Background: Twins are thought to maintain discrete circulations, but blood group typing suggests that this is not always the case.

Subjects: Cord blood samples were collected at delivery from 18 pairs of twins, one pair of conjoined twins and 25 normal infants (all>35/40).

Interventions: Blood films were stained using fluorescent labels for CD3 (UCHT-1) and CD45R0 (UCHL-1). Double-staining cells were expressed as a proportion of CD3+ cells; 200 cells counted per infant. Three pairs of twins also had blood films examined after delivery.

Results: The proportion of T-cells expressing CD45R0 was lowest in the normal infants (median 5%±1%, range 3-8%). Expression of the marker was similar in both members of all twin pairs. Monozygotic twins showed low levels of expression (4%±1.5% and 6%). Dizygotic twins had raised values from 4% to 25% (median 12%±1%). Conjoined twins had a low level of expression (4%±1%). In two pairs of dizygotic twins observed longitudinally, the level of CD45R0 expression decreased to normal within three weeks.

Conclusion: T-cell activation in some dizygotic twins suggests that there is a degree of mixing of circulating cells, or chimerism, in otherwise normal twins.