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Exercise Immunology is a relatively new field in the study of the immune system of people mainly associated with some kind of physical training. This article will study the impact of exercise on people who do regular endurance training at various levels mainly easy, moderate, and intense. We will also study the impact of a single episode of exercise on our immune system and how it behaves. We must understand the relationship to be prepared for the effects and try and find out ways in which the same can be mitigated. We are aware the Human Immune System is extremely complex. Hence I will restrict myself to only its effect and relationship with exercise and training. 

immune system

Our immune system is working 24 X 7 to fight several pathogens and infectious agents that could make us potentially ill. Viruses, bacteria, parasites, fungi, toxins, pollution, etc are attacking us all the time. Although our immune system is always alert, we fall sick. Various factors change the behavior and effectiveness of our immune system. As mentioned earlier, we will restrict it to exercise only.

 Immunity Classification

There are various ways in which Immune System is classified and studied. We will study under the system where there are two main branches of Immune System namely ‘Innate Immunity and ‘Adaptive Immunity’. Although exercise will affect them both however broad changes happen in adaptive immunity because of exercise. Hence we will discuss Adaptive Immunity at length and its impact.

Innate Immunity 

Innate Immunity is non-specific to any particular kind of pathogen or infectious agent. It is more general and the response is immediate and quick. It is important to note here that it has no immunological memory and hence its behavior is the same even if the same infection occurs again and again.

Adaptive Immunity 

Adaptive immunity on the other hand is more specific to an antigen such as a flu virus. It has a delay in response and hence takes a long time from exposure to the response. It has an excellent immunological memory once exposure happens. It makes specific immune cells that are designed to fight and neutralize a specific virus. Further, we can divide adaptive immunity into two components, a humoral component, and a cellular component. 

Humoral arm makes antibodies that circulate in the blood, fight and neutralize all agents which can make us ill. The cellular arm, on the other hand, is responsible to make T-cells and B-cells that can kill cells that have already got infected thereby giving space to healthy cells again. Response to exercise hence affects the ability of both that is making of antibodies as well as killer T-cells. 

Impact Of Exercise

A single session of exercise will impact immunity depending on the intensity of the exercise and training. It is important to note here that a single moderate episode of training/ exercise has only a marginal effect on our immunity. On the other hand, heavy and intense exercise can transiently suppress our immune system for up to three hours or more post-exercise. A study was conducted in which well-trained and physically fit cyclists were asked to do intense training for two hours. They were told to function at 75% of VO2Max. It was noticed that their adaptive immunity was transiently suppressed for over an hour. 

In another study done on marathon runners, a single run significantly suppressed the production of T-cells for up to three hours after they crossed the finish line. The transient suppression came back to normal after six hours though. 

Open Window Theory

The suppression of both humoral and cellular immune systems soon after the exercise is completed has given rise to Open Window Theory. Basically, this theory states that after an intense exercise period, the immune system is suppressed transiently. This gives an opportunity to various viruses, bacteria, etc to get a foothold within a person and infect them. It appeared that the athlete was already exposed to the pathogen before training but the immune system was fighting well. However, as soon as the person enters the Open Window period, infection wins. This shows that susceptibility to various types of infections is at its peak post-exercise recovery period. 

The possible explanation is the suppression of the immune system due to the elevation of several stress hormones. These include the adrenal hormones of cortisol, epinephrine, and norepinephrine which are also immunosuppressant. Besides this, a rise in internal body temperature also plays an important role in immunosuppression. 

Now the question which comes to our mind is that these transient suppressions of the immune system translates into an increased susceptibility to infection for a longer period or not?

Unfortunately, the answer to this question is ‘YES’. It was noticed in various studies that in the preparation phase of various competitions such as marathons, Iron Man or Tour de-France, the incidents of infections were up to five times higher. 

Some people ask if they should exercise when they are sick or there is an infectious disease or virus around. There is a common belief that workouts will make our bodies excrete toxins and will prepare them better to fight viruses or bacteria. This is false. A single episode of exercise may suppress the immune system for many hours thus giving an open window to the infecting agents to get a foothold inside the body or make it worse if it is already infected. The general rule is to rest if the symptoms are below the neck.

The Final Call 

However, there have been many studies showing that in case of moderate to light exercise regularly, our immune system improves. While moderate training can improve immunity, intense sessions will have the opposite effect. High frequency of intense workouts will chronically suppress our immune function, making us more vulnerable to infections. In fact frequency of infections is very common in long-distance athletes.

Article by: Capt Jitender Harjai fitness Expert H2F Care

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