Registering with the National Marrow Donor Program takes only minutes. You receive a kit in the mail afterwards which includes some swabs. You swab your cheek with these swabs and put them into the kit and mail it back to the National Marrow Donor Program. Eventually you enter their records, and one day in the future you may be fortunate enough to be a match for a person desperately in need of blood stem cells.
There are many, many risks associated with being the recipient of another person's blood stem cells, including death.
But being a donor? There is almost no risk to a donor's health at all.
Donors may be asked to donate either bone marrow or peripheral blood stem cells (stem cells in circulation in your body). Donating bone marrow is much easier—health professionals give the patient either local or general anesthetic, then stick a needle into the back of the pelvic bone and draw out some sponge-like marrow. That's the extent of the procedure for the donor, who might have some minor back pain for a couple of days.
Donating peripheral stem cells is a little more involved. For about a week prior to the actual day of donation, the donor is required to be given an injection of a medication called filgrastim that boosts the body's production of those stem cells so that in a week's time, there are plenty in the body to donate. When that day comes, an IV goes into each arm. One removes blood from the body and passes it through a large device that takes out the blood stem cells. The remaining blood is then returned to the patient via the other IV.
Prior to the collection date, the donor might experience a headache or some bone or muscle aches. All this passes shortly following the collection date.
The risks associated with donating blood stem cells are the same as donating blood in a blood drive—maybe some swelling or redness at the injection site. Complications are otherwise so minimal as to almost be a non-issue, especially considering the potential gift of life the donor is offering a needy patient.