ABSTRACT
Being mammals ourselves, it is not surprising that the immune system of this phylogenetic group holds our special interest and has been studied to the greatest extent. However, the immensely complex immune system we observe in mammals arose through the stepwise accumulation of small changes over billions of years. Although some form of innate immunity has been found in all species thus far examined, adaptive immunity, centered upon highly antigen-specific receptors generated via somatic recombination, emerged after the divergence of the vertebrates. The cartilaginous fishes (e.g., chimera, sharks, skates, and rays) are the oldest phylogenetic lineage to have a mammalianlike adaptive immune system based upon immunoglobulin super-family domains (immunoglobulins, T-cell receptors, and major histocompatibility complex) that are somatically recombined by the RAG proteins. Their location at this pivotal point in phylogeny means that the cartilaginous fishes are vital in furthering our understanding regarding the evolution of the immune system in general and the emergence of adaptive immunity specifically. In this chapter, I will discuss some of the findings regarding immunity in the cartilaginous fishes and other nonhuman species, which have improved our understanding regarding the evolution of the immune system.
