My discussion of the evolution of immune function emphasized 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 (Restif and Koella, 2003), 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. An example of a surprising result from a co-evolutionary model is that, as the potential of transmission increases, the host's investment in immunity and its resistance to a parasite do not increase, but rather decrease (Koella and Boete, 2003b). Thus, simplistic interpretations of immune function are dangerous; co-evolutionary dynamics can give counterintuitive outcomes.

Although these points may not be surprising, they are often neglected in studies of immune function. Evolutionary biologists, for example, try to understand the variability of the efficacy of the immune response among individuals and among populations. We have studies, for example, on the genetic underpinning of the melanization response (Cotter and Wilson, 2002), on the cost of inducing an immune response (Robb and Forbes, 2006), and on the tendency for females to choose males with effective immune responses as mating partners (Rantala et al., 2002). Both of these questions implicitly assume that the measured immune response is related to resistance. But, little effort has been put into estimating the relationship between immune responses and resistance against the predominant parasites in natural populations, so that it is difficult to reach strong conclusions. Indeed, immune responses can be negatively related to sexual attractiveness (Rantala and Kortet, 2003), and a recent overview of sexual selection of immune responses (Lawniczak et al, 2007) emphasizes that any correlation between immune response and partner choice can be expected and the correlation is influenced by the trade-offs within the immune system and those between immune function and other traits. Of course, I am not arguing that immune responses are not associated with resistance, and a given immune response may well reflect the host's quality in some circumstances. But we need detailed description of immune function and how it is linked to resistance and, ultimately, reproductive success and we need a better understanding of co-evolutionary dynamics before we can reach strong conclusions. Because of these problems in interpreting immune function, it is reassuring that many evolutionary studies continue to focus on explicit measures of resistance rather than immune function (e.g. studies on Drosophila and it parasites and parasitoids; Kraaijeveld and Godfray, 1997; Kraaijeveld et al., 2001b; Rolff and Kraaijeveld, 2003; Lazzaro et al., 2006; Vijenravarma et al., 2008), including co-evolutionary aspects of the host and its parasite. (Kraaijeveld and Godfray, 1999; Kraaijeveld et al., 2001a).

Acknowledging co-evolution and the complexity of immunity is also critical in more applied contexts, for example the genetic manipulation of mosquitoes for the control of malaria. A key step is to identify the genes most relevant in determining resistance to malaria. While considerable progress has been made in the past decade or so, most efforts have considered mosquito-parasite associations that are neither natural nor relevant for human health. Will the identified genes be important in natural systems, against all parasite genotypes, in all genetic backgrounds of the mosquito? Will the parasite have the ability to counteract the mosquito's resistance? Our limited knowledge, some of which is reviewed above, suggests that the answer to the first question is no and to the second may be yes (Boete and Koella, 2003; Lambrechts et al., 2006, 2008). If so, attempts at manipulating the mosquito's immune response for malaria control may be futile.

Overall, this chapter argues that to understand the evolutionary pressures on immune function, we must understand in much more detail its complex relationship with resistance. On the one hand, the immune system that helps to influence resistance is complex and involves trade-offs among its components. On the other hand, resistance is partly determined by the parasite, so that the evolutionary patterns of resistance can only be understood with a co-evolutionary approach.

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