People in most parts of the world do not think about their blood group much, unless they have an operation or an accident and need a transfusion.
But in Japan, whether someone is A, B, O or AB is a topic of everyday conversation.
There is a widespread belief that blood type determines personality, with implications for life, work and love.
It is Saturday night and a speed dating session is under way in a small building in the backstreets of Tokyo.
Men and women are sitting nervously at tables hoping to find that special someone.
The room is brightly painted in red and white, the staff upbeat and enthusiastic, but the conversations are rather stilted.
The couples have just a few minutes to try to sound each other out before a bell rings and they have to move on to the next lonely single.
It is a scene repeated in cities across the world but this speed dating session in Japan has a twist.
It is for women who want to meet men with blood group A or AB.
One says she decided to narrow down her search for a boyfriend after a bad experience with a man with type B.
"Looking back it seems trivial," she said. "But I couldn't help getting annoyed by how disorganised he was."
"I really would like someone with type A blood," added her friend. "My image is of someone who is down to earth, something like that."
Interest in blood type is widespread in Japan, particularly which combinations are best for romance.
Women's magazines run scores of articles on the subject, which has also inspired best-selling self-help books.
The received wisdom is that As are dependable and self sacrificing, but reserved and prone to worry.
Decisive and confident - that is people with type O.
ABs are well balanced, clear-sighted and logical, but also high-maintenance and distant.
The black sheep though seem to be blood group B - flamboyant free-thinkers, but selfish.
"At the interview for my first job they asked me about my blood type," said a man with blood group B, who wanted to identify himself only as Kouichi.
"The surprise was written on my face. Why? It turned out the company president really cared. She'd obviously had a bad experience with a B type blood person. But somehow I got the job anyway."
Later, though, the issue of his blood came up again.
"The president was the kind of person who couldn't take her drink and at one company party she got drunk. So she sent B people home before the others. 'You are blood type B,' she said. 'Get out.'"
There is even a term for such behaviour in Japan, burahara, which translates as blood group harassment.
The preoccupation with blood ultimately dates back to theories of eugenics during the inter-war years.
One study compared the blood of people in Taiwan, who had rebelled against Japanese colonial rule, with the Ainu from Japan's northern island of Hokkaido, thought to be more peaceable.
Stripped of its racial overtones, the idea emerged again in the 1970s.
Now, blood typecasting is as common as horoscopes in the West, with the whiff of science - although dubious - giving it added credibility.
Some firms organise work teams by blood type to try to ensure office harmony.
And people going on a date or meeting someone for the first time are liable to be asked: "What is your blood group?"
"This particular thing about blood types is a clever way of telling people what you think about them, but indirectly," said Jeff Kingston, professor of Asian Studies at Temple University in Japan.
"Here people don't like to be upfront and open about their opinions. So if you can hide behind blood types you can then tell someone indirectly what you think about them."
Scientists regularly debunk the blood group theory but it retains its hold - some believe because, in a largely homogenous society, it provides an easy framework to divide people up into easily recognisable groups.
The last Prime Minister, Taro Aso, even put the fact that he was a type A in his official profile on the internet.
If he had hoped that having a favoured blood group would give him a boost at the polls he was disappointed.
When the election came around, he lost.