I give blood all the time. A friend who had never given asked to go with me (he needed the moral support - admitted he was scared. ) They had to type his blood as he had never had it done. All of a sudden the nurses all seemed excited. A doctor from the local hospital came in the door and made a bee line for the table my friend was at (he still was not bleeding into the bag)
It turns out that he has a rare rare blood type. The nurses said probably one in seventy million had his exact type. She said there were less than a dozen with that type in the US
They put him on a list, said they didn't want to take his blood (it would be outdated by the time it was used) They asked if he would be available when and if someone with that type blood needed it. He still has never given but is on the rare rare rare blood list.