Pretty much as above, but stronger. People who need a bone marrow transplant generally have something like leukaemia, and need a very precise match at 6 "HLA" sites: 2xA, 2xB and 2xD, where each can have something like 5-20 common possible variations, and many more uncommon ones. Note that the A and B are not the same A's and B's in your blood group.
To enable a reasonable chance of finding a matched unrelated donor, as ~60% of the time there is no related match or near-match, one needs 100,000 to 1,000,000 people on what is effectively a world wide registry. These individuals on teh registry are typed at the A and B sites. If there is a needy person, about 10-100 people with A and B matches will be called and further tissue typed at the more complex D locus. If they then match the potential donor would then be asked if he/she was willing to donate. The old way donation is actually a reasonable sort of operation and carries a small risk, although things are moving quickly to a more minor means of extracting the cells from the bloodstream. Obviously the donation is generally the only chance for the recipients long-term survival.
The australian red X quotes 1:1000 chance per year of being called for a second round typing.
I'd go for it. The chance of actually being required is low but it could just make a big difference to somebody.
