His experiments led to the understanding of A, B, AB and O blood groups.
Landsteiner found that there are substances in the blood, antigens and antibodies, that induce clumping of red blood cells when red cells of one type are added to those of a second type. He recognized three groups namely A, B and O based on their reactions with each other. A fourth group, AB, was identified a year later by another research team.
Today application of knowledge of the ABO system in blood transfusion practice is of enormous importance, because mistakes can have fatal consequences.