Until the early twentieth century, it was always unsuccessful to transfuse blood（输血） from one person to another person. Then in 1901, Karl Landsteiner, an Austrian scientist, discovered that there are four types of blood. These four blood types were named A, B, AB, and O. People have one of these four types. Blood type O is the most around the world because the number of the people with type O is the largest, while type AB is the least. If people with type A blood are given type B blood,  or people with type B blood are given type A blood,  they will die. Karl Landsteiner's research made it possible to be sure that people who need blood get the correct type.

    The study of blood didn't stop. In 1972, a Japanese doctor, Furukawa Takeji, another research about blood. And he came up with the idea that it was possible to tell one person's only by his blood type, which can help you know about the person quickly.