In contrast to a vast number of investigations on immune cell responses to a single bout of exercise, there is limited information on how repeated bouts of exercise on the same day affect the immune system. The few studies that have measured changes in concentration and function of leukocytes associated with repeated bouts of exercise have all used different exercise and recovery protocols, as well as subjects with varying training status [43,146,147]. A methodological limitation that applies to all of the previous investigations is the lack of control for diurnal variations in various blood constituents, as the response to a first bout of exercise in the morning has been compared with a second or third bout later on the same day. Besides, the exercise and recovery protocols used in the aforementioned studies hardly reflect the daily exercise and recovery regime practised by most elite endurance athletes. A recent study used a study design that eliminated the effect of diurnal variations. 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. 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 approximately 75% of maximal O, uptake on a cycle ergometer. The second bout of exercise in the Two trial was associated with significantly increased concentrations of total leukocytes, neutrophils and CD56+ NK cells. These differences suggest a "carry-over" effect in the immune system from a first to a second bout of exercise on the same day, whereas immune changes in response to five repetitive bouts of cycling over five days (each separated by a rest of 24 hours) were not different from those elicited by the first bout , Ronsen et al.  further described enhanced plasma IL-6 and IL-lra responses to repeated versus single bouts of cycling in athletes. The response was more pronounced if the recovery between the two exercise episodes was 3 hours versus 6 hours.
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