| Summary: | The kinetic responses of leukocyte subsets to exercise and their recovery may serve as indicators of immunological resilience. These time-dependent responses were investigated in healthy young men using a bicycle ergometer test. Fifteen recreationally active male cyclists (20–35 years, VO<sub>2max</sub> 56.9 ± 3.9 mL kg<sup>−1</sup> min<sup>−1</sup>) performed four exercise protocols with a 1 h duration in a cross-over design: at 70% of the maximal workload (Wmax) in a hydrated and a mildly dehydrated state, at 50% of the Wmax, and intermittently at 85/55% of the Wmax in blocks of 2 min. The numbers of lymphocytes, monocytes, neutrophils, eosinophils, basophils, thrombocytes, and NK cells (CD16 and CD56) were measured at different time points up to 24 h post-exercise. The total leukocyte counts and those of most subsets increased from the start of the exercise, peaking after 30–60 min of exercising. The neutrophil numbers, however, peaked 3 h post-exercise. The CD16<sup>bright</sup>CD56<sup>dim</sup> NK cells showed a 1.5-fold increase compared to the CD16<sup>bright</sup>CD56<sup>bright</sup> NK cells. Other than for MCP-1, no significant differences were found in the serum cytokine levels. Our results show that exercise intensity is reflected in different time-dependent changes in leukocyte subsets, which supports the concept that the exchange of immune cells between peripheral blood and tissues contributes to enhanced immune surveillance during strenuous exercise.
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