Human Chimeras

To the ancients, a chimera was a fabulous monster with a lion's head, a serpent's tail and often an extra head in the middle of its back. To the botanist, it means a plant combining growths of differing genetic makeup--usually the result of grafting. Now British medical scientists are discovering human chimeras, in which one person has some of the body cells of another, invariably a twin.

First case so reported was that of a Mrs. McK., whose red cells were 60% type O and 40% type A (TIME, July 20, 1953). It seemed clear that she had derived them in the womb from a twin brother, but medical detectives could get only circumstantial evidence because the brother died in infancy. Now, in the British Medical Journal, research teams report two cases with both twins alive.

Mrs. W., a healthy woman of 29, then mother of two, presented herself for blood grouping (as a precaution before birth of a third child) at the Essex County Hospital, Colchester, last July. She was found to have 49% type O cells, 51% type A. Her twin brother was called in, found to have 61% type A, 39% type O. Mrs. W.'s husband is type O; so are all three of their children.

When Miss W., 21, made a donation at the North London Blood Transfusion Centre, the typing of her blood proved tricky: it turned out to be 99% type O, 1% type A. Her twin brother showed 86% type A, 14% type O.

More than a dozen assorted medical specialists worked on the cases, came to the unanimous conclusion that there had been connections between the placental blood vessels in each pair of twins during gestation. Blood cells in both men contained certain knobs (known as drumsticks, because of shape) usually found only in females. When chimerism occurs in cattle, the female is sterile--a freemartin. Mrs. McK. and Mrs. W. have each had three children, so human chimeras are not freemartins. Despite their originating from separate eggs, they may enjoy at least one of the advantages of identical (one-egg) twins: skin grafts between chimerical twins should "take" permanently.

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