You’ll always be free to decline to donate, and your answer will remain confidential. However, saying no can have very serious consequences if you withdraw once you’ve been identified as someone’s match. Because being someone’s match means YOU are their best hope of channelling their inner Bee Gees and staying alive.
So before signing up all gung-ho because your cousin is sick, take the time to think things through. And when you do sign up, do it to save anyone, anywhere, from the moment you register until your 60th birthday. Yep, it’s one long commitment.
Also, if you’ve signed up and realize you’re not up for it, contact your country’s registry and withdraw your name. That way, we’ll all avoid an extremely awkward situation.