I just wanted to bump this up again.
An anonymous bone marrow donor saved my life two years ago, and I was, without exaggeration, several weeks from death.
A lot of people heard stories that donating bone marrow is painful, but in 80% of cases these days, donation is done via an IV in each arm, and is NOT the painful procedure it used to be. Your own immune system is completely back to normal in 3 weeks.
You can get on the registry with a cheek swab via the post (so you don't even need to get blood drawn), and you could save a life. Note that it's NOT the same registry as being an organ donor - this is a separate step.
PLEASE PLEASE join the UK registry at Anthony Nolan if you're eligible. It's free, you can save a life by sharing your marrow, and continue on living yours without any harm to yourself.