The cartilaginous fishes are known as the oldest phylogenetic group from evolutionary perspective in which possess a “modern” human-type adaptive immune system and immunoglobulins. As one typical species in this family, sharks have been around for over 500 million years. Due to survive in marine environment, large quantities of urea are contained in sharks’ blood to prevent dehydration, while also make their antibodies much more robust than that of other vertebrates. Comparing with the various immunoglobulin isotypes produced by other species, sharks, on the other hand, mainly produce IgM, IgW and IgNAR (immunoglobulin new antigen receptor). IgNAR is a heavy chain homodimer, which consists of five constant domains and one variable region (VNAR) for each heavy chain.

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