Last month, when the champion American cyclist Tyler Hamilton was accused of blood doping, or transfusing himself with another person’s blood to increase his oxygen-carrying red cells, he offered a surprising defense: The small amount of different blood found mixed in with his own must have come from a “vanishing twin.”
In other words, his scientific expert argued, Hamilton had a twin that died in utero but, before dying, contributed some blood cells to him during fetal life. And those cells remained in his body, producing blood that matched the dead twin and not Hamilton. Or perhaps it was his mother’s blood that got mixed in during fetal life.
An arbitration panel did not believe those hypotheses and said there was a “negligible probability” that Hamilton was anything but guilty.
The test, they concluded in a 2-1 decision, shows a blood transfusion and that meant that Hamilton was suspended from racing for two years; he is the first and only person in cycling convicted for that offense. At age 34, near the end of his career, it could mean his championship days are over.
Hamilton has said he will appeal the decision. If he can prove the test was flawed, then not only might he return to cycling and his multimillion-dollar career, but other athletes could use the same defense. The new test, developed over two years by the World Anti-Doping Agency and the U.S. Anti-Doping Agency, would be all but useless.
Travis Tygart, the general counsel for the U.S. Anti-Doping Agency, which prosecuted Hamilton, says the scientific evidence was against the cyclist.”Our interest is only justice,” Tygart said. “We don’t blindly bring doping cases.”
Whether Hamilton is guilty or innocent, his defense does refer to a real phenomenon. Researchers who have no involvement in Hamilton’s case say it actually is possible for someone to have two types of blood in his body, without doping. They emphasize that they do not know whether this is the case with Hamilton.
One route to this odd state, called chimerism, is the vanishing twin. Dr. Helain Landy of Georgetown University, who has no involvement in the Hamilton case, has found that 20 to 30 percent of pregnancies that start out as twins end up as single babies, with one twin being absorbed by the mother during the first trimester.
Others researchers have found that in some cases, before the twin is absorbed, some of its cells enter the body of the other fetus and remain there for life. The cells can include bone marrow stem cells, the progenitors of blood cells.
Another route to chimerism is through the cells that routinely pass from a mother to fetus and remain there for life.
Dr. Ann Reed, chairwoman of rheumatology research at the Mayo Clinic, who uses sensitive DNA tests to look for chimerism, finds that about 50 percent to 70 percent of healthy people are chimeras. The more scientists look for chimerism, the more they find it. It seemed not to exist in the past, she said, because no one was looking for small amounts of foreign cells in people’s bodies.
“Some believe that if you look hard enough, you can find chimerism in anybody,” said Reed, who also has not been involved in the Hamilton case. It is so common that she thinks there must be a biological reason for it. It also may cause problems, she and others say.
Chimerism may be why bone marrow from a seemingly perfectly matched donor relentlessly attacks a patient who receives it in a transplant – the attackers may be a small percentage of cells in the marrow that come from someone else. It also may help explain autoimmune diseases, when the body’s own immune cells attack. The attacking cells may be the foreign ones that arise from someone else.
The Hamilton case involves a test developed by Dr. Margaret Nelson and her colleagues at the Royal Prince Alfred Hospital in Australia. It was based on a simple idea: If an athlete got a transfusion, he would have to make sure the blood was the right match using the blood antigens A, B and O. But blood cells have other surface proteins, so-called minor antigens, that do not matter in blood typing for transfusions but can be used to distinguish one person’s blood from another’s. The investigators said they could use a sensitive test, flow cytometry, to search for small amounts of blood with minor antigens different from those in the athlete’s own blood.