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In Greek mythology, the Chimera was an awesome fire-breathing monster with the head of a lion, the body of a goat, and the tail of a serpent, but in medicine, a chimera is a person composed of two genetically distinct types of cells. I learned from a fascinating article on immunity (Holloway, 2007) that human chimeras were first discovered when it was found that some people had more than one blood type. Most of them proved to be “blood chimeras,” that is, nonidentical twins who shared a blood supply in the uterus. But many more people are microchimeras and carry smaller numbers of foreign blood cells that may have passed across the placenta from their mother, or persist from a blood transfusion or in vitro fertilization.
When patients need a new heart or other organ transplant, they are put on a lifelong regimen of drugs to suppress their immune system, because otherwise the immune system would reject the transplant as a foreign organism. But although these drugs permit transplants and save lives, they also have debilitating and sometimes deadly side effects, because the weakened immune system has trouble fighting off viruses and cancers.
Some years ago a well-known transplant surgeon named Thomas Starzl made an interesting discovery. He had brought together many of his former patients, including some he had operated on in the early 1960s. He learned that some of them had stopped taking their immunosuppressant drugs a long time ago, but were still in very good health. Starzl tested these patients and discovered that they were microchimeras, that is, that they had foreign donor cells in various tissues and blood.
For Starzl, these shared cells were the key to implant tolerance