Chimerism

Chimerism is the presence of two or more genetically distinct cell populations within a single organism, originating from different zygotes. In humans, chimerism can be natural or acquired and may involve any tissue, though blood and immune cells are often assessed for detection. The concept derives from the mythical Chimera, but in biology it describes a real mixture of cell lineages rather than mythic composition.

Natural chimerism in humans typically results from the exchange of cells between twins during early pregnancy (twin-to-twin transfusion or shared circulation). Maternal-fetal cell exchange can also lead to fetal or maternal microchimerism, in which minority cell populations persist for years. Acquired chimerism occurs when donor cells are introduced through organ or bone marrow transplantation or, less frequently, through blood transfusions.

Chimerism is distinct from mosaicism. Mosaicism arises from genetic variation within cells that originated from a single zygote after a post-zygotic mutation, whereas chimerism reflects the presence of cells from two or more zygotes.

Detection often requires genetic testing of multiple tissues or high-resolution DNA analysis, because one cell lineage may predominate in some tissues but not others. Clinically, chimerism can complicate paternity testing, forensic analysis, transplant compatibility assessments, and blood typing, but many individuals with chimerism are asymptomatic and unaware of it.

In plants, chimeras can occur naturally or through grafting and have practical applications in horticulture and breeding.