Blood transfusion is a newly recognized cause of microchimerism, the stable persistence of a minor population of allogeneic cells. Relatively recent advances in polymerase chain reaction technology have spawned new information about the frequency and aetiology of transfusion-associated microchimerism (TA-MC). Although conceptually related to fetal-maternal microchimerism, TA-MC is a distinct and separate entity. Evidence of TA-MC has been strongest among patients with severe traumatic injuries who receive relatively fresh blood products shortly after an episode of massive haemorrhage. The presence of a focal deficit in the cellular immunologic repertoire prior to transfusion that happens to match a blood donor's human leucocyte antigen type also appears to be an important predisposing factor. TA-MC seems to be common (affecting approximately 10% of transfused injured patients), enduring (lasting years to decades) and pronounced (involving up to 5% of circulating leucocytes and multiple immunophenotypic lineages suggestive of haematopoietic engraftment). Further study of this topic may reveal important information regarding potential clinical consequences of TA-MC, as well as basic haematologic and immunologic processes.

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