I refer the reader to detailed descriptions of the individual components of insect immune systems provided in other chapters of this volume and just summarize them here. I count seven layers to the immune response, moving from the outside of an insect to the inside. First, is the native microbiota, which occupies niches on the surfaces of insects (both on the outside of the body and within the gut, for example) and can prevent colonization by other microbes. Second is the barrier epithelial immune response, which is induced to produce antimicrobial peptides (AMPs) when it recognizes a pathological event. Third is the clotting response, which can entrap microbes in a fibrous net. Fourth is the haemocyte-driven immune response, which can lead to phagocytosis, encapsulation, or nodu-lation of invading parasites. Fifth is the melaniza-tion response, which can produce reactive oxygen that presumably kills microbes. Sixth is the AMP response, in which the fat body releases large quantities of AMPs into the circulation. Finally there is the RNA interference (RNAi) response that can limit viral growth. Alongside all of this lies tolerance and we do not yet have a good picture of the physiological mechanisms that underlie tolerance.
I concentrate on realized immune responses rather than potential immune responses in this chapter. By that is meant that most attention will be devoted to experiments that challenged insects with microbes that cause pathology and measure the effects of the immune response on this pathology. This is in contrast to experiments that treat the insect with an elicitor and measure a transcriptional output. We took this approach because this is where we find the greatest number of examples of interactions between immunity and gross physiology. Not enough studies have been performed linking molecular mechanisms to gross changes in physiology to tell a meaningful story.
I draw my examples from the world of insects but focus on work done in Drosophila melanogaster using injected microbes. This is done for the sake of consistency because different insects may have come up with different evolutionary solutions for a problem. One should avoid the trap of oversimplification by saying 'insects work like this . . .' and try to limit this problem by focusing on one insect. Injected microbes are the focus because this type of experiment makes up the bulk of the literature. So-called natural infection models in the fly require that larvae or adults be fed a paste of the infecting bacteria and, as it will become clear below, this is predicted to have spectacular effects of the physiology of the insects that are difficult to control.
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