chimerisme

Chimerism, or chimerisme in French, is a biological condition in which an individual carries two or more genetically distinct populations of cells that originate from different zygotes. In humans this most often arises before birth when two fertilized eggs contribute cells to a single developing organism, or when a twin’s cells persist in the surviving sibling after a vanished twin.

Two main forms are recognized: natural chimerism, typically arising from post-zygotic exchange of cells between twins or from a vanished twin; and acquired chimerism, resulting from medical interventions such as bone marrow or organ transplantation, or from transfusion.

Microchimerism is a related, less extensive form in which only a small number of cells from another individual are present in the body, often detectable only by sensitive genetic testing. Gonadal or reproductive chimerism refers to the presence of two genetically distinct germ lines in the same person.

Detection relies on genetic analysis of multiple tissues. A person who is a chimera may show different DNA profiles in blood, saliva, or skin samples, and standard paternity or forensic tests can be affected. However, chimerism is usually asymptomatic and discovered incidentally during testing.

Clinical significance is variable. In some cases, chimerism can complicate organ transplantation or fertility assessments. In pregnancy, fetal microchimerism has been studied for potential associations with autoimmune diseases, though evidence of causation remains inconclusive and research is ongoing.