Your immune system is capped in its use of resources; it can only have a set number of T cells in operation at one time. A reserve of naive T cells is needed to effectively respond to new threats. These are untrained cells that will be educated and drafted to combat new intrusions. A small reserve of memory cells is needed to respond effectively to previously encountered threats - one reserve per threat. The more threats you have encountered, the more cells become devoted to memory; eventually you don't have enough naive T cells left to mount any sort of effective defense.
This contributes to a range of problems, starting with a lack of resistance to disease, one of the roots of age-related frailty, and most likely including buildup of damage-inducing senescent cells and increased risk of cancer with age. The immune system is responsible for destroying errant cells of all sorts, and so it is reasonable to speculate on the degree to which cancer risks can be blamed on immune system dysfunction:
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.
That's an open access paper, so don't miss the PDF link underneath the abstract. It is pleasing to see researchers outside the normal pro-longevity groups thinking in terms of restoring youthful function - that's very necessary to the future of medicine I'd like to see.