Like humans, elves have several different blood types, with the key differentiation being the type of H antigen and the presence or absence of Diego-A antigen. The types of H antigen common among elves are classified as hX, hY, hZ, and hW, with the Diego-A antigen’s presence or absence indicated by a + or – symbol. As such, elves have been known to express the blood types X+, X-, Y+, Y-, Z+, Z-, W+, W-, and ZW+ or ZW-. Approximately 50% of elves have type X blood, 25% have type Y blood, 12.5% have type Z blood, 8.33% have type W blood, and the remaining 4.66% have type ZW.
Due to their close genetic relationship, the blood type is similar to humans and transfusions are possible. However, the H antigen is extremely rare in humans, occurring in less than one in a million people, with the result that while elves can and do donate blood to humans, only very rare and specially screened humans may donate blood to elves. The Diego-A antigen is relatively rare in humans as well, meaning that humans of certain backgrounds will need to be screened additionally to make sure there is not a mismatch.
Human/elf hybrids, while inveriably sterile, do exist. They will generally inherit either a human or an elven blood type, with or without Diego-A or Rh antigens. This can cause pregnancy complications if, for instance, a human mother is carrying a child of an elven father. Modern solutions to this problem exist, but it led to the folk legend that elven women could not bear children with humans but that human women could bear them with elven males.
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