Our immune system ages: it reacts more slowly to pathogens, vaccines become less effective, and the risk of cancer increases. At the same time, the immune system follows a 24-hour rhythm, as the number and activity of many immune cells fluctuate throughout the day. Researchers at the Leibniz Research Centre for Working Environment and Human Factors (IfADo) in Dortmund have now investigated whether this daily rhythm influences the aging of the immune system and whether the immune system reacts “younger” or “older” at certain times of the day.