National transfusion registries from twenty-two countries
Blood Doesn't Choose
Your blood type is decided before you're born, from your parents' genes, with no choice of your own — yet it decides, with near-arithmetic precision, whom you can help and whom you can turn to. In Romania, over a third of people carry A positive; AB negative, the rarest, belongs to one person in a hundred. Pick your type on the dial below to see who you can give to, then watch how the proportions shift from one country to the next.
What a blood type actually marks
Every red cell carries a combination of proteins on its surface, like a label attached at birth. The ABO system reads two of them: the A antigen and the B antigen. Carry only A and you're type A; carry only B and you're type B; carry both and you're AB; carry neither and you're O. A second, independent label sits on top: the Rh factor, present or absent, which splits each of the four groups again, into positive and negative. Eight combinations result, and all the arithmetic of a transfusion lives inside them.
A body attacks any antigen it doesn't recognise as its own. That's the rule behind the numbers: type O blood, carrying no antigen label at all, can be received by anyone, while AB positive blood, already carrying every possible label, isn't alarmed by any it receives from outside. O negative is the universal donor; AB positive is the universal recipient. The other six types sit somewhere between these two ends, each with its own circle of compatibility, wider or narrower.
Pick your type
The dial has eight sectors, one per type: the red half holds the Rh-positive types, the blue half the Rh-negative ones. Touch a sector to see who you can give to, who you can receive from, and how common your type is in Romania.
Touch a sector
The corridor of type B
Type B isn't spread evenly. In Ireland and the Netherlands, under one in ten people carry it; in Germany and Austria, a few more do; and from Czechia and Slovakia eastward, through Romania and Moldova, toward Turkey, Israel and India, the share climbs almost without a break. Romania sits at 16%, 45% above Ireland; its neighbour Moldova climbs past 20%; India reaches 34%, more than double Romania's share.
Population genetics ties this gradient to the history of migrations out of the Central Asian steppe toward Europe: the wave that carried the B allele furthest west faded gradually along the way. Sitting at the crossroads between steppe and Mediterranean, Romania kept more of it than the Atlantic did, but less than Asia.
What nearly vanishes toward the east
Rh-negative traces an even steeper gradient. In Ireland, nearly one person in six is Rh-negative; in France and Germany, nearly one in seven; in Romania, also one in seven; toward Turkey, one in eight. In India it drops to one in seventeen, and across East Asia it almost disappears: one in two hundred in Japan, one in three hundred thirty in South Korea. An Irish person is 53 times more likely than a Japanese person to be Rh-negative.
The explanation is historical too: the allele that leaves the Rh antigen absent is old in Europe and persisted there through relative genetic isolation, while East Asian populations evolved almost entirely without it. Hence the clinical rarity of Rh-negative blood beyond the Urals, where a hospital can't rely on local reserves and has to bring them in from elsewhere.
Method and sources
Compatibility
The rules for giving and receiving are standard immunohaematology: type O carries no ABO antigen on its red cells, so it triggers no immune response on receipt, whatever the recipient's type; type AB carries both, so its body doesn't react to anyone's blood. The Rh factor is added separately: an Rh-negative person can only receive Rh-negative blood, while an Rh-positive person can receive either. The same rules, valid everywhere, govern the dial above.
Sources
The country percentages come from national transfusion services and local red cross societies — the Irish Blood Transfusion Service, the Japanese Red Cross Society, Sanquin (Netherlands), Blutspendedienst (Germany), Národná transfúzna služba (Slovakia), Magen David Adom (Israel), Donor.ro for Romania, "Nicolae Testemițanu" State University of Medicine for Moldova — supplemented by South Korea's official statistics service (KOSIS) and two peer-reviewed surveys, for India and Jordan. Every set of eight percentages was checked to add up to nearly exactly 100%; where the total drifted noticeably, as for the United Kingdom or Iceland, the country was left out of the numeric comparisons.
Limits
The estimates come from donor samples, not from testing entire populations, and the methodology and collection year differ from country to country. They agree, though, on the same order of magnitude, year after year, which makes the gradients above — type B moving east, Rh-negative fading east — hard to write off as coincidence. Converting percentages into people, for Romania, uses a rounded figure of 19 million residents.