yt antigen system yt antigen system the yt antigen system (also known as cartwright) is present on the membrane of red blood cells and helps determine a person's blood type. the antigens are found on the protein acetylcholinesterase, an enzyme which helps break down acetylcholine. the yt system features two alleles, yt(a) and yt(b). antibodies against the yt system can lead to transfusion reactions such as hemolytic anemia.