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:

  1. 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);
  2. 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.
  3. 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.
  4. 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.

  • $\begingroup$ Where are these numbers coming from? $\endgroup$
    – Daron
    Apr 8, 2022 at 0:37
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    $\begingroup$ Since A and B are alleles, the percentages require Mendelian genetic tracking. If EVERYONE in your population started with AB blood, then you'll have 50% AB, 25% A, 25% B children. The ratios of A and B must be balanced to achieve a high percentage of AB individuals. $\endgroup$
    – DWKraus
    Apr 8, 2022 at 2:35
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    $\begingroup$ I guess size does not matter. You can transfer blood from a great dane to a chihuahua when the antigenes match. $\endgroup$ Apr 8, 2022 at 13:37
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    $\begingroup$ This is one of the weirdest FRP questions and supporting data I have ever seen here (not counting the non-sensical questions), and that's saying a lot. I am seriously stumped trying to figure out how this level of effort and detail on this subject could ever payoff in an actual game world. $\endgroup$ Apr 8, 2022 at 15:16

2 Answers 2


Following the rules of Blood Banking:

Shameless plug, but I have 18 years of blood banking experience.

You are still maintaining the same basic blood types for your species, which is good. It means there are still transfusions possible. You will need to know the general rules of transfusions for the blood to be supported by blood banks. I suspect, however, the real problem will be exotic antigens causing antibody problems.

Antibodies attack the corresponding antigens, so A antigens are bound by A antibodies. If you lack antigens, it doesn't matter if you have the antibodies. So A and B are the common antigens, while O is the lack of those antigens. A and B resemble bacteria to your immune system, so you always produce antibodies if you DON'T have the antigen. If you have the antigen, the body recognizes the antigen as "self" and doesn't produce the antibodies to that.

Your basic rules for ABO are that you make antibodies against things you DON'T have antigens for. So AB people can get red cells (antigens) from any source and not have a reaction. O people can only get O blood. In a crisis, you want to give O blood because you know it will be compatible, since it has no antigens.

blood typing

Plasma rules are opposite. Plasma doesn't contain antigens (cells), but does contain antibodies (in plasma), so giving AB plasma (lacking any antibodies to A or B) is the universal plasma donor. O people can get plasma from anybody, since they have no A or B antigens for the antibodies in donor plasma to attack.

RH (the D, or plus/minus) operates slightly differently. It doesn't look like bacteria, so you only develop antibodies if you are exposed to the rh(D) factor. Rh negatives lack the rh antigen, while rh positive have the antigen. Anyone is safe for ONE exposure, but after that, you may develop antibodies. So an rh negative mom who has an rh positive child and is exposed to the child's blood during pregnancy or birth develops anti-D antibodies. Because of the specific type of antibodies that form to D, the antibodies can then attack subsequent rh positive babies the mom might have.

So a good blood bank will track all this and match the specific types together. It might be complicated in a multi-racial blood bank, but workable.


There are a significant number of OTHER antigens which are highly variable by ethnic group. Blood banking is one of the few areas where race really DOES make a difference. For people rarely transfused, the problems of these antigens is small - like the rh(D) factor, you only make antibodies against things you DON'T have yourself but are exposed to. While they are less antigenic (causing antibody development) than the rh(D) antigen, enough exposure (or a hyperactive immune system) will mean that the frequently transfused will need more and more specific typing. So a person with sickle-cell anemia getting regular transfusions might need A Pos, Fy(a) negative, Kell negative,JkA negative units. Since these antigens aren't routinely typed for, it means they have a hard time finding blood, especially in an emergency. You need to have complex charts to track and identify which antibodies someone has, which they MIGHT produce, and what blood they will be compatible with.


So a person in a foreign land, getting regular transfusions, is more likely to develop exotic antibodies to exotic antigens. An O person in a land of AB people will already have a hard time getting transfusions due to the shortage of O units. But if they lack the Fy(a) antigen and everyone in that region is Fy(a) positive, they are likely to develop antibodies to Fy(a) and then they start having real problems.

As For Size:

In a modern blood bank, this isn't an issue. Blood donations are standardized into discreet units, and more units are needed for bigger bleeds. A larger human would need more units, while a smaller one would get by with fewer. A VERY small human might get part of an oversized unit (if that was all that was available) and there might be some wastage. For transfusions into children, this is how it gets handled.

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    $\begingroup$ You failed to mention Whole Blood transfusions. (Probably because a blood bank doesn't do them.) I believe these count as both cells and plasma, so the ABO system must match exactly, and the other antigens must be OK both ways as well. Since this is where transfusion technology begins, I think it worth mentioning. $\endgroup$
    – David G.
    Apr 8, 2022 at 15:07
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    $\begingroup$ You know, I thought the level of detail and implied expertise in the question was crazy, but this answer just blows me away. For all of the non-sensical, surreal questions and answers on this site, it is the occasional masterful article like this that convinces me of the real value of Wordlbuilding.SE. Just, WOW. $\endgroup$ Apr 8, 2022 at 15:23
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    $\begingroup$ @DavidG. Fair point, since the tech level wasn't specified. $\endgroup$
    – DWKraus
    Apr 9, 2022 at 0:18

One way to approach this would be to chart it out. You could then match your humanoids and find which are more likely to be compatible.

However, I think you have license to make it as simple or as complicated as you want to. For example even among us ordinary humans there are "subgroups" or phenotypes where some people have different combinations of A and B antigens. Who is to say that some of your humanoids have some special blood characteristic? I think it is up to you to be how human specific it.

When I first read your question, I thought you might also be asking about Rh factor and pregnancies. That might be an additional complication in interbreeding, if the baby is Rh positive and the mother is Rh negative, the mother immune system could damage the baby.

Blood type compatibility Blood Cell Compatibility

  • $\begingroup$ I think you put two versions of the same chart. $\endgroup$
    – Daron
    Apr 8, 2022 at 2:01
  • $\begingroup$ Yes I did, I was going to edit and remove it, but then got distracted. $\endgroup$
    – UVphoton
    Apr 8, 2022 at 2:35

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