This was interesting. I'm myself 0- or O-, which I now learned actually means I'm O type in the ABO system and D- in the Rh system, but there are many other factors at play. And I've considered myself a "universal donor", meaning I could give blood to anyone, but I myself can receive blood only from someone with the same group as myself.

This was significant when I lived in China, where O- type is very rare (something like 0.3 % of population) so foreigners would register with a private clinic to be able to donate/receive just in case someone needs a blood transfusion of this type.

But, apparently, there are even much rarer types where just being O- is not enough to donate.

Another rare group is the Bombay blood group: https://en.m.wikipedia.org/wiki/Hh_blood_group

(interestingly I can't find it in the submission using quick search)

Amazing article. And for some crazy reason I found myself tearing up when it considered the sacrifices of the donars.