Chimerism and the Vanishing Twin

From the New York Times:

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..."

Whether Mr. Hamilton is guilty or innocent, his defense does refer to a real phenomenon. Researchers who have no involvement in Mr. 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 Mr. 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 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 explicitly 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 Dr. Reed...

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