This comes from a discussion with a friend about how few people on the Lost island actually knew their blood type. Because I know mine I just assumed it was common knowledge for people until I discovered that hellbob didn't know his. Not only do I know my blood type, but I also know hellbobs, and I know my father is AB, but don't know if it is positive or negative.
I also found a website that has all the facts about blood types:
Do you know your blood type?
What is your blood type?
Edit Hey, mle292, if I get in an accident & need blood I'm coming after you!