I got something in the mail from Be The Match bone marrow donor registry which told me that I have a rather rare human leukocyte antigen (HLA) type. Matching HLA is the best selector of a donor for a patient. It says that for some patients, I may be the only one on the registry who can save their life. The idea is that it is unlikely that I will be called upon to be a donor, but following through and actually donating is critical in the case that I do.
Bone marrow transplants are used to treat people with blood cancers such as leukemia and lymphoma. Apparently umbilical cord blood is also very valuable. I do not know how that works, but you can donate your (child's) umbilical cord here.
On a less "I'm special" note, the letter fails to mention that it may be very difficult for me to find a donor should I need a bone marrow transplant. I guess they want to avoid scaring people about things like that, but I feel like that should be mentioned.