In Greek mythology, a chimera is a fire-breathing monster with the head of a lion, the body of a goat, and the tail of a serpent. In biology, a chimera is an organism consisting of tissues whose cells derive from two or more genetic sources. An apple tree, for example, can include grafts that enable it to produce a variety of apple types. In humans, such genetic mosaics reflect specific, intimate circumstances. The phenomenon is termed microchimerism because a one-cell population is typically much smaller than the other.

An organ transplant is an unnatural procedure that suddenly deposits many cells from one individual into another whose own organ has failed. It is similar to grafting in plants, and in fact, if the donor tissue reacts against the recipient's body, the condition is termed "graft-versus-host disease" (GVHD). When a transplant works, a peaceful co-existence evolves as recipient cells infiltrate...