SPOTLIGHT Critical review interactions with other cells or with soluble factors (11). The same mechanisms act in acquired transplantation tolerance (4, 12) and might be harnessed to achieve donor-specific tolerance by blunting the effects of alloreactive T cells. A further possible mechanism of immunologic tolerance that is unique to the transplant setting is microchimerism, the persistence of a small number of donor-derived bone marrow cells in recipients (13). However, debate continues regarding methods for detection of microchimerism, the importance of the anatomical sites in which it is found (14), as well as its clinical relevance (15). Whether tolerance or rejection is associated with microchimerism may depend on the state of maturity of the host immune system and the degree of antigenicity of the donor organ (16).