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Competitive training demands strenuous prolonged exercise that may modulate the number and activity of circulating immune cells. Over demanding programs can lead to fatigue and increased risk of infection and susceptibility to diseases. This dissertation presents three studies developed within the Exercise Immunology scope, considering the analysis of acute and chronic systemic immune cell responses to exercise applied in real situations of competitive training, controlling for factors that may affect immune responses. We aimed to evaluate the immune response to a high intensity prolonged swimming training session (Study 1) and to a 7-month swimming training season (Study 2), and the influence of a 4-month training macrocycle on the immune response to a swimming session (Study 3). Subjects characteristics, namely sex, menstrual cycle phase, maturity, swimming age group, and distance specialty were controlled, and training load, performance improvements and Upper Respiratory Symptoms (URS) were monitored. The swimming session induced an impaired acquired immune surveillance immediately and at least throughout 2h post-exercise, however, 24h after, senior and youth swimmers had totally recovered but not juniors. This supports the idea of an open window to infection after prolonged intense exercise, suggesting also a more difficult recovery of juniors. During the season’s high intensity training periods immune depression and higher URS prevalence were observed. When the season ended, innate immunity was decreased, appearing to have been more affected by cumulative training loads, while acquired immunity …