Optimal immune defence in the light of variation in lifespan

There is good evidence for costs to both the uses of immune defences and their development and maintenance. The optimal defence will be a balance of these costs with the risk of infection and the virulence of the disease. It is therefore clear that the life-history characteristics of both host and parasite will impact the optimal level of defence, and that this may in part explain the variation in immune defence against different pathogens and parasites. For instance, it has traditionally been suggested that long-lived hosts should invest in immune memory. Ecological evolutionary theory can be used to examine in detail how different host characteristics will affect the optimal immune response that evolves. Here, we review theoretical studies on the impact of host lifespan on various immune defence characteristics including acquired immunity and highlight the importance of population-level epidemiological feedbacks on the outcome. In particular, we discuss when longer-lived hosts may invest less in acquired immunity and develop new theory to highlight the importance of the mechanism of host population regulation to the outcome. We finish by discussing where more theory is needed and how comparative and experimental studies may test the theory.