I last gave blood (the only time I've ever done it) 20 years ago, when I was 22. I passed out and the doctor asked if I'd had a history of fits, because of the way I behaved when I was out, iyswim. I mentioned that I had fainted a few times as a child (not much, never knew why, dr at the time didn't seem too worried) and I later got a letter from the blood service saying "thanks but don't come back". Tbh, I've always been rather glad.... 
I'm now looking at the bone marrow registry because a friend's daughter has leukaemia and whilst I don't imagine I would be a match for her, I do think it would be amazing to donate and to actually know "I am saving someone's life".
But you have to be a blood donor.
Should I just go along and ask? It makes me feel very nervous.
Any thoughts here?