The Colton antigen system (Co) is present on the membranes of red blood cells and in the tubules of the kidney^[1] and helps determine a person's blood type. The Co antigen is found on a protein called aquaporin-1 which is responsible for water homeostasis and urine concentration.^[2]

The Co antigen is important in transfusion medicine. 99.8% of people possess the Co(a) allele. Individuals with Co(b) allele or who are missing the Colton antigen are at risk for a transfusion reaction such as hemolytic anemia or alloimmunization. Antibodies against the Colton antigen may also cause hemolytic disease of the newborn, in which a pregnant woman's body creates antibodies against the blood of her fetus, leading to destruction of the fetal blood cells.^[3]