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. 2007 Dec 4;2007:83671. doi: 10.1155/2007/83671

Figure 1.

Figure 1

Phylogenetic tree of the animal kingdom highlighting the evolution of key immunological elements. Two arrows on the left side of Figure 1 indicate possible appearance of the two branches of immunity. Innate immunity may be observed along the entire animal kingdom. Traditionally accepted adaptive immunity appeared only in vertebrates, while certain adaptive immune mechanisms may have appeared early at the level of arthropods and molluscs illustrated by dots (below the arrow).