I haven't in a while because of scheduling conflicts, but I do donate blood. I can't remember how much I've donated so far, but it's not nearly a gallon. I am O+, which is the most common blood time in the US, and parts of Europe. I donate so people don't die.

EDIT: Moles are caused by concentrated pockets of pigmentation and sunlight. Not marks. You can have dark scars because of shots, though.