In the early twentieth century by the Austrian scientist Karl Landsteiner, testing blood taken from different individuals has revealed certain patterns (group signs), which he described in his classification.
it was Later determined that the group signs are due to two different molecules located on the surface of red blood cells. Landsteiner called them "A" and "B". In human blood can contain one of them or both together, or completely absent. On this basis, the blood was divided into four groups.
The first group took blood, which was absent and molecules "A" molecules "B", identified it as 0. Blood that contains only the molecule "A" was included in the second group, she received a letter A. Accordingly, the blood containing only the molecule "B" attributed to the third group and identified by the letter V. finally, blood containing, and a molecule "A" molecule and "B" attributed to the fourth group and designated AB.
For the discovery of blood groups Karl Landsteiner was awarded the Nobel prize.
The study of blood continued and it was soon revealed that the blood has other differences that are due to the presence of a particular protein, called the RH factor. People in the erythrocytes which are present, this protein was considered to be RH positive (Rh+) when the protein was absent - RH negative (Rh-).
Today the group and rhesus belonging of blood are the most important characteristics that are always considered in blood transfusion. Donor blood that is transfused to a sick person have to match (be compatible) with his blood group and RH factor. Otherwise the recipient's body treats the donor blood as a foreign object and produces antibodies that attack the donor blood cells destroy them. This phenomenon is called "hemolysis" and could lead to exceptionally serious consequences, until the death of the recipient.