The purpose of this study was to review results on exercise induced changes in the immune system during and after strenuous long term exercise in athletes and well-trained individuals. By performing a literature search using PubMed I have studied changes in the blood of some lymphocytes and neutrophil granulocytes, some immunoglobulins, some important cytokines and cortisol during and in the hours after longlasting strenuous exercise. I have also studied changes of immunoglubulins in saliva. Further, I tried to find out if athletes doing long-distance sports have an increased risk of infections and if changes in the immune system following long term strenuous exercise could explain this phenomenon. My main findings are that during intense long term exercise there is an increase of some lymphocytes (NK-cells and T cells) and neutrophil granulocytes in the blood. After exercise there is a “window” of hours with decreased number of these lymphocytes and different immunoglobulins in the saliva. Different cytokines increase in concentration during exercise and are increased for a long time after the exercise. The same is the case for cortisol. Thus, based on observations mainly in the blood, in the hours (between 3 and 24(72)) after long term strenuous exercise, parts of the immune system in athletes may be impaired. Athletes more often get upper airways infections. Downregulations of the immune system after long term strenuous exercise among athletes may contribute to the increased risk of infections.