What do you think would happen if a nation gave blood or organ transplant? I like the idea that their blood/tissue would conform to the person it's given to and in some cases make them healthier (though they may get some...odd side effects, like knowing the emotions of those close by or a strange craving for the nation's favorite food). Mostly I like the idea of Arthur not drinking for a month so he can give it away, slamming himself on the operation table and saying "Let's hurry this up, I want a drink." Cause their organs will grow back so it's no big deal, just might take a few days.

.... isn’t the “organ donations can make you more similar to the donator, including taste in food” thing actually scientifically documented? not to that degree, but still. that would totally work as a headcanon. 

but okay. I have tried to answer this for like an actual hour. partly because, uh, not to be super cringe, but you caught me neck-deep in a Hetalia/Vampyr crossover where the character 'Arthur' in Vampyr (who never actually appears in-game) actually refers to Arthur Kirkland. this is a problem because your ask that was supposed to be just Hetalia is now fueling this crossover. 

without the crossover, I’m split between like. I really like your idea that the nations just don’t have blood types etc, and that the donated material would just conform to the host. but I also lean in the opposite direction, in that I don’t think normal human beings even could receive donations from a country? like... Arthur’s blood type is probably some immortal eldritch bullshit rather than like, type O, and receiving it would just kill most human beings. but my answer with the Vampyr crossover is that it straight up makes you temporarily immortal, which I suppose is technically compatible with “make them healthier” lol. which I’ll put under the cut because no one came here for Vampyr content. >_>

re: Vampyr. okay. I know no one asked but okay. at one point. without giving away any major spoilers for the game. at one point in Vampyr, a character who is at one point referred to as the Defender of Britain drinks the “blood of Arthur” and becomes the single most difficult boss in the entire game, bar none. mind you, his competition includes, without spoilers, vampire-werewolves and a character that is basically the male version of Lady Dimitrescu. he is 100% a mortal human being and he can still wipe the floor with all of them. 

and the reason my headcanon is “Arthur’s blood makes you briefly immortal” is because upon drinking this blood, it initially doesn’t look like this character changes at all? because it doesn’t make him a vampire or anything, he just kinda seems to get like, a caffeine boost... until you do something like [checks notes] shoot him point-blank in the chest with a sawed-off shotgun or make all of the blood in his body literally boil and combust and he shrugs it off like it’s no big deal. like it’s never explicit that the blood gives brief immortality, but even with game mechanics/health making human enemies unrealistically tough, I don’t remember any of them being anywhere near “shoot him 12 times with a shotgun and make his blood explode via vampire magic” tough. 

like good lord, he nearly kicked my ass on easy mode, and he’s legendary in the fandom for being an absolute fucking nightmare if you’re doing a pacifist run (ie hardest mode). and just to add insult to injury, he fights you using a medieval broadsword and a crossbow. literally everyone else, including the player, uses modern weapons and/or vampire powers, but Geoffrey "Defender of Britain” McCullum can and will kick your ass with a fucking sword, your shotgun and fancy vampire magic be damned. and I, for one, think Arthur Kirkland would be incredibly proud of him. >_>