A review of the biology and classification of human chimeras

Chimeras are organisms composed of cells derived from two or more zygotes. Clinicians, blood group serologists, and cytogeneticists have recognized natural human chimeras for more than 60 years and molecular biologists are now able to recognize them using more sensitive and definitive tests.

Human chimeras are divided into two major classes, man-made and natural. Man-made chimeras include transplanted patients and several kinds of iatrogenic chimeras including those that develop after in vitro fertilization (IVF). Natural chimeras have historically included twin chimeras and fusion chimeras. Recently described microchimeras are primarily natural ones as well. Updated terminology and classification are suggested to account for information gleaned from natural and experimental animal chimeras.

Many human chimeras remain undetected. The states of health and disease of human chimeras remain largely unknown. Of four ways to detect human chimeras, molecular typing is the most sensitive and specific. Before systematic and temporal studies can be undertaken, improved cell sampling and better analytical detection methods are necessary. Chimeras may be sought among dizygotic twins and children born after IVF procedures.