignant and last for about 1-2 week. Sore throat, cough, nasal congestion, headaches, body aches, and fever are some symptoms of upper respiratory tract infection.
The most common cause of upper respiratory tract infection is viruses like rhinoviruses, coronaviruses, adenovirus, and Para-influenza virus.
Upper respiratory tract infections can also be caused by Human respiratory syncytial virus and metapneumovirus, two viruses that can cause more severe illness and also immune compromised among athletes. A survey was done on athletes who had classic symptoms of
upper respiratory tract infection .only 30% of causative agents wear found using various techniques that includes serology, culture, and polymerase chain reaction (PCR) from nasopharyngeal,
Saliva and blood samples
A study assessing athletes who had 48 hours of typical URI symptoms only found causative agents in 30% of cases using a variety of isolation techniques including serology, culture, and polymerase chain reaction (PCR) from nasopharyngeal, saliva and blood samples. Viral upper respiratory tract infections can be transmitted by different ways. This infection can be transmitted by person to person depends on the time people spend together. However most common way of transmission is hand to hand contact because the spread of infection through the inoculation of virus in the nose or eyes of susceptible persons. (Catherine F. Decker, 2010). So to prevent the infection the athletes are highly encouraged proper hand washing.
Especially in the winter season athletes can be at slightly higher risk for pneumonia. But regular lower respiratory tract infections should prompt evaluation for primary immunodeficiency. Bacterial, viral, and, hardly fungal causes should all be considered with endemic fungi. The most common cause of bacterial pneumonia is S. pneumonia. the most common symptoms in pneumonia are cough, sputum production, shortness of breath, chest pain, and fever are the most common symptoms in pneumonia. In athletes its essential to study the symptoms carefully because that will help to decide if outpatient therapy is suitable or inpatient observing is necessary.
Exercise Induce Asthma
The most common causes of chronic cough in athletes are asthma and allergies. Asthma usually develops in athletes within 5 to 10 minutes after exercise and may resolve in 30 to 90 minutes while resting or even still exercising. The common cause seems to be the inhalation of cold or dry air via mouth.
Pharmacologic mediators are release from mast cell is due to the mucosal drying which brings about bronchospasm. Mediator that is released includes histamine, leukotriene, kinins, serotonin, prostaglandins and cytokines. Those mediators produce vasodilation and also increased the permeability of vessels. It also causes smooth muscle contraction and activates other cells. Due to the inhibitory prostaglandins bronchospasm is not activated during this period. There is not much known about the effect of exercise on bronchospasm, but ventilatory effect should be decreased by the better fitness that is needed for exercise and should also help in reducing the degree of cooling and drying of bronchial mucosa.
In conclusion, both acute and chronic exercise causes harm in various aspect of immune function. Dietary deficiencies of nutrients are also related with weak immune function and cause more risk of infection in athletes. Athletes should take proper amount of vitamins A, E, B6, and B12 to maintain their immune system. Glutamine intake is beneficial to immune system.
Athletes should be immunized against seasonal influenza.
Athletes should take a prevention infection control that includes regular hand washing, minimal contact with sick persons, and the avoidance of sharing personal items.
Lots of studies have been done that confirms the both acute and chronic exercise affects the immune function by various ways. This is very important issue that is essential to be addressed in further studies.