Speculations on the origin of the vertebrate immune system

As one studies more and more the immune systems of invertebrates their multiplicity and diversity increase. On one hand, the number of different local and systemic innate mechanisms (or cascades of mechanisms) increases with every phylum studied, on the other hand the diversity increases within each system as multiple loci and polymorphisms are discovered. Even somatic variation is not restricted to vertebrates. Yet no immune system similar to that of vertebrates characterized by the usage of RAG enzymes, has been found below jawed vertebrates. The availability of genome projects in early chordates allows understanding better the different causal lines that led to the generation of the vertebrate system with its diverse repertoire of antigen receptors generated somatically. This paper singles out a linkage group in human, gathering genes of the Ig superfamily with structural relationships to the antigen receptor JAM/CTX, nectin, or to some haematopoietic cells CD that have homologues in protochordates such as Branchiostoma and Ciona or in more primitive protostomes. The role of virus receptor for many of these clustered genes products provides perhaps one way of envisaging the recruitment of this family in the adaptive immune system from an ancient form of innate antiviral immunity.