Why study the evolution of immunity?

Abstract

Investigations of immune recognition in nonmammalian species provide new insights into the evolution of immunity and the inner workings of the mammalian immune system. Very diverse mechanisms are used by different multicellular organisms to recognize and cope with the rapidly evolving microbial world.

Figure 1

Present ideas of immune diversity in alternative model systems. Left to right: fly (drosophila), snail (biomphalaria) sea urchin (strongylocentrotus), amphioxus (branchiostoma), lamprey (petromyzon), shark (heterodontus), chicken (gallus) and mouse-human (mus-homo). Molecules such as RAG-1 and RAG-2 have been identified in sea urchin, but only a gene like RAG1 has been characterized thus far in amphioxus. Amphioxus VCBPs (variable (V) region–containing chitin-binding proteins) have a ‘head-to-tail’ organization, which defines a unique potential binding surface, and have a considerable genetic polymorphism. Hypermutation has been noted in genes encoding snail fibrinogen-related proteins (FREPs). Three independent phyletic lineages are presented in red and yellow (protostomes) and in blue (deuterostomes); phylogenetic relationships are not drawn to evolutionary time scale. This diagram emphasizes some of the findings that have influenced the understanding of immune function and/or suggest alternative mechanisms of immune recognition. They are not intended to be inclusive for any given system or to include all systems in which notable findings relevant to the subject of this commentary have been made. Findings not in the text are in refs. 4,7,16. Dscam, Down's syndrome cell adhesion molecule; CDR3, complementarity-determining region 3; FR2, framework region 2; XRCC4, DNA-repair protein; Ku70 and Ku80, DNA-binding proteins; DNA-PK, DNA-dependent protein kinase; TCR, T cell antigen receptor; Ig, immunoglobulin; Ψ, pseudo.