A neat review discussing the evolution of the adaptive immune system. Plenty of interesting stories on fish!
Origin and evolution of the adaptive immune system: genetic events and selective pressures. Flajnik & Kasahara, Nature Rev. Genetics 2010
Another interesting paper that puts the genome sequence of the Atlantic cod in the context of immune system evolution
The genome sequence of Atlantic cod reveals a unique immune system. Star et al. and Jakobsen, Nature 2011