Specific immune responses will have implications for a number of ecological and evolutionary aspects of host-parasite relationships. Such immune specificity may be observed on two non-mutually exclusive levels. On one hand, genetically encoded responses against distinct parasite types can be discrete. Coupled with genetic diversity underlying these responses, this can lead to specific interactions between host and parasite genotypes. In addition, individual host experiences of parasites within their environment will shape immunity in response to the encountered circumstances. These primed levels of immunity can also show specificity. While vertebrates are well known for possessing specific immunity, and the associated mechanisms are well understood, specific immunity in invertebrates has been something of an enigma due to the lack of a clear mechanistic basis. Recently, studies in evolutionary ecology have shown that specific responses are widespread in invertebrates. In this chapter evidence for specific immune responses is presented, including both the cases where the specificity is observed in genetically determined hostparasite interactions and where specific responses are acquired depending on encounters with particular parasite types. Specific immune responses have implications for topics such as the evolution of virulence, the maintenance of diversity in natural populations, and the evolution and maintenance of sexual reproduction.

The study of ecological immunology has been motivated by a number of different questions that have emerged from various fields of enquiry. In particular, the field was strongly influenced by mathematically oriented population biologists interested in the dynamics of interacting hostparasite populations (Anderson, 1994a, 1994b; Anderson and May, 1979; May and Anderson, 1979) and the structure of host and parasite populations (Lythgoe, 2002). Regardless of how they come about, specific interactions between hosts and parasites are often assumed in these analyses. This population biology-centred view has focused the studies primarily on vertebrate host systems. Within these systems, the necessary specific immune responses have a clear and relatively well understood immunological basis in the highly specific adaptive immune system with a diverse repertoire of T and B cells (Lythgoe, 2002; Berding et al, 1987). The interest in specific host-parasite interactions, and thus by implication in specific responses, has also been of considerable interest for the development of models to explain the maintenance of genetic diversity in host populations. If processes that maintain genetic diversity, such as sex and meiotic recombination, are driven by antagonistic co-evolution (the Red Queen hypothesis; Salathe et al., 2008), then they will only be selected for if specific interactions of an appropriate kind do indeed exist (Box 14.1).

The second major influence on the developing field of ecological immunology came from

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