“The rarest blood type in existence is Rh null blood. This stuff is characterized by a complete lack of antigens in the Rh system, which is the largest blood group system. Besides making someone feel really special, Rhnull and other rare bloods are extremely valuable. Because Rh null blood can be considered ‘universal’ blood for anyone with rare blood types within the Rh system, its life-saving capability is enormous. As such, it’s also highly prized by doctors – although it will be given to patients only in extreme circumstances, and after very careful consideration, because it may be nigh on impossible to replacef”

Discovery (https://www.discovery.com/science/Rhnull-Rarest-Blood-Type-on-Earth)