On the Origin of The Immune System

darwinDid the immune system evolve to keep out harmful organisms, or is it like a bouncer at a nightclub, trained to allow the right microbes in and kick the less desirable ones out? In the fifth essay in Science‘s series in honor of the Year of Darwin, John Travis explores the evolution of the immune system.
http://www.sciencemag.org/content/324/5927/580.full