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Evolutionary tree representing the emergence of specific immunological components. Innate immunity is observed among all members of the animal kingdom, while adaptive immune system is typically restricted to vertebrates. Some adaptive immune characteristics do appear in invertebrates, however. (Reproduced with permission from Kvell K, Cooper EL, Engelmann P, Bovari J, Nemeth P. Blurring borders: Innate immunity with adaptive features. Clin Dev Immunol 2007:836-71)
