White blood cells: what the test shows and why it matters for athletes

White blood cells 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 it is more interesting: intense training in itself changes the white blood cells more than a mild cold, and the ratio of individual cell types can hint at both an infection and excessive load. The editorial team explains how to read this section of the test.

What white blood cells are

White blood cells, or leukocytes, 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 red blood cells, they have a nucleus and are able to leave the vessels into the tissues and move toward the focus of inflammation.

White blood cells form in the red bone marrow from the same stem cells as red blood cells and platelets. Some of them then mature in the lymph nodes, spleen and thymus. At any given moment only a small fraction of all the body's white blood cells circulates in the blood: most are in the tissues, the bone marrow and the «marginal» pool — adhering to the walls of the vessels.

It is precisely the presence of the marginal pool that explains why the number of white blood cells 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 white blood cells form in this case — only their distribution changes.

The total number of white blood cells in adults is usually within roughly 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 white blood cell count may have a substantial shift among individual cell types.

The white blood cell differential: who is responsible for what

The white blood cell differential is the distribution of white blood cells by type in percentages and, more importantly, in absolute numbers. Doctors increasingly rely precisely on the absolute values, because the percentage of one type automatically changes when another rises or falls.

The percentages given are approximate: each laboratory states its own limits on the form. For assessment it is important to look simultaneously at the total number, the absolute numbers 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 research 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 white blood cells

The reaction of white blood cells 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 white blood cells rises at the expense of neutrophils and lymphocytes, which are washed out of the marginal pool. After exhausting competition, 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, however, drop below baseline 1–2 hours after prolonged intense work. It was precisely this temporary «dip» that once gave rise to the «open window» hypothesis — a period of increased vulnerability to infections.

Modern researchers are reconsidering this idea. Campbell and Turner (2018) argue that lymphocytes do not disappear after exertion but move into the tissues — the lungs, mucous membranes, intestine — to «patrol». That is, a temporary decrease in the blood may reflect a 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, in most people the indicators return to the individual norm.

Why an athlete needs this test

First, white blood cells help decide whether to train today. Sports doctors often use the ºbove the neck» rule: with a mild runny nose one can easily train, but with a fever, muscle aches or enlarged lymph nodes it is better to cancel the training. A blood test in doubtful cases helps to understand the nature of the infection.

Second, repeated infections are one of the signs of a disturbed balance of load and recovery. The IOC consensus on load and the risk of illness (Schwellnus et al., 2016) emphasizes that a sharp increase in volumes, lack of sleep, stress and travel raise the frequency of respiratory infections in athletes. A regular differential test is one element of such monitoring.

Third, the white blood cell differential helps to detect problems the athlete does not suspect: elevated eosinophils may point to uncontrolled allergy or exercise-induced asthma, which is important for track-and-field athletes, skiers and swimmers. Persistently reduced neutrophils require a search for the cause.

• Before the season:a baseline test for future comparison.
• Frequent colds and slow recovery:assessment of the differential together with ferritin, vitamin D and the training 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 interpretation errors

White blood cells respond not only to training but also to eating, smoking, emotional stress and the circadian rhythm. Therefore the editorial team advises giving the test in the morning, after 24 hours without intense exertion, on an empty stomach or after a light breakfast as required by the laboratory, and sitting quietly for a few minutes before blood is drawn.

A common mistake is comparing percentages instead of absolute numbers. For example, «elevated lymphocytes 45%» with a total white blood cell count near the lower limit may mean an entirely 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 joint injections or inhaled forms in high doses, raise neutrophils and lower eosinophils and lymphocytes. Some antibiotics, anti-inflammatory and other drugs can reduce the number of neutrophils.

The third mistake is drawing 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

White blood cells are not one but several groups of immune cells, and the real information is given by the white blood cell differential in absolute numbers, not just the total count.

Intense training temporarily changes the differential: neutrophils rise for hours, while lymphocytes fall after a short rise. So that the test reflects the state of health rather than yesterday's load, blood is given after a day of rest.

Regular monitoring of white blood cells helps to notice infection, allergy or signs of overload, but it should be interpreted together with a doctor and with symptoms in mind.

We also recommend reading our materials on the causes of white blood cell deviations from normal, on platelets in an athlete's test, and on overtraining syndrome.

References

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5. 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.
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