I've never been able to. We have a blood drive in honor of my uncle every Christmas time, and they've never let me. Though they told me that it wasn't because my blood was bad, but solely because they can't take any due to the risk of "depleting my system." They take red blood cells, and those carry the oxygen. Stuff like that. But yeah, as Julie said, instead of donating the blood every year, I volunteer at the blood drive for the few hours its held.