Review paper
Convergent evolution of acquired immunity

An acquired immunity is connected with clonal proliferation of antigen-specific immune cells leading to the formation of memory cells responsible for efficient response to successive encounter of the same antigen but not to an unrelated one. Recently it turned out that such ability emerged independently at least three times in the course of evolution of the animal kingdom. 1) An acquired immunity of human and other jawed vertebrates (fish, amphibians, reptiles, birds, and mammals) is based on lymphocytes, each of them with particular set of TCR or BCR antigen receptors, which vast diversity (up to 1018) is dependent on somatic DNA rearrangement of limited numbers v(d)j genes belonging to immunoglobulin superfamily (IgSF). 2) Only in 2004-2006 it was evidenced that an acquired immunity exists also in jawless vertebrates (hagfish and lampreys), but evolved independently as vast diversity (1014) of their VLR lymphocyte receptors is connected with somatic DNA rearrangement of leucine-rich repeats (LRRs), unrelated to IgSF. 3) Quite recently (2005-2007) it was evidenced that the fruit fly, Drosophila melanogaster, is able to survive the lethal dose of injected pathogen after preceding “vaccination” with the sublethal dose of the same pathogen, but not the unrelated one. Variability of IgSF-type Dscam receptors (Down’s syndrome cell adhesion molecule) (up to 18,000 isoforms) on insect immunocytes is created by alternative splicing. In conclusion, recent achievements of evolutionary immunobiology indicate that an acquire immunity emerged by convergence from various elements of pre-exiting innate immune systems of particular taxonomic groups.