Agglutinogen is an antigen[1] that causes the formation of agglutinins in the body and leads to agglutination, such as hemagglutination, which involves red blood cells (RBCs).
If the blood is of type AB, then both agglutinogens A and B are present.
The agglutinogens are made by specific enzymes, which are encoded in genes.
A third version of this gene, the O allele, codes for a protein that is not functional; it makes no surface molecules at all.
The existence of agglutinogens on the surface of red blood cells has been inferred from the reactions, hemolysis and agglutination, which occur on contact with specific antisera.