In 1900, a German scientist, Karl Landsteiner, discovered that blood groups differ from one individual to another. He took blood samples from five associates and himself, allowed them to clot, and then separated the serum from the cells. Landsteiner found that when he mixed the serum and red cells from different individuals, some samples clumped, and some didn’t. Our present-day classification of the ABO system is based on Landsteiner’s realization that agglutination occurred because of highly reactive antigens present on red blood cells, which corresponded to antibodies present in the serum. Landsteiner isolated and named the red cell antigens “A” and “B” and the corresponding antibodies “Anti-A” and “Anti-B.” If the red cells contained neither antigen, he called these cells “O,” representing zero antigens present. The fourth type of red cell, “AB,” was discovered in 1902 by Von Decastello and Sturli, associates of Landsteiner. “AB” cells contained both A and B antigens on their surface.