As if understanding this complex evolutionary puzzle were not already sufficiently challenging, we have learned recently that two types of adaptive immune system have evolved in vertebrates: a recently recognized system in jawless vertebrates (hagfish and lamprey) and the more familiar adaptive immune system of jawed vertebrates. … This leads to the conjecture that two interactive lymphocyte arms are a fundamental feature of the adaptive immune system that was selected to provide balance and self-regulation.

Cooper, M., & Herrin, B. (2010). How did our complex immune system evolve? Nature Reviews Immunology, 10 (1), 2-3 DOI: 10.1038/nri2686