in approximately 50,000 donors. collecting, storing, freezing the blood from those few donors, we can secure a repository of these very rare blood groups. we have a frozen repository here so that we can always find the blood. >> one stranger saving another. it's what happened in celia hutchinson's case. in boston, she and her husband walter are back at massachusetts general hospital for a check-up. dr. chen performed celia's stem cell transplant three years ago. >> good morning. celia, great to see you. >> same here. >> take some deep breaths. >> celia has been cancer-free for the past two years. the transplant did its job. and celia credits her donor stuart with saving her life. >> i hear from my donor almost every day. >> do you really?