Compromised immunity contributes to the decreased ability of the elderly to control infectious disease and to their generally poor response to vaccination. It is controversial as to how far this phenomenon contributes to the well-known age-associated increase in the occurrence of many cancers in the elderly. However, should the immune system be important in controlling cancer, for which there is a great deal of evidence, it is logical to propose that dysfunctional immunity in the elderly would contribute to compromised immunosurveillance and increased cancer occurrence. The chronological age at which immunosenescence becomes clinically important is known to be influenced by many factors, including the pathogen load to which individuals are exposed throughout life. It is proposed here that the cancer antigen load may have a similar effect on "immune exhaustion" and that pathogen load and tumor load may act additively to accelerate immunosenescence. Understanding how and why immune responsiveness changes in humans as they age is essential for developing strategies to prevent or restore dysregulated immunity and assure healthy longevity, clearly possible only if cancer is avoided. Here, we provide an overview of the impact of age on human immune competence, emphasizing T-cell-dependent adaptive immunity, which is the most sensitive to ageing. This knowledge will pave the way for rational interventions to maintain or restore appropriate immune function not only in the elderly but also in the cancer patient.