This article traces the ubiquitous geopolitical metaphors used by researchers in the field of pregnancy-related microchimerism. In this research domain, immunologists and medical geneticists locate ‘non-self’ cells in women by marking Y chromosomes in cells derived from their sons. In the course of this research trajectory, experiments have yielded a number of surprises, beginning with the very presence of these cells in women decades after pregnancy. This finding confounded the expectations predicted by classical immunology, which posits the destruction of such (...) ‘foreign’ entities. Once they were known to be present, fetal cells were implicated in pathological and destructive activity. In a series of reorientations in which metaphor, theory and practice are entangled with matter, the characterization of these cells has shifted from invaders to insurgent foreigners to assimilated productive immigrants. The article reflects on this rich domain of material-semiotic practice to demonstrate that the case of fetomaternal cell trafficking contributes to a growing uneasiness with dominant ontologies of bodies as bounded, defended, pure collectives of homogeneous inhabitants. (shrink)