Here is a good link on this. In the old days it was a very painful experience to have bone marrow taken from the patient ( big syringe and the use of small sledge hammer to get the syringe into the pelvic bone...been there done that) but as I understand the procedure has evolved into a non painful experience but it is still no walk in the park.
There is a lot of preparation for the person who is to be put in the registry as this just isn't something where you walk in and give a sample. Bone marrow is one of those substances in our body that gives Doctors a lot to digest as far as what goes on inside of our own and how it matches up with somebody else. I have to take my hat off to you about being a donor because the end result is that if there is a match between yourself and the prospective patient you will undoubtedly save a life. This is a very admirable thing to do.
Link,,,,
http://www.marrow.org/DONOR/When_You...ion/index.html