Immune responses and the evolution of resistance

Abstract This chapter discusses the evolution of immune function. It emphasizes two points. First, the immune system is complex, with many responses that may act together or inhibit each other to determine the outcome of an infection. Using an immune response as an indicator of the host's resistance (or, more generally, its quality) is therefore problematic, as increased investment in a given immune response may well indicate increased susceptibility to a parasite. Second, resistance is a product of the interaction between a host and a parasite. Thus, we cannot understand the evolution of immune function without considering the co-evolution of the host's and the parasite's contributions to resistance. Indeed, as found in a more general context, mathematical models of the evolution of the host that do not consider the co-evolutionary response by the parasite can be misleading as their predictions can differ qualitatively from the co-evolutionary dynamics and equilibrium.