Some 160 of the 342 blood group antigens are ‘high-prevalence’, which means that they are found on the red blood cells of most people. If you lack an antigen that 99 per cent of people in the world are positive for, then your blood is considered rare. If you lack one that 99.99 per cent of people are positive for, then you have very rare blood.
If a particular high-prevalence antigen is missing from your red blood cells, then you are ‘negative’ for that blood group. If you receive blood from a ‘positive’ donor, then your own antibodies may react with the incompatible donor blood cells, triggering a further response from the immune system. These transfusion reactions can be lethal.
Source: Inside the Science of Sharing Rare Blood
Similarly, there are rare people born with reversed organs and such. The word had been that if you are a rare case, China would likely have someone with the same issue given the population size. India would be another likely source…
Knowing that frozen blood has a 48-hour lifespan, compared to four weeks for fresh blood, makes me understand why there are constantly blood drives. But the last time I checked, those on blood thinners are not allowed to donate. If you can, please consider donating or encouraging those who can to donate blood.
Related: How Many Undiscovered Blood Groups Are There?