AbstractThe purpose of this study was to review the literature concerning how physical strain among elite athletes in endurance sports influences their immune system. By literature search in Pubmed, I have studied exercise induced changes in several cytokines, hormones and immune cells in the blood, as well as cytokines in the urine, and immunoglobulines in salvia. Further, I tried to find out if there is a link between these changes and upper respiratory tract infection, that I found is the most common cause of illness among the athletes. Since many studies do not base the diagnose on detected pathogens, other mechanisms related to the upper respiratory symptoms are discussed. I also tried to find out if there is a link between training load and the risk of infection. The main finding in this study is that bouts of endurance exercise causes small changes in the studied parameters of both the innate and adaptive immune system. The changes is correlated to both the lenght and the intensity of the exercise and to the quality of restitution. The relation between the recorded alterations in immune function and clinical observed illness is, however, uncertain. That is because the reported episodes of ilness, seldom is based on objective diagnoses in the althletes that display altered immune activety following exercise. Therefore, further studies need to specifically address the relationship between clinical illness, exercise and immunological parameters.