Leukocytes: What the Test Shows and Why It Matters for an Athlete
Leukocytes are cells of the immune system, and their number in a complete blood count is often perceived as a simple indicator of "whether there is inflammation or not." For an athlete everything is more interesting: intense training in itself changes leukocytes more than a mild cold, and the ratio of individual cell types can hint at both an infection and excessive exertion. Our editorial team explains how to read this section of the analysis.
What Leukocytes Are
Leukocytes, or white blood cells, are a collective name for several types of cells that protect the body from bacteria, viruses, fungi and parasites, and also clear away damaged tissue. Unlike erythrocytes, they have a nucleus, are able to leave the blood vessels into the tissues and move toward the focus of inflammation.
Leukocytes are formed in the red bone marrow from the same stem cells as erythrocytes and platelets. Then some of them mature in the lymph nodes, spleen and thymus. At any given moment only a small fraction of all the body's leukocytes circulates in the blood: the majority are in the tissues, the bone marrow and the "marginal" pool — stuck to the walls of the blood vessels.
It is precisely the presence of the marginal pool that explains why the number of leukocytes in the blood can change within minutes. Adrenaline, cortisol and increased blood flow during exertion wash the cells off the vessel walls into circulation. No new leukocytes are formed in the process — only their distribution changes.
The total number of leukocytes in adults is usually within the range of approximately 4–9 or 4–10 ×10⁹/L, depending on the laboratory. However, the total number says little without a breakdown: a person with a normal number of leukocytes may have a substantial shift among individual cell types.
The Leukocyte Formula: Who Is Responsible for What
The leukocyte formula is the distribution of leukocytes by type in percentages and, more importantly, in absolute numbers. Doctors increasingly rely specifically on absolute values, because the percentage of one type automatically changes when another rises or falls.
| Cell type | Approximate proportion | Main function | When it changes |
|---|---|---|---|
| Neutrophils | ≈40–70% | Destruction of bacteria, the first line of defense | Bacterial infections, stress, exertion, corticosteroids |
| Lymphocytes | ≈20–40% | Immune memory, antibodies, antiviral defense | Viral infections; temporarily drop after hard training |
| Monocytes | ≈2–10% | Turn into macrophages, "clean up" tissues | Protracted infections, recovery after damage |
| Eosinophils | ≈1–5% | Allergic reactions, defense against parasites | Allergy, bronchial asthma, helminthiases |
| Basophils | up to ≈1% | Participation in allergic reactions | Rarely have independent significance |
The percentages given are approximate: each laboratory indicates its own limits on the form. For assessment it is important to look simultaneously at the total number, the absolute figures of each type and the clinical picture — the presence of fever, sore throat, cough, rash.
Some laboratories additionally calculate the neutrophil-to-lymphocyte ratio (NLR). This index is used in studies as a marker of physiological stress and inflammation. For athletes it is interesting in that it rises sharply after exhausting exertion, but as an independent diagnostic tool in sports medicine it is not yet established.
Automatic analyzers count the main cell types well, but do not always recognize immature or atypical forms. If the device "signals" such cells, the laboratory reviews the smear under a microscope — for example, atypical mononuclear cells are characteristic of infectious mononucleosis.
How Exertion Changes Leukocytes
The reaction of leukocytes to physical exertion is one of the best described in exercise physiology (Pedersen, Hoffman-Goetz, 2000). During and immediately after intense work, the total number of leukocytes rises due to neutrophils and lymphocytes that are washed out of the marginal pool. After exhausting competitions, such as a marathon, leukocytosis can be significant (Kratz et al., 2002).
Then the trajectories diverge. Neutrophils, under the action of cortisol, continue to rise for several more hours after exertion. Lymphocytes, on the other hand, 1–2 hours after prolonged intense work drop below the baseline level. It was precisely this temporary "dip" that once gave grounds for the "open window" hypothesis — a period of increased vulnerability to infections.
Modern researchers are reconsidering this idea. Campbell and Turner (2018) argue that after exertion lymphocytes do not disappear but move into the tissues — the lungs, mucous membranes, intestine — to "patrol." That is, the temporary decrease in the blood may reflect the mobilization of immunity rather than its suppression.
The practical conclusion for the test is simple: blood taken within a few hours after intense training will show a picture that may resemble a bacterial infection or stress. After 24 hours of rest the indicators in most people return to their individual norm.
