Leukocyte counts and lymphocyte responsiveness associated with repeated bouts of strenuous endurance exercise

This study compared leukocyte counts and lymphocyte responsiveness during and after a second bout of high-intensity endurance exercise on the same day with the response to a similar but single bout of exercise. Nine athletes participated in three 24-h trials: 1) rest in bed (Rest); 2) one bout of exercise (One); and 3) two bouts of exercise (Two). All bouts consisted of 75 min at ∼75% of maximal O₂ uptake on a cycle ergometer. Lymphocytes in whole blood were stimulated with monoclonal antibodies against CD2 and assessed by flow cytometry for expression of the early activation molecule CD69. The second bout of exercise in the Two trial was associated with significantly increased concentrations of total leukocytes, neutrophils, lymphocytes, CD4⁺, CD8⁺, and CD56⁺ cells and a significantly decreased percentage of CD56⁺ cells expressing CD69 compared with a single bout. Additionally, there was a significantly decreased CD69 fluorescence in CD56⁺ cells postexercise. These differences suggest a "carry-over" effect in the immune system from a first to a second bout of exercise on the same day.