The origins of vertebrate adaptive immunity

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Abstract
Innate immunity has a long phylogenetic history, encompassing species as diverse as sea anemones, insects and mammals. By contrast, adaptive immunity, which involves lymphocyte-like cells and the antigen-binding receptors they express, is restricted to vertebrates. A major departure in adaptive immunity is evident within vertebrate species. Jawed vertebrates use V(D)J recombination of immunoglobulin and T cell receptors, whereas jawless vertebrates rearrange variable lymphocyte receptors encoding leucine-rich repeats to form an alternative type of immune receptor. Homologues of molecules that previously were thought to be unique to the rearrangement and diversification of immunoglobulin and T cell receptors have been identified in invertebrate species, in which these forms of immune recognition molecules are absent. The enormously complex, multifactorial and highly regulated cellular processes that generate receptor variation in somatic cells arose through the integration of molecular systems that are not exclusively associated with immune diversity. Reconstructing the nature of ancestral forms of adaptive immune receptors is compromised by the absence of crucial intermediates; however, it is possible to infer some of the main steps that gave rise to the antigen receptor-bearing immunocytes of jawed vertebrates.