It is almost certain that specific immunity plays a crucial role in natural systems, and it would be an important advance to understand specific immunity in insects to a level that allows the development of field-based assays. While it is clear that the presence of specific responses adds an extra dimension to studies of ecological immunology in insects, caution must be exercised when considering simple measures of immunity in the field. Standard assays of immunity in insects tend to consider relatively general effector systems, and these are usually measured against standardized microbes or immune elicitors. While measuring important aspects of the insect's immune defence, these assays overlook specificity, and may therefore not give an appropriate or full picture of the defence capabilities of a certain individual. This incomplete picture of resistance, together with the potential importance of tolerance (see Box 14.1), may explain results that fail to find clear correlations between measured immune parameters and levels of infection or survival on exposure to parasites and pathogens (Adamo, 2004; Mucklow et al, 2004).
The presence of specific immune responses in insects has the potential to open up avenues in the study of more general questions in evolutionary ecology research. It is clear that specific interactions between a host's immune system and its parasites will have profound implications for many ecological and evolutionary processes. A selection of the potential consequences has been discussed in this chapter, but this list is by no means exhaustive. The presence of specific immune responses in insects, while essentially mechanistically distinct from those occurring in vertebrates, offers the opportunity to further experimentally investigate this phenomenon. The potential for experimental manipulation of systems used in the study of insect immunity and host-parasite interactions will enable the true nature of specific immune defence and its ecological and evolutionary implications to be uncovered.
Integration of the whole-organism approach— addressed prominently in this chapter—with more mechanistic studies is vital for further understanding of the importance of specific immune responses in insects. However, functional assays that allow organisms themselves to show what the immune system is capable of have proven to be a valuable tool in gaining insights into the existence of specific responses in insect immunity. These insights have demonstrated that the conclusion that the insect immune system is devoid of adaptive immune reactions and only capable of coarse specificity (Hoffmann, 2003) is incomplete and unjustified.
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