The blood plasma contains special proteins involved in immune reactions, agglutination (gluing of cells, microbial particles). And they are called agglutinins. There are only two types of them. They were conventionally named - alpha and beta. But they are found in everyone in different ways - someone has two types of proteins at once, someone has one of them, and someone has none. Erythrocytes contain agglutinogens - their function is to replace missing agglutinins. Conventionally - A and B. That is, if there is no alpha in the plasma, then there is A in the erythrocytes, if there is no beta in the plasma, there is B. If there is no alpha or beta in the plasma, there are A and B in the erythrocytes, if plasma has both alpha and beta - there are no agglutinogens in erythrocytes.
So we get blood groups:
* α and β: first (0)
* A and β: second (A)
* B and α: third (B)
* A and B: fourth (AB)
That is, in all In four groups, we essentially have two proteins we need.
Division into 4 groups is not some kind of fundamental law, it is a systematization adopted by a person, the purpose of which is to explain to whom what can be poured. I heard this version of why it happened in nature this way:
Previously, all people had one blood group (0), but then their diet began to change, people began to switch to growing plants, a second blood group appeared ( A), the third group (B) appeared with the repeated change of the diet - people began to engage in animal husbandry. The fourth group (AB - the latest), was formed during the incest of the second and third. At the same time, the child's blood group, knowing the group of parents, is almost impossible to predict with accuracy, but it is safe to say which group the child will not have.
Group 1 (00)
Group 2 (AA, A0)
Group 3 (BB, B0)
Group 4 (AB)
So when crossing, one character is taken from one parent and the second from another, for example AA + B0 = AB (A0 or B0), but not 00 (that is, the first group cannot be)
The story may not be the most true, since monkeys have the same blood groups like a human, but beautiful.
Because the blood type is determined by the presence or absence of two proteins (not counting the Rh factor). So we get 4 groups (conditionally 00, 01, 10, 11).