There is a weight requirement and I don't meet it. I am slightly underweight so I can't donate.
I don't think I would though anyway...unless a family member needed it. They take about 1/7th of your blood and...well...thats 1/7th of your blood that isn't in your system until it builds up again...what is it, two weeks before it builds up?
I'm pretty sure I would not be able to function
A friend of mine donated and she met the weight requirement only because she was muscular...the loss of blood caused her to pass out and she was extremely ill for about a week...