How can you find out what your blood type is?

All people do not have the same blood. It was only recently discovered, and until then, the blood, in emergency was given by immediate transfusion regardless of the so-called blood type.
This is why half of the patients were worse and many of them died.
To determine the blood groups, are needed the red blood cells of the donor and recipient's blood serum. Serum is one part of blood plasma.

What happens if we mix different blood groups?

If one drop of blood from one type is mixed with blood or serum from other blood type, red blood cells are glued to each other and this is called agglutination. In that case, the red blood cells usually fall apart and cause hemolysis of the blood.
This is why it is important to determine blood groups.

Using agglutination tests, it is showed that the people have four blood types: O, A, B and AB.

Red blood cells of O do not agglutinate with the serum of any group.
In other words, the blood group O can be given to anyone for the purposes of transfusion, regardless of their blood type.

That's why people with a group O are known as general blood donors.
In the group AB the serum does not agglutinate the red blood cells of any group, so that people with this blood type can receive blood of any group.

These are called “common” recipients of blood.

We all inherit our blood type, and we can’t change it in the course of our life.
An interesting observation is that across the planet there is a certain scheme of blood groups. From west toward east Europe the number of people with blood group A declines, while the number of people who have group B rises.
In England, 43 percent of people have group A, and 30 percent of the Russia, and Iran, only 15 percent.