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Chapter 18 : Innate Immunity to Parasitic Infections
Category: Immunology; Clinical Microbiology
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The highly divergent lifestyles and patterns of adaptive immune responses associated with control of helminth and protozoan parasites are reflected in the distinct innate responses they trigger and in their consequences for the pathogen. In the case of protozoa, innate immunity has an important role in limiting early parasite replication until cognitive T- and B-lymphocyte responses can dominate. Alternatively, the innate recognition of helminths may function to limit the number of invading larvae and perhaps slow their development prior to the onset of adaptive immunity. Consequently, in order to be successful, parasites have to survive the innate response and this hard-wired system provides a powerful selective pressure on host and microorganism. This is evident from the numerous strategies employed by protozoa and helminths to evade these antimicrobial mechanisms and underscores the importance of studying innate immunity in order to understand the host/parasite adaptation. Perhaps the simplest forms of innate immunity are represented by the presence of preexisting, soluble factors that can recognize and destroy invading parasites. In addition to complement, there are other soluble factors that mediate innate immunity to parasitic infections. Natural killer (NK) cells can produce IL-5, this may represent a mechanism whereby NK cells can influence the development of effector mechanisms required for resistance to worms. Although innate immunity has an important role in resistance to acute infections, it is the adaptive response, characterized by the selective expansion of T and B cells, that is required to provide long-term immunological control of many chronic parasitic infections.