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Animal immunity is under intense evolutionary pressure, and the mechanisms that carry out recognition and elimination of pathogens are among the most rapidly evolving genetic systems. It is increasingly apparent that this has led to the emergence of novel molecular mechanisms not only among vertebrates, where immunity is by far best characterized, but also across invertebrate phyla. This propensity for rapid divergence has been a serious obstacle for progress in the field of comparative immunology. The variety of recent genome sequences from marine invertebrates representing new phyla offers a means to move forward in this area. Genome sequences provide much improved sensitivity for the detection of gene homologs and a framework for unbiased computational and experimental searches for novel immune mediators. Furthermore, new genomes now offer a more complete and unbiased view of immunity across bilaterian phyla, especially among deuterostomes. In this review we summarize these findings with particular attention toward immunity in Strongylocentrotus purpuratus, the purple sea urchin, and outline the changing perspective on the evolution of deuterostome immunity.

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