Broadly speaking, the insect innate immune system is encoded by three major functional categories of genes that are involved in (1) recognition of invading microbes, (2) immune-signal amplification and transduction, and (3) effector mechanisms that mediate the killing and clearance of infectious micro-organisms. Despite its lack of adaptive immune mechanisms and antibody-mediated defences similar to those found in vertebrates, the innate immune system in insects is quite specific in its antimicrobial action. Once invading microbes are recognized through specific interaction between pattern-recognition receptors (PRRs) and pathogen-associated molecular patterns (PAMPs), a variety of defence reactions can be activated (Medzhitov and Janeway, 2002). The activation of immune responses can either occur directly, as in the case of phagocytosis and melanization, or indirectly, through intracellular immune-signalling pathways that initiate the transcriptional activation of appropriate antimicrobial peptides and other immune effector genes (Dimopoulos, 2003; Christophides et al., 2004; Osta et al., 2004). In this chapter, we will specifically focus on the specificity of the innate immune responses at the level of the PRRs, with a major focus on the mosquito Anopheles gambiae as a model system. We will first provide a general overview of the insects' PRR repertoire and highlight some of its most interesting features with regard to antimicrobial defence (section 5.2). We will then provide detailed molecular and functional descriptions of some of the best-characterized PRR families (sections 5.3-5.7).
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