colton antigen system colton antigen system the colton antigen system (co) is present on the membranes of red blood cells and in the tubules of the kidney 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. interpretation of antibody panel to detect patient antibodies towards the most relevant human blood group systems. further information: blood compatibility testing 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.