In my universe, there are sixteen species from the Homo genus: anatomically modern humans, demons, angels, merfolk, ogres, giants, halflings, dwarves, elves, goblins, gnomes, orcs, trolls, vampires, therianthropes, and wizards. They can all interbreed with each other, and the resulting offspring is less fertile than their parents, but still fertile.
Some species have a weird blood type repartition compared to anatomically modern humans:
- Gnomes are MUCH more likely to be rhesus negative than anatomically modern humans (62 % of gnomes are rhesus negative) (I do not mean rhesus null, I mean at least the simple absence of D antigen), and they are also more likely than anatomically modern humans to have blood type O (70 % of gnomes are blood type O, 14 % are type A, 14 % are type B, and 2 % are type AB) (to be exact, 46.2 % of gnomes are O negative, 7.3 % are A negative, 7.3 % are B negative, 1.8 % are AB negative, 24.4 % are O positive, 6.7 % are A positive, 6.7 % are B positive and 0.2 % are AB positive);
- Ogres mostly have B negative blood type (the simple absence of D antigen), 60 % of ogres are rhesus negative, and 40 % are Rhesus positive, 35 % of ogres are B, 25 % are A, 30 % are O, and 10 % are AB, in other words, 20 % of ogres are B negative, 15 % are B positive, 16 % are A negative, 9 % are A positive, 18 % are O negative, 12 % are O positive, 6 % are AB negative, and 4 % are AB positive.
- Goblins mostly have A negative blood type (again, at least only the simple absence of D antigen), 43 % of goblins are A negative, 9 % are B negative, 4 % are O negative, 2 % are AB negative, 30 % are A positive, 8 % are B positive, 3 % are O positive, and 1 % are AB positive.
- Vampires are MUCH more likely than anatomically modern humans to be AB, and they are the species that is the most likely to be AB-cis, 35 % of vampires are AB positive, 5 % are AB negative, 26 % are A positive, 4 % are A negative, 18 % are B positive, 2 % are B negative, 9 % are O positive, and 1 % are O negative.
Also, my species are EXTREMELY variously sized: gnomes are as small as the average adult domestic cat, halflings are as large as the largest domestic rabbit breed, goblins are as short as mandrills, vampires are as small as common chimpanzees, ogres are as tall as the average real life NBA player, and as heavy as adult male gorillas, giants are as massive as polar bears, and merfolk are as massive as belugas.
So, I wonder if interspecies blood transfusion would be a problem. Shall you know that the technological level is contemporary.