I have always found the topic of blood/hematology, fascinating, and even moreso, the discussions regarding: rh compatibility.
Are you rh negative? If so, do you know your alpha designation: O, A, B, or AB ?
Even if someone is not interested, nor willing to take a blood transfusion, if you are a woman, and you are rh negative, and your husband/partner is RH Positive, you can bet your bottom dollar, it'll matter in the near future. Trust me on that one.
It's always good to know. Flying High Now, posted a really good remark about the dangers of receiving rh incapatible blood. ie: same alpha, but if RH Positive is infused to an rh negative: problems arise. You can survive it, but it sends your immune response into overload.
Myself: a negative (tattooed on my upper right arm).
Who else?