"I have absolutely no idea how the tests are done. You can put it on your list of things to consult Wikipedia about. For some reason the types are A, B, AB, and O, with each coming in 'positive' and 'negative' for some other parameter, and if I recall right, which I might not so you'll want to check Wikipedia, you can get blood from anybody who has the exact same or strictly fewer of things than you - with 'things' being A, and B, and positivity for the parameter. So AB+ can get blood from anybody, O- can give it to anybody. There are probably exceptions with rare blood types that crop up in only a handful of people. I have no idea if your people will have the same kinds of blood types."