Parasites and tissue microenvironment.

Human parasitic diseases cause hundreds of thousands of deaths per year, in addition to the significant morbidity and socioeconomic suffering experienced by millions of people worldwide. The development of vaccines and new treatments has been impeded by a lack of basic understanding of the infection and survival mechanisms utilized by parasites. Most parasites have multifaceted life cycles with numerous morphological stages that infect discrete host cell types and tissues. This enormous variety of life cycles and tissue niches result in even greater diversity in the host immune response, targeting not only the different types of parasitic organisms but also the different life cycle stages of the same species within the mammalian host. Parasitic organisms have evolved together with the mammalian immune system over many millennia and hence have become remarkably efficient modulators of host immunity in order to promote their own survival. Indeed, the multiplicity of developmental stages, combined with distinct tissue tropisms, and the ensuing elaborate mechanisms of immune evasion, vastly compound the complexity of host-parasite interactions.