Self/nonself discrimination at the basis of chordate evolution: limits on molecular conservation

All organisms rely on their capacity of self/nonself discrimination to rapidly detect approaching allogeneic cells as well as invading pathogenic microbes as foreign and to eliminate them. Failure to recognize nonself causes self-mating, germline parasitism and disease. Recent findings indicate that, in urochordates — the closest living relatives of vertebrates — different species use completely different molecules for allorecognition. Thanks to their phylogenetic position, these organisms might help us to understand the evolutionary origin of the vertebrate immune system.