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 fac

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.