CELLULAR PERSISTENCE BEYOND BOUNDARIES: A COMPREHENSIVE REVIEW OF HUMAN MICROCHIMERISM, ITS BIOLOGICAL FOUNDATIONS, AND CLINICAL IMPLICATIONS

Microchimerism refers to the sustained coexistence of a genetically distinct, numerically minor cell population within a host organism. Although the phenomenon has been recognised for several decades, its biological significance remained underappreciated until advances in molecular detection made it possible to track rare cell lineages with high sensitivity and specificity. The present review synthesises current knowledge regarding the origin, trafficking, and long-term fate of microchimeric cells in humans, with particular emphasis on fetomaternal exchange during pregnancy, the role of stem-cell-like progenitors in sustaining engraftment, and the dual — protective versus pathological — consequences that these foreign cells may exert on host tissues. Evidence drawn from epidemiological studies, in-vitro experiments, and animal models collectively suggests that microchimerism is not a passive residual phenomenon but rather an active participant in immune modulation, tissue homeostasis, and the pathogenesis of certain autoimmune conditions. Methodological considerations relevant to reliable detection are also addressed, alongside perspectives on therapeutic exploitation of microchimeric cell populations.