as king said, that cost you nothing, but can save some1s life..
i have that rh factor negative and not many people have that blood, and it can only be given to people who have same antigen, so its precious and 2 times at year i donate blood.
My friend had accident, and once he need blood, and he had rh neg factor, and they couldnt find any1 to give him, then i heard it and donate my blood and he got cured cuz of that...