Why an Athlete Needs This Test
First, leukocytes help decide whether to train today. Sports doctors often use the "above the neck" rule: with a mild runny nose you can easily train, but with a fever, aching muscles or enlarged lymph nodes it is better to cancel training. A blood test in doubtful cases helps to understand the nature of the infection.
Second, recurring infections are one of the signs of a disturbed balance of exertion and recovery. The IOC consensus on exertion and the risk of illness (Schwellnus et al., 2016) emphasizes that a sharp increase in volume, lack of sleep, stress and travel increase the frequency of respiratory infections in athletes. Regular analysis of the formula is one of the elements of such monitoring.
Third, the leukocyte formula helps to detect problems that an athlete does not suspect: elevated eosinophils may indicate uncontrolled allergy or exercise-induced asthma, which is important for track-and-field athletes, skiers and swimmers. Persistently low neutrophils require a search for the cause.
- Before the season:a baseline test for future comparison.
- Frequent colds and long recovery:assessment of the formula together with ferritin, vitamin D and the exertion regimen.
- Prolonged fatigue without an obvious cause:ruling out infectious mononucleosis and other conditions.
- Taking drugs that affect the bone marrow or immunity:monitoring as prescribed by a doctor.
Preparation and Typical Mistakes of Interpretation
Leukocytes react not only to training but also to food intake, smoking, emotional stress and the circadian rhythm. Therefore our editorial team advises taking the test in the morning, after 24 hours without intense exertion, on an empty stomach or after a light breakfast at the laboratory's request, and sitting quietly for a few minutes before the blood draw.
A common mistake is to compare percentages instead of absolute numbers. For example, "elevated lymphocytes 45%" with a total leukocyte count near the lower limit may mean a completely normal absolute number of lymphocytes and only a slightly reduced number of neutrophils.
The second mistake is not taking medications into account. Corticosteroids, in particular injections into joints or inhaled forms at high doses, raise neutrophils and lower eosinophils and lymphocytes. Some antibiotics, anti-inflammatory and other drugs are able to lower the number of neutrophils.
The third mistake is to draw a conclusion from a single test. Indicators slightly above or below normal without symptoms are best assessed over time. Small deviations that recur from year to year in a healthy person are often an individual feature of theirs.
Editorial Conclusions
Leukocytes are not one but several groups of immune cells, and the real information is provided by the leukocyte formula in absolute numbers, not just the total count.
Intense training temporarily changes the formula: neutrophils rise for hours, lymphocytes fall after a brief rise. So that the test reflects the state of health rather than yesterday's exertion, blood is drawn after a day of rest.
Regular monitoring of leukocytes helps to notice an infection, allergy or signs of overload, but it should be interpreted together with a doctor and taking symptoms into account.
We also recommend reading our materials about the causes of leukocyte deviations from normal, about platelets in an athlete's test and about overtraining syndrome.
References
- Pedersen BK, Hoffman-Goetz L. Exercise and the immune system: regulation, integration, and adaptation. Physiol Rev. 2000;80(3):1055–1081.
- Walsh NP, Gleeson M, Shephard RJ, et al. Position statement. Part one: Immune function and exercise. Exerc Immunol Rev. 2011;17:6–63.
- Campbell JP, Turner JE. Debunking the myth of exercise-induced immune suppression: redefining the impact of exercise on immunological health across the lifespan. Front Immunol. 2018;9:648.
- Kratz A, Lewandrowski KB, Siegel AJ, et al. Effect of marathon running on hematologic and biochemical laboratory parameters, including cardiac markers. Am J Clin Pathol. 2002;118(6):856–863.
- Schwellnus M, Soligard T, Alonso JM, et al. How much is too much? (Part 2) International Olympic Committee consensus statement on load in sport and risk of illness. Br J Sports Med. 2016;50(17):1043–1052.
- Nieman DC, Wentz LM. The compelling link between physical activity and the body's defense system. J Sport Health Sci. 2019;8(3):201–217.
- Sanchis-Gomar F, Lippi G. Physical activity – an important preanalytical variable. Biochem Med (Zagreb). 2014;24(1):68–79.
Andriy Melnyk
A strength-sports coach and author of programs for beginner and intermediate levels. Writes about training planning.