Blood types can get really complicated, people typically only know about A, B, AB, O, and the + or - versions, but there are way more variants you can have if you include the whole Rh system. The Rh system has more than 50 antigens, all of which can cause complications if they don’t match. The universal donor is considered to be O-, but the most universal is if you have Rh null blood (no Rh antibodies), because that blood can be compatible with literally anyone. But in all of known history, only 43 people have ever been recorded with that blood type, and if one of those people need a blood transfusion, they have to use Rh null blood. So they’re doomed unless one of the few people on the planet with that blood type somehow donate blood to this person on short notice. I certainly would never want to live in fear of getting an accident or something requiring a transfusion, knowing that getting a compatible transfusion isn’t likely