For most people, working out strengthens the immune system rather than weakening it. Moderate exercise enhances your body’s ability to detect and fight off infections. But there is a real caveat: very high training volumes, like those seen in marathon runners and elite athletes, can temporarily increase susceptibility to respiratory infections. The answer depends almost entirely on how much and how hard you’re training.
What Happens to Immune Cells During Exercise
When you exercise, your body floods the bloodstream with immune cells. White blood cell counts rise sharply, including the cells responsible for killing viruses and bacteria. This is a normal stress response, and it’s actually beneficial. Moderate exercise enhances the ability of neutrophils (your first-responder immune cells) to engulf pathogens and produce the chemical bursts that destroy them. It also delays the natural death of these cells, keeping them active longer.
Natural killer cells, which patrol for virus-infected and cancerous cells, increase in both number and activity during and after moderate workouts. Muscle tissue itself releases signaling molecules that attract immune cells, essentially recruiting your body’s defense system to perform a sweep. Over time, regular moderate exercise shifts your immune cells toward a profile that’s better at resolving inflammation and repairing tissue.
The “Open Window” Theory: Mostly a Myth
For decades, exercise scientists warned about an “open window” of immune suppression in the hours after a hard workout. The idea was based on a well-documented observation: about one to two hours after vigorous exercise, the number of lymphocytes circulating in your blood drops sharply. This looked like the immune system was temporarily shutting down, leaving you vulnerable to infections.
That interpretation has largely been overturned. A major 2018 review in Frontiers in Immunology concluded that the drop in blood lymphocytes doesn’t represent suppression at all. Instead, those cells are being redeployed to the tissues where infections actually start: the lungs, the gut lining, the skin. The temporary disappearance from the bloodstream reflects a heightened state of immune surveillance, not a weakened one. The review’s authors stated plainly that “limited reliable evidence exists to support the claim that exercise suppresses cellular or soluble immune competency.”
This doesn’t mean hard exercise carries zero risk. But the original framework overstated the danger for the average gym-goer by a wide margin.
Where the Real Risk Lives: Extreme Training
The people who do face genuine immune consequences are those training at elite or ultra-endurance levels. In a study of 2,311 endurance runners, 13% reported illness in the week after completing the Los Angeles Marathon, compared with just 2.2% of similarly trained runners who didn’t race. Marathon runners in general face a two- to six-fold increased risk of upper respiratory infections in the one to two weeks after a race.
Volume matters as much as intensity. Runners logging more than 96 kilometers (about 60 miles) per week doubled their odds of getting sick compared to those running less than 32 kilometers per week. At the Winter Olympic Games, 45% of Finnish athletes experienced cold symptoms during a 21-day stay, versus 32% of staff members, suggesting that competitive athletic stress provides a measurable additional risk.
At the cellular level, prolonged intensive training reduces the proportion of natural killer cells and a related cell type called NKT cells in both the blood and the spleen. These reductions persisted across multiple time points in longer training programs, making them a potential biomarker of exercise-related immune suppression. Athletes in heavy training also show drops in salivary immunoglobulin-A, an antibody that protects the mucous membranes of the mouth, nose, and throat. Lower levels correlate with more frequent respiratory infections.
How Exercise Slows Immune Aging
One of the strongest arguments for regular exercise is its effect on the aging immune system. As you get older, your body produces fewer naive T cells (the ones capable of recognizing new threats) and accumulates more senescent T cells that have essentially retired from active duty. This process, called immunosenescence, is why older adults are more vulnerable to infections and respond less robustly to vaccines.
Exercise pushes back against this decline in measurable ways. Elderly runners with an average of 17 years of training showed improved T cell proliferation compared to sedentary peers. A study of healthy, physically active adults aged 55 to 79 found that their thymus function markers, a proxy for the ability to produce new T cells, were comparable to those of younger people. These active older adults also had lower levels of chronic low-grade inflammation, which is a hallmark of immune aging.
Higher cardiovascular fitness specifically correlates with fewer senescent T cells and a greater proportion of naive T cells in the bloodstream. Research on master athletes over age 40 found they had lower proportions of exhausted memory T cells across multiple categories. The researchers proposed that regular exercise not only prevents these worn-out cells from accumulating but may actively promote their clearance.
Fueling Your Immune System Around Workouts
If you’re training hard enough to worry about immune suppression, nutrition during and after exercise can help. The most well-supported strategy is consuming a carbohydrate drink of at least 6% concentration during prolonged exercise. This maintains blood sugar levels, which in turn blunts the cortisol spike that contributes to post-exercise immune changes. Studies testing higher concentrations (12% or more) found no additional benefit, so 6% appears to be the effective threshold.
After exercise, consuming 1.0 to 1.2 grams of carbohydrate per kilogram of body weight, along with some protein, is generally recommended for recovery. For a 70-kilogram person, that’s roughly 70 to 84 grams of carbohydrate. While this post-exercise intake doesn’t dramatically reverse immune cell changes, it does help prevent certain aspects of immune activation like neutrophil degranulation, and it replenishes the energy stores your immune cells depend on.
Cold Weather and Heat Don’t Add Much Risk
A common concern is that exercising in extreme temperatures further suppresses immunity. The evidence doesn’t support this for most situations. Exercising in the heat does increase stress hormones and circulating immune cells more than exercising in comfortable temperatures, but studies measuring actual immune cell function (neutrophil activity, natural killer cell activity, mucosal immunity) show limited additional impairment. The exception is exertional heat illness, where core temperature exceeds 40°C, which is a medical emergency for many reasons beyond immune function.
Cold exposure is even less concerning. Despite the persistent belief that being cold makes you sick, controlled laboratory studies actually show immune-stimulating effects from cold exposure, whether combined with exercise or not.
How Much Exercise Is the Sweet Spot
The current guidelines from the American College of Sports Medicine recommend 30 minutes of moderate-intensity aerobic activity on five days per week, or 20 minutes of vigorous activity on three days per week, plus two days of strength training. This volume falls comfortably in the range that enhances immune function without the risks associated with extreme training loads.
The relationship between exercise volume and immune health follows a pattern: sedentary individuals have baseline immune function, moderate exercisers have enhanced function, and only those at the extreme high end of training volume tip into increased infection risk. For the vast majority of people wondering whether their gym habit is hurting their immune system, the answer is that it’s almost certainly helping it.