White blood cells (WBCs) are a core component of the body’s immune system, circulating through the bloodstream to help defend against infection and disease. Physical activity is known to influence nearly every physiological system, prompting questions about its effect on these immune cells. This article explores the relationship between exercise and white blood cell counts, examining the immediate and long-term changes that occur in response to physical exertion.
Immediate White Blood Cell Changes During Exercise
A single session of exercise causes a significant and immediate increase in circulating white blood cells. This response, known as exercise-induced leukocytosis, is a documented physiological reaction where the total WBC count can double or more. The primary drivers of this rapid surge are stress hormones, such as adrenaline (epinephrine) and noradrenaline (norepinephrine), which are released in greater amounts during physical exertion.
These hormones, combined with increased cardiac output and blood flow, mobilize WBCs from various areas in the body. The cells are drawn from the “marginal pool,” which includes cells adhering to the walls of blood vessels, as well as from organs like the spleen. This mobilization affects several types of white blood cells, including lymphocytes, monocytes, and neutrophils. Natural Killer (NK) cells, a type of lymphocyte, are particularly responsive, with their numbers capable of increasing by 150–400%.
Exercise Variables Affecting White Blood Cell Counts
Exercise intensity is a dominant factor; vigorous and high-intensity exercise provokes a much larger and more rapid increase in WBCs compared to low-intensity activity. For instance, high-intensity aerobic exercise causes a greater elevation in total leukocytes, lymphocytes, and monocytes immediately after the session compared to exercising at a low intensity. The duration of the physical activity also modulates this immune response.
Longer periods of exercise can amplify the increase in WBC counts, particularly for neutrophils. Studies show that endurance exercise lasting over 90 minutes at a moderate to high intensity produces a particularly pronounced effect. The type of exercise performed also contributes to the specific changes observed. Endurance activities like running and cycling often lead to a greater increase in both neutrophils and lymphocytes compared to resistance training, though resistance exercise still induces a significant leukocytosis.
Chronic Exercise and Baseline White Blood Cell Levels
Regular, long-term exercise can lead to more persistent adjustments in the immune system, with effects on baseline white blood cell counts that are more subtle than short-term spikes. Some research indicates that trained individuals may have resting leukocyte levels that are slightly lower than those of sedentary people. Specifically, consistent aerobic exercise has been associated with decreases in resting total WBC and neutrophil counts, which may reflect lower levels of systemic inflammation.
For example, one study found that postmenopausal women who engaged in a high amount of aerobic exercise over six months saw a significant decrease in their baseline total WBC and neutrophil counts. This effect was most noticeable in individuals who started with higher baseline WBC counts. While baseline levels may adapt over time, the immediate mobilization of immune cells during exercise remains a consistent physiological response. Studies on this aspect have shown that trained athletes exhibit a similar leukocytic response to acute exercise compared to untrained individuals.
Interpreting Exercise-Induced White Blood Cell Fluctuations
The temporary rise in white blood cells during and after exercise is a normal adaptive process, not a sign of illness. This mobilization of immune cells is interpreted as a mechanism for enhanced “immune surveillance.” By increasing the circulation of cells like lymphocytes and neutrophils, the body is better prepared to detect and manage potential threats, such as pathogens or minor muscle tissue damage from strenuous activity. This exercise-induced leukocytosis is distinct from pathological leukocytosis, which is a sustained high WBC count caused by infection or disease, as exercise-related changes are transient and return to normal within hours.
Following particularly strenuous and prolonged exercise, such as a marathon, there can be a temporary period of altered immune function called the “open window.” This phase can last from a few hours to several days, during which some immune cell functions might be temporarily reduced. Some theories suggest this could increase susceptibility to infections like the common cold, highlighting the complexity of the immune response to extreme exertion.