Microchimerism is a medical term and the best way to understand the word is to break it down. The term comes from the word chimera which, in Greek mythology was a fire-breathing monster with the head of a lion, the body of a goat, and the tail of a serpent. In other words it is an animal that is made up of three genetically distinct species. The medical definition of a chimera is a person who is composed of two genetically distinct types of cells. Adding micro in front indicates that one of the genetically distinct types is quite small, which makes sense because it is not possible or desirable to breed two genetically distinct species, especially humans with other animals.
The medical definition of microchimerism is therefore the presence of two genetically distinct and separately derived populations of cells in an individual, with one population being at a low or micro concentration. It was first discovered in non-identical twins who had not one but two blood types. It is not found in identical twins because they always have to same blood types but non-identical twins are just siblings that shared the womb at the same time and therefore shared a common blood supply.
When twin embryos share a blood supply, stem cells can pass from one to the other and settle in the bone marrow. This results in that twin producing both types of blood cells, its own and its twin's type. About 8 percent of non-identical twin pairs are microchimeras. Occasionally a person who is not a twin is found to be a microchimera. At first it was thought that this usually occurred because there was originally a twin but it died in the womb. However microchimerism may also be caused by transfer of cells between mother and fetus. This can go both ways with the child ending up with maternal cells and the mother receiving foetal cells. Because a small number of cells from prior pregnancies persist in mothers for many years, they may also be passed to another child through a later pregnancy.
Many more people are now becoming microchimeras due to blood transfusions passing a small number of foreign blood cells from the donor. In-vitro fertilization (IVF) procedures are also contributing to the increase in microchimerism. To improve fertilization success rates, two or more embryos are placed in the uterus so women who have IVF have more non-identical twin pregnancies than usual. More twins will inevitably mean more chimeras. Organ transplant is another modern way to create microchimeras.
Many questions are still to be answered about this newly discovered condition. Is it harmful? It may be implicated in auto-immune diseases where the body attacks cells that it considers to be foreign. Is microchimerism linked to other diseases such as cancer? Does microchimerism have benefits as well? Can it protect the host against some conditions? For more information, there is now an organisation with a website dedicated to investigating and answering these vital and intriguing questions.