Have you ever been in a raging bad mood, snapping at friends for no reason, hating the world? Maybe all of that pent up energy eventually motivated you to get out of your toxic mind and into the gym or out for a run? And then, poof! Your mood has been reset, you’re calmer, your anger has dissipated. There’s a plethora of reasons why staying physically active is beneficial, and it just so happens that tempering the immune system can be added to the list.
As with lots of things however, it’s possible to have too much of a good thing. The effects of exercise on the immune system can be pictured as an upside down “J” as seen in the figure below.
Too little or too much and immune function is poor. Like Goldilocks, you need just the right amount for optimum immune function. I myself try to stay pretty active with volleyball, yoga, biking to work sometimes, hiking, and walking my dog. But you’ll never see me training for a marathon because I want my immune system to be at its best. (That’s not true at all. I hate running and the thought of 26.2 miles screams Hell to me, and I don’t think the human body was made for such insane distances! And apparently my method isn’t fail-safe as I’m fighting a cold while writing this!)
The overall view is that the positive effect of exercise on your immune system is to keep inflammation under control. Inflammation is the root cause of a lot of diseases including cardiovascular diseases, chronic obstructive pulmonary disease, various cancers, dementia, and even depression. In this post, I’ll talk about a few of the ways exercise turns down the immune system’s anger (inflammation) including decreasing fatty tissue, stimulating muscle cells to make anti-inflammatory molecules, and preventing infiltration of macrophages (those important cells in the innate immune system named, ironically in this case, for their “big eating” habit).
Fatty tissue and inflammation
Of course the main motivating factor behind all of us exercising is to trim down and burn away some of that fat. I always know when my pants start filling out a bit too much that it’s time for some yoga. It turns out though that growing waist bands (even more so than growing bums or legs or arms) are not just bad for our vanity but also for the immune system. That’s because the fat tissue, also called adipose tissue, causes chronic, low level inflammation. This comes in the form of chemical messengers called cytokines whose slow, steady release into the bloodstream can be damaging to tissue and cause the immune system to get all fired up. It’s like that low level annoyance that you have towards a coworker that initially is no big deal but its continued presence leads to an explosion….or in this case the development of Type 2 Diabetes or cardiovascular disease. So battling the bulge is good for the health in part because it prevents the accumulation of fat cells that can cause systemic inflammation.
Muscles and anti-inflammatory messages
While lack of exercise causes the accumulation of pro-inflammatory adipose tissue, engaging in physical activity promotes the opposite of that or an anti-inflammatory state. Even after short periods of moderate exercise, muscles (not tissue we normally think of as having an immune role) start to produce signals that can calm down the immune cells. This comes in the form of a cytokine or message called IL-6 which is released quickly after a workout and then starts a cascade of production of other anti-inflammatory signals.
In obese patients, the fatty tissue has lots of macrophages that can instigate an inflammatory state. Physical activity can prevent these cells from infiltrating into the fatty tissue. One way this might occur is through decreased production of the docking site for macrophages. For immune cells to go from traveling through the bloodstream at high speeds to entering a tissue they need to go through several steps that slow the cells down. Exercise can interrupt this process and therefore block inflammatory macrophages from entering fat tissue.
Too much exercise!
So it seems like exercise decreases inflammation but of course inflammation has its purposes, particularly in protecting us from infection. So does that mean exercise makes us more susceptible to infections? It turns out that elite athletes oftentimes get more upper respiratory tract infections especially after intense training. Within our salivary glands are lots of antibodies (called soluble IgA) that are the first barrier that prevent infections from occurring. These antibodies act like bouncers at a bar; they bind to viruses and bacteria entering through our airways and stop them from getting into our bodies. Intense workouts in elite athletes have decreased levels of these antibodies and are therefore more susceptible to respiratory infections. Their susceptibility may also be affected by the anti-inflammatory state created by intense workouts.
Moderate exercise therefore has an overall positive effect on our immune systems. It prevents the accumulation of pro-inflammatory fat cells and promotes muscle cells to create an overall anti-inflammatory environment. These are pretty cool examples of non immune cells (fat and muscle) playing a role in our immune system. The importance of the immune system is demonstrated here, where exercise influences inflammation and inflammation influences the progression to disease. Happy workouts lead to happy immune systems